THE CHOICE AND USE OF APPARATUS 291
LIST OF ILLUSTRATIONS
PAGE
NANDA KOT--AN EXAMPLE OF
TELE-PHOTOGRAPHY _Frontispiece_
HEAD OF DOG FLEA 56
STARCH GRAINS OF POTATO 72
PHOSPHORESCENCE ANIMALCULÆ 72
PROTEUS ANIMALCULE 72
CYCLOPS 72
BLADDERWORT 88
SPORES OF HORSE-TAIL 88
HAIRS ON A POTATO LEAF 88
SPIROGYRA 88
THORN INSECT 120
HEAD OF PALM WEEVIL 120
LEAF INSECT 120
HEAD OF STICK INSECT 120
FORAMINIFERA 128
DIATOMS 128
STINGING HAIRS OF NETTLE 144
BUTTERFLY WING SCALES 144
CRYSTALS FROM HUMAN BLOOD 168
CRYSTALS FROM THE BLOOD OF THE BABOON 168
CLUSTER CUPS 184
RUST OF WHEAT 184
POLLEN GRAINS ON A GRASS FLOWER 184
LOWER SIDE OF A FERN FROND 184
HEAD OF A BEETLE 200
HEAD OF HERCULES BEETLE 200
A CICADA 200
HEAD OF MANTIS 200
FACE OF A FLY 216
SECTION OF HUMAN SKIN 216
FEELER OF COCKCHAFER 232
VIEW WITH ORDINARY CAMERA 240
VIEW WITH MICRO-TELESCOPE 240
EYE OF A COCKCHAFER 256
HOOKS ON A BEE’S WING 256
SPIDER’S FOOT 264
FLY’S FOOT 264
FLY’S EYE 296
IMAGES SEEN BY A FLY 296
* * * * *
The Romance of the Microscope
CHAPTER I
THE EARLY DAYS OF THE MICROSCOPE
It is certain that lenses were used as early as the thirteenth century,
and it is probable that they date back to far earlier times. The
ancient gem cutters probably used spheres of glass filled with water as
magnifiers, their work could hardly have been accomplished without some
artificial aid. We know, from early writings, that burning glasses were
used by physicians in their work, and Seneca, the author, who wrote in
A.D. 63, says: “Letters, however small and dim, are comparatively large
and distinct when seen through a glass globe filled with water.”
Euclid, whose name at least is familiar to everyone, was, as shown
by his writings, perfectly well acquainted with the fact that curved
mirrors may be used to magnify objects, and that was so long ago as
the third century B.C. Convex glasses, used as spectacles, were first
mentioned by Bernard de Gordon, about 1307, but, as far as we know,
they were never used for the purpose of studying minute living objects.
To Leonardo da Vinci belongs the honour of seriously investigating,
for the first time, the properties of concave and convex lenses,
and several alchemists, as the early chemists were called, used
flasks filled with water, concave mirrors or glass balls to gather
together the rays of the sun. “Long before the dawn of the seventeenth
century, the principle of the lens was both comprehended and applied
to scientific matters by the Englishmen, Leonard Digges and his son
Thomas, and by the Italian, Giambattista Porta.”
Towards the end of the sixteenth century and the early part of the
seventeenth century, interest in the minute structure of natural
objects appears to have developed. As early as 1590, Thomas Mouffet
used magnifying glasses in studying small mites, and in 1637 Descartes
invented a single lens microscope in which the rays of light were
reflected on to the object by means of a concave mirror. This method of
illumination, it is interesting to note, is still used in some forms of
pocket magnifiers. Most of the early discoveries were made with single
lenses, for in the compound microscopes which were first made, it was
only possible to view such a small portion of an object at one time
that the advantage lay with the less complicated instrument.
The earliest microscopes were simply short tubes of any material which
would not admit light; at one end there was a lens, at the other a
glass plate on which the object to be examined was placed. Because
these crude instruments were chiefly used for the examination of
insects they were known as “Vitrea pulicaria” or “Vitrea muscaria.”
Later they were called “Engyoscopes,” and, after the invention of
compound microscopes, they were described as “Microscopia ludicra,” as
opposed to the latter instruments, known as “Microscopia seria.”
The next stage in the development of the microscope consisted in the
introduction of lenses of very short focal length, and, in 1665, Robert
Hooke used small glass balls, formed by fusing threads of drawn glass,
for this purpose.
It was Antony van Leeuwenhoek, however, who perfected these
instruments. He brought an extraordinary skill and industry to bear
on the grinding and polishing of minute lenses of short focal length.
Already in 1673 Regnier de Graaf wrote to the Royal Society in London
that Leeuwenhoek was making glasses far superior to those of the great
Italian lens maker, Eustachio Divini. Leeuwenhoek’s success was largely
due not only to his method of grinding, but also to the skill with
which he mounted his lenses, which were accurately fitted into a minute
hole in a metal plate. The object to be examined was firmly held in a
stand and adjusted by means of a screw movement. By this means, and by
the use of hollow metal reflectors, he succeeded in availing himself
of transmitted light in the case of transparent objects. Leeuwenhoek
was able to make immense advances with these instruments, the minute
pond animals he could see with ease, and by 1683 he had even attained a
sight of the bacteria. His researches represented the high-water mark
of work done with the simple microscope, most of the later work was
carried out with the compound instrument.
The earliest history of the compound microscope is difficult to
separate from that of the telescope and, in any complete account, the
two instruments must be considered together. It appears that the first
scientist that conceived the idea of using a series of lenses, rather
than a single lens, was Leonard Digges, whom we have already mentioned.
In a book by Porta, a writer who though not himself original, was
gifted with great curiosity and industry in the collection of the ideas
of others, we read: “How to make plain a letter held far away by means
of a lens of crystal,” and also that “with a concave lens you see
things afar smaller but plainer, with a convex lens you see them larger
but less distinct. If, however, you know how to combine the two sorts
properly you will see near and far both large and clear.”
Shortly after the publication of Porta’s book the method of combining
two lenses into a microscope or telescope was discovered, quite
accidentally, by a Dutch boy named Zacharias, who worked in the shop
of his father, a spectacle maker. The event was described by Willem
Boreel, Dutch Ambassador to France, in a letter written in 1655. He
wrote: “I am a native of Middleburg, the capital of Zeeland, and close
to the house where I was born, there lived in the year 1591 a certain
spectacle maker, Hans by name. His wife, Maria, had a son, Zacharias,
whom I knew very well, because I constantly as a neighbour and from
a tender age went in and out playing with him. This Hans or Johannes
with his son Zacharias, as I have often heard, were the first to invent
microscopes, which they presented to Prince Maurice, the governor and
supreme commander of the United Dutch forces, and were rewarded with
some honorarium. Similarly they afterwards offered a microscope to
the Austrian Archduke Albert, supreme governor of Holland. When I was
Ambassador to England in the year 1619, the Dutchman Cornelius Drebbel
of Alkomar, a man familiar with many secrets of nature, who was serving
there as a mathematician to King James, and was well known to me,
showed me that very instrument which the Archduke had presented as a
gift to Drebbel, namely, the microscope of Zacharias himself. Nor was
it (as they are most seen) with a short tube, but nearly two and a half
feet long, and the tube was of gilded brass two fingers’ breadth in
diameter, and supported on three dolphins formed also of brass. At its
base was an ebony disc, containing shreds or some minute objects which
we inspected from above, and their forms were so magnified as to seem
almost miraculous.” So this was the first compound microscope!
Although Zacharias invented the microscope, it was Galileo who
introduced it to the scientific world. He published a book in 1610
in which he wrote: “About ten months ago a rumour reached me of an
ocular instrument made by a certain Dutchman, by means of which an
object could be made to appear distinct and near to an eye that looked
through it, although it was really far away. And so I considered the
desirability of investigating the method, and reflected on the means by
which I might come to the invention of a similar instrument. I first
prepared a leather tube at the ends of which one placed two lenses
each of them flat on one side, and as to the other side I fashioned
one concave and the other convex. Then holding the eye to the concave
one, I saw the objects fairly large and nearer, for they appeared three
times nearer and nine times larger than when they were observed by
the naked eye. Soon after I made another more exactly, representing
objects more than sixty times larger. At length, sparing no labour and
no expense, I got to the point that I could construct an excellent
instrument so that things seen through it appeared a thousand times
greater and more than thirty-fold nearer than if observed by the
naked eye.” Galileo had his enemies, who accused him of having picked
Zacharias’s brains; he admitted that he had taken his idea from the
Dutchman’s invention, but further than that he would not go; in fact,
he replied that the invention of Zacharias was a mere accident but that
his own instrument was discovered by a process of reasoning.
It would serve no good purpose to tell the story of all the scientists
who have helped to bring the microscope to its present state of
perfection, although many of their descriptions of objects and
apparatus are as quaint as the latter. Scheiner, for example, who wrote
in 1630, mentions “that wonderful instrument the microscope, by means
of which a fly is magnified into an elephant, and a flea into a camel.”
To Kircher belongs the credit of being the first worker to construct
an instrument with coarse and fine adjustment and with a substage
condenser, which could be used either for concentrating the sun’s rays
or those from a lamp. With an instrument of this pattern Malpighi saw
the circulation of blood in a frog’s lung. By 1685, when instruments
with four and six lenses were being used, the compound microscope was
firmly established as a help to scientists, and the simple lens was
used thereafter as an adjunct but not a rival to the newer instrument.
History makes a strong appeal to many people, and those who are
fascinated thereby will find endless amusement in reading old books on
the microscope and its objects. In the preface to Mouffet’s _Insectorum
Theatrum_, one of the earliest books on insects, we read the following
quaint lines: “If you will take lenticular object glasses of Crystal
(for though you have Lynx his eyes, they are necessary in searching
for atoms) you will admire to see the Fleas that are curasheers, and
their hollow trunk to torture men, which is a bitter plague to maids,
you shall see the eyes of Lice sticking forth, and their horns, their
bodies crammed all over, their whole substance diaphanous, and through
that, the motion of their heart and blood. Also little Handworms, which
are indivisible, they are so small, being with a needle prickt forth
from their trenches near the pool of water which they have made in the
skin, and being laid upon one’s nail, will discover by the sunlight
their red heads and feet they creep withal.” The creatures called
Handworms are itch mites, which tunnel in the human skin.
In our chapter on Nature Study and the Microscope we refer to the brown
patches to be found on the backs of fern fronds; it is interesting to
note that so long ago as 1646 Sir Thomas Browne had quite a good idea
of their structure. Describing them, he said: “Whether these little
dusty particles, upon the lower side of the leaves be seeds we have
not yet been able to determine by any germination. But, by the help
of magnifying glasses we find these dusty atoms to be round at first
and fully representing seeds out of which at last proceed little
mites, almost invisible, so that such as are old stand open, as being
emptied of some bodies firstly included, which though discernible in
Hartstongue, is more notoriously discoverable, in some differences, of
Brake or Fern.”
Two years earlier a noted scientist, Hodierna, had made a special study
of the eyes of insects and, considering the crude instruments with
which he must have worked, his descriptions are wonderfully accurate.
Of the house fly he wrote: “The head is all eyes, prominent and without
lids, lashes or brows. It is plumed with hairs like that of an ostrich
and has two little pear shaped bodies hanging from the middle of the
forehead. The proboscis which arises from the snout can be extended
freely and stretched forth to suck up humours and can afterwards
be directed back through the mouth and taken into the gullet. This
instinct nature has given the creature according to its need, for it is
without a neck and cannot stretch forth its head to obtain its food,
as is also the case with the elephant.” The author’s knowledge of the
house fly was evidently greater than his knowledge of the ostrich,
for the bird has anything but a plumed head. The eye of the insect he
compares to a white mulberry.
Another of these early workers, writing about the same time, gives
a concise account of cheese mites, heading his description “On the
creatures which arise in powdery cheese,” he wrote: “The powder
examined by means of this instrument (the Compound Microscope) does not
present the aspect of dirt, but teems with animalcula. It can be seen
that these creatures have claws and talons and are furnished with eyes.
The whole surface of their body is beautifully and distinctly coloured
in such sort as I have never seen before, and which indeed, cannot be
seen without wonder. They may be observed to crawl, eat and work and
are equal in apparent size to a man’s nail. Their backs are all spiny
and pricked out with various starlike markings and surrounded by a
rampart of hairs, all of such marvellous kind that you would say they
are a work of art rather than of nature.”
At about this period the microscope was used for the first time for
medical work and, as far as can be ascertained, Pierre Borel was the
first to use it for this purpose, and he learned a great deal about the
structure of flesh and the appearance of blood.
Of all the early writers on microscopy the man who spread abroad his
knowledge of the instrument and its capabilities, more than anyone
else, was Kircher, who died in 1680. He was an energetic writer, and
wrote on a large number of subjects. His books dealt with magnetism,
designs for a calculating machine, light, sound, history of plague,
the philosopher’s stone, Egyptian antiquities, a history of China and
a grammar. To all who read his book on the plague, it is clear that
he had a good idea of infection; he was, in fact, the first writer
who suspected it, though the microscope he used could not show him
bacteria. In his book he wrote: “Everyone knows that decomposing bodies
breed worms, but only since the wonderful discovery of the microscope
has it been known that every putrid body swarms with innumerable
vermicules, a statement which I should not have believed had I not
tested its truth by experiments during many years.” The experiments he
performed to prove his statement are so quaint that we give them in his
own words.
_Experiment I._--“Take a piece of meat which you have exposed by
night until the following dawn to the lunar moisture. Then examine
it carefully with the smicroscope and you will find the contracted
putridity to have been altered by the moon into innumerable wormlets
of diverse size, which, however, would escape the sharpness of vision
without a good smicroscope. The same is true of cheese, milk, vinegar
and similar bodies of a putrifiable nature. The smicroscope, however,
must be no ordinary one, but constructed with no less skill than
diligence, as is mine which represents objects one thousand times
greater than their true size.”
_Experiment II._--“If you cut up a snake into small parts and macerate
with rain water, and then expose it for several days to the sun and
again bury it under the earth for a whole day and night and lastly
examine the parts, separated and softened by putridity, by means of a
smicroscope you will find the whole mass swarm with innumerable little
multiplying serpents so that even the sharpest eyes cannot count them.”
_Experiment III._--“Many authors claim that unwashed sage is injurious,
but I have discovered the cause of this. For when, by means of the
sun, I minutely examined the nature of the plant, I found the back of
the leaves completely covered by raised work as with the figure of a
spider’s web, and within the water appeared infinitesimal animalcules,
which moving constantly came out of little buds or eggs.”
_Experiment IV._--“If you examine a particle of rotten wood under the
sun, you will see an immense progeny of tiny worms, some with horns,
some with wings, others with many feet. They have little black dots of
eyes. What must their little livers and stomachs be like?”
In the light of modern discovery much of the writing of these early
microscopists seems absurd. Kircher’s experiments, for example, prove
nothing, and he is often hopelessly vague and sometimes incorrect in
his statements. We must not be too critical, however, for some of this
early work was excellent, the microscopes in use would not be tolerated
at the present day, and without these pioneers microscopy would not
have reached the stage it has. Rather than laugh at their efforts, we
should marvel that they did so well.
CHAPTER II
SOME EARLY MICROSCOPISTS
Of the early British microscopists, Robert Hooke must not pass
unnoticed. He was appointed Curator of the Royal Society two years
after its formation, and the terms of his appointment were somewhat
one-sided. He was required to “furnish the Society every day they meet
with three or four experiments”; for this no pay was to be his till the
Society accumulated sufficient funds to reward him.
Although compound microscopes had been invented in Hooke’s day, it is
noteworthy that he remained faithful to the single lens, in fact it was
not till very many years later that the simple lens was supplanted, in
general use by the more complicated, if more perfect instrument.
In his book on Microscopy, entitled _Micrographia_, Hooke gives
a quaint account of the making of a microscope. “Could we make a
microscope,” he writes, “to have only one refraction, it would _cæteris
paribus_, far excel any other that had a greater number. And hence it
is, that if you take a very clear piece of a broken Venice glass, and
in a Lamp draw it out into very small hairs or threads, then holding
the ends of these threads in the flame, till they melt and run into a
small round Globul, or drop, which will hang at the end of the thread;
and if further you stick several of these upon the end of a stick with
a little sealing wax, so that the threads stand upwards, and then on
a whetstone first grind off a good part of them, and afterward on a
smooth Metal plate, with a little Tripoly, rub them till they come
to be very smooth; if one of these be fixt with a little soft wax
against a small needle hole, prick’d through a thin Plate of Brass,
Lead, Pewter, or any other Metal, and an Object, plac’d very near, be
look’d at through it, it will both magnifie and make some Objects more
distinct than any of the great Microscopes.”
This early worker was noted for the variety of his investigations
rather than for the depths of his learning. Amongst the so-called
Observations, in his book are many that are not connected with
microscopic work. The following are interesting and, in the curious old
book _Micrographia_, there are an extraordinary number of well executed
illustrations. Early in his book Hooke compares various man-made
objects, such as a razor edge, the point of a needle and a piece of
cloth, with various natural objects, and always to the detriment of
the former. He examined _Foraminifera_ with his microscope, and was
probably the first man to draw these beautiful little creatures.
Petrified wood and charcoal also came under his notice. When he studied
cork, he observed that it was made up of “little boxes or cells,” and
the name cell has survived to this day despite the fact that it is by
no means an appropriate term. That Hooke’s knowledge was not very deep
is shown by the fact that he presumed cork to be a fungus growing on
the bark of trees.
Many of the objects we have described in our pages were described and
illustrated by Hooke more than two hundred years ago. The sea mat,
despite his accurate observations, he mistook for a seaweed, as many
later naturalists have done. The stinging hairs of nettle he made
out in every detail. Fish scales, bee stings and birds’ feathers all
came under his notice. The foot of a fly he described with wonderful
accuracy; the scales of a butterfly’s wing and the head of a fly were
all studied and described in detail. On the life history of the gnat
he made many blunders, but he saved his reputation by remarkable
observations upon the _Chelifer_, a curious parasite of the fly which
we mention in our pages, and upon the silver fish, a little creature
which frequents sugar and starch. Neither of these organisms had been
described before. Fleas, lice, vinegar-eels and spiders were also
studied by this indefatigable worker, a worthy collection indeed, but
Hooke, like others of his time, was an observer first and foremost. As
a methodical, scientific worker he was of little account.
Living about the same time as Hooke, the celebrated Italian, Malpighi,
laid the foundations of much of our present-day knowledge of plant
structure. Various romantic stories have been told concerning certain
imaginary events which led Malpighi to take up the study of plant
structure, but the scientist himself refuted these picturesque stories.
Suffice it to say that his book on the subject, _Anatome Plantarum_,
though imperfect in many respects and, as might be conjectured
in so early a work, often inaccurate, contains a large number of
astonishingly good drawings; many of the original drawings, by the way,
executed in red chalk, are in the possession of the Royal Society.
It is interesting to note that this botanist compared the falling of
leaves to the shedding of an insect’s skin, in this respect at any rate
he had advanced no further than Aristotle, who compared leaf-fall to
the moulting of a bird. On the other hand, the Italian was the first
scientist to describe the pores (stomata) of leaves, though he never
discovered that they occurred on all leaves. He, first of all men,
showed that nectar was formed by the flower and not transferred thence
from other sources as had previously been believed; he too explained
accurately for the first time the process of germination in the seed.
It was not alone as a botanist, however, that Malpighi was celebrated.
He elucidated the various changes which take place during the hatching
of an egg; he was the first man to give an accurate account of the
structure of an insect, and this he did in his work on the Anatomy of
the Silkworm. Using a simple microscope for his investigations, he
contracted an eye affliction during this period from which he suffered
more or less severely all the rest of his life. He discovered the
breathing tubes of insects and that when they are covered with grease
the insect will die “in the time that one can say the Lord’s Prayer”;
the heart, the silk glands, the development of wings and legs were all
discovered for the first time by this untiring worker, aided by his
simple microscope.
Pages could be filled with accounts of Malpighi’s other scientific work
on the structure of the lung, the liver and kidney, the life of the
liver fluke and a hundred and one other subjects. Though undoubtedly a
great and clever microscopist, the general estimate seems to be that
his work had little influence upon the scientific world. The main
reason is that he was ahead of his time; men of the day concluded, for
instance, that in his Anatomy of Plants he had said the last word on
the subject, that there was no more to be learned. An English worker,
Nehemiah Grew, carried the Italian scientist’s studies of plant
structure a little further and his Anatomy of Plants contains many new
and often accurate observations. His studies also led him to discover
the structure of the ridges and sweat pores of the human hand, in fact
Grew may be looked upon as the originator of the study of finger prints.
A Dutchman, Jan Jacobz Swammerdam by name, and a contemporary of Grew,
was undoubtedly the most accurate observer amongst these old-time
microscopists. Despite ill health, his enthusiasm was unbounded, and a
friend wrote concerning him: “Swammerdam’s labours were superhuman.
Through the day he observed incessantly, and at night described and
drew what he had seen. By six o’clock in the morning in summer he
began to find enough light to enable him to trace the minutiæ of
natural objects. He was hard at work till noon, in full sunlight, and
bareheaded, so as not to obstruct the light, and his head steamed with
profuse sweat. His eyes, by reason of the blaze of light, became so
weakened that he could not observe minute objects in the afternoon,
for his eyes were weary.” If only for the fact that the Dutchman made
clear the processes involved in the transformations of insects, his
name would be famous. He described the structure and habits of the hive
bees, male, female and drone with wonderful accuracy, and illustrated
his work with plates which “would do credit to the most skilful
anatomists of any age.” Swammerdam was sarcastic at times; he had
shown that the facets of a bee’s eye are six-sided and, as so commonly
happened in those days, some naturalists jumped to a conclusion, in
this case that the fact explained the six-sidedness of the cells in the
honey comb. By the same reasoning Swammerdam remarked that men, having
round pupils, should build round houses. It is not only for his study
of the minute structure of insects that this microscopist is noted,
he worked upon the tadpole and the snail. He it was who discovered
the red blood corpuscles of the frog, and he described his discovery
in the following terms: “In the blood I perceived the serum in which
floated an immense number of rounded particles, possessing the shape
of, as it were, a flat oval, but nevertheless wholly regular. These
particles seemed, however, to contain within themselves the humour[1]
of other particles. When they were looked at sideways, they resembled
transparent rods, as it were, and many other figures, according, no
doubt, to the different ways in which they were rolled about in the
serum of the blood. I remarked besides that the colour of the objects
was the paler the more highly they were magnified by means of the
microscope.” Of the snail he made a number of strikingly accurate
studies, in all of which he was aided by his lenses, so that it is the
more remarkable that he considered snails to be insects.
Leeuwenhoek, another Dutchman, we have already mentioned in our
previous chapter. He of all men brought the simple microscope to its
highest state of development. His instruments were one of the sights
of Holland, and many eminent personages made a point of seeing them.
Though he had not the advantage of any scientific training and spoke
no other language than his own, he made some remarkable additions
to the scientific knowledge of the time. Like Hooke, he was not a
methodical worker, he was impelled by an unbounded curiosity. “When
we are inclined to disparage Leeuwenhoek’s hasty methods it is well
to recollect that he initiated biological inquiries of the greatest
interest, _e.g._, the parthenogenesis of aphids and the revivification
of dried microscopic organisms, while he gave the first notices, or the
first worth mention, of rotifers, Hydra, infusorians, yeast cells and
bacteria.”
We may here explain the meaning of the term “parthenogenesis of
aphids.” The female aphids or green flies are able to bring forth
generation after generation during the first two-thirds or so of each
year without the assistance of males. This form of increase, which by
the way accounts for the extraordinary numbers of green fly, is known
as parthenogenesis.
Leeuwenhoek thought that no one but himself could use his lenses
properly, in consequence, when he sent any interesting object to a
friend for him to examine, a lens was always affixed in place so that
the object could be seen to the best advantage. He gave a set of his
lenses and objects to the Royal Society, and described his gift as
“a small black cabinet, lackered and gilded, which has five little
drawers in it, wherein are contained thirteen long and square tin
boxes, covered with black leather. In each of these boxes are two
ground microscopes, in all six and twenty; which I did grind myself,
and set in silver; and most of the silver was what I had extracted from
minerals, and separated from the gold that was mixed with it; and an
account of each glass goes along with them.”
Kircher, whose work we mentioned in our last chapter, was overwhelmed
with the notion that various living creatures are generated from
non-living matter. Fleas, for example, he was certain, came from dirt,
and it remained for Leeuwenhoek to prove that they arise from eggs and
grubs, in the manner now so well understood.
He carefully studied the structure of a garden spider, and for the
first time explained its wonderful feet, its jaws and poison gland,
its spinnerets and silk. He studied _Hydra_ first of all men, and
said that, under the microscope, its tentacles appeared to be several
fathoms long. Although sadly at sea over the correct position of his
snails in the animal world, he was clever enough to include _Volvox_
amongst the plants and fortunate enough to see the young forms escape
from the parent colony.
Concerning this microscopist’s early studies in bacteriology we may
quote from Professor Miall’s _The Early Naturalists_, a book by the
way of the greatest interest to those who would learn something of
the struggles of the men who laid the foundations of our present-day
biological knowledge.
Professor Miall says: “In 1683 Leeuwenhoek wrote a letter to the Royal
Society which contains the first mention of bacteria. He had been
writing and speculating upon saliva, and had searched the saliva of
the human mouth for animalcules without finding any. It then occurred
to him to ask whether the teeth might lodge animalcules discharged
from the salivary ducts. He tells us that, though his own teeth were
scrupulously clean and particularly sound for his age (about fifty),
the lens revealed a white deposit upon them. This deposit was found to
contain minute rods, some of which showed either a steady or gyratory
movement. Others were very minute, of rounded form, and moved with
remarkable velocity. The largest of all, which were either straight
or bent were motionless. The teeth of an old man, which were never
cleansed, contained among others large rods which exhibited snake-like
undulations. Rubbing the teeth with strong vinegar did not kill the
moving bodies, but they became quiescent when detached and placed in a
mixture of vinegar and saliva, or vinegar and water. Nine years later
Leeuwenhoek returned to the subject. Living particles were no longer
met with in his teeth, and he was at a loss to explain why, until
it occurred to him that he was accustomed to drink hot coffee every
morning. This, he thought might have killed the animalcules, and his
conclusion was confirmed by finding that on the back teeth, which were
less exposed to the hot drink, plenty of them were still to be found.
In 1697 he tells how he pulled out a decayed tooth, and found that the
cavity abounded in moving particles.” Nearly a hundred years elapsed
before anyone else took up the study of bacteria.
From the time of Leeuwenhoek onwards, scientific discoveries were
announced in rapid succession, so that in one short chapter it is
impossible to keep pace with the progress that was made. Among the
great men who owe much of their success to the microscope we may
mention the Frenchman Réaumur, whose memory is kept green for all
time by his thermometer; as a worker upon problems of insect life
he was indefatigable; the Swede, Linnæus, to whose early efforts we
owe the orderly arrangement of living creatures and plants, known as
classification. This arrangement has been considerably modified, more
modern ideas have upset much that he initiated, yet he remains the
parent of orderly arrangement.
Buffon, a great naturalist, was followed by Cuvier, the first serious
student of fossils; by Humboldt, naturalist and traveller; by Robert
Brown, the founder of modern Botany; by Darwin and by Pasteur in turn.
How much these men owe to the microscope can never be known; certain
it is that without its assistance our world, the world we know and can
see, would have been smaller than it is to-day.
FOOTNOTE:
[1] Humour is here used in its original sense, meaning moisture or a
liquid.
CHAPTER III
THE ACTION OF LIGHT
It is hardly necessary to remark that the wonderful properties of the
microscope depend upon light. Without light, lenses would be useless,
objects could not be illuminated and we could not see them. In this
short chapter we propose to give a brief outline of the action of
light; if our words appear to savour of the school-book, we shall try
to avoid it, but, we repeat, if they do so we would remind our readers
that the more one knows of the action of light the better use one can
make of one’s instrument. As a well-known microscopist has remarked
we may be able to afford a costly harp or a costly microscope, but
although we may be able to strike a few notes on the former and examine
a few objects with the latter, we can only make the best use of either
by thoroughly understanding and practising upon it.
The first thing we learn when we study light is that it travels in
straight lines. The chief source of light to the inhabitants of this
earth is the sun. Now the sun is so far away that, for all practical
purposes, the rays of light coming from it may be looked upon as being
parallel to one another. That we must always remember, when dealing
with the sun, though, of course, it does not apply when we are dealing
with lights near at hand, unless they are specially constructed to
throw parallel beams or rays, whichever we elect to call them. To
prove that light travels in straight lines is not difficult, and we
may devise a number of experiments for the purpose. The doors and
ventilators of many dark rooms, in which photographic operations are
carried on, are constructed on the assumption that light cannot travel
round corners. An arrangement as shown in the diagram will allow air,
but no light, to pass. If light were capable of going round corners,
some other arrangement would have to be devised for the ventilation of
dark rooms.
[Illustration]
Having learned so much about light, we come to the most important fact
of all, as far as the action of light concerns microscopic work. When
rays of light travel, from a substance like air into a substance like
water, they are bent out of their straight course. Without any desire
to introduce a number of unfamiliar words, we may venture to remark
that, any substance through which light passes is called a medium.
Some media are clearly more dense, more compact or solid--dense is the
proper word--than others. Water is more dense than air and glass than
either. The bending of light rays is known as refraction. So now we may
state our second law a little more concisely, thus:--When light passes
from a medium into one more dense, or _vice versa_, it is refracted,
and the more dense the medium into which or from which the light passes
the greater the refraction.
[Illustration]
A diagram and an experiment should make matters clear. Suppose AB is a
ray of light traveling in air and that it falls on a sheet of water,
WXYZ, the ray will be bent along BC and its course from air to water
may be represented by ABC. Suppose again, WXYZ represents, not water
but glass; as glass is more dense than water the course of the ray AB
is represented by ABD, it is refracted or bent to a greater extent
than the ray which passed from air into water.
For our experiment we need only plunge a stick into water and notice
that, owing to this property of light, the stick appears bent, from the
point where it comes into contact with the surface of the water.
[Illustration]
Some of us may be old enough to remember that once, on either corner of
nearly every mantlepiece, there stood an ornament of doubtful utility
from which there hung a dozen or more glass prisms. Now the only beauty
about these otherwise hideous contraptions was to be seen when light
played upon them. Then patches of violet, green, yellow and red were
thrown upon neighbouring objects. White light, ordinary sunlight that
is to say, is really composed of various colours--violet, indigo, blue,
green, yellow, orange and red--which, when combined together, make
light as we know it. When white light passes through a prism of glass,
it is not only bent out of its course, but broken up into all these
colours. A prism, as we all know, when examined at either end, is seen
to be triangular in shape. Putting aside for a moment the question of
the breaking up of light into its component parts, the path of a ray
of light through a prism is shown in the diagram. As the ray passes
from air into glass it is bent, because glass is more dense than air;
it is bent once more on leaving the prism because air is less dense
than glass.
[Illustration]
Now lenses are made of various shapes, and those with two outwardly
curved surfaces are known as double convex lenses. A double convex lens
is usually made with both its surfaces equally curved and in the finer
optical work great care is taken to ensure that this is the case. For
certain purposes, however, as we shall learn in a moment, one or other
of the faces only may be much more curved than its companion and this
may be carried to such an extreme that one face is flat, the lens is
then known as plano-convex. Lenses may also have inwardly curved faces,
if both are of this design they are called double concave; if one face
is flat and the other inwardly curved they are known as plano-concave.
There are other combinations, for example, one face may be inwardly
curved and the other outwardly curved, but the four kinds we have
described are all that need trouble us.
[Illustration]
It does not require a great amount of imagination to recognise that
the double convex lens, that is the lens with two outwardly curved
faces is little more than a pair of prisms placed base to base, or more
accurately, a number of prisms so arranged as shown in the diagram.
Parallel rays of light falling upon such an arrangement of prisms
would be bent from their course, as shown by the arrows, and this is
just what happens with a double convex lens. Now rays of light from
an object, passing through a lens of this shape may follow any one of
three courses, according to the position of the object with regard to
the lens. In one position and one only the rays after passing through
the lens will be parallel to one another, as shown in the diagram.
The only position of the object for the above to take place is when
it coincides with a point known as the principal focus of the lens,
conversely the parallel rays of light from the sun, after passing
through a double convex lens, will come to a point at its principal
focus.
Suppose now that the object be placed at a point beyond the principal
focus of the lens, the light rays therefrom will, after passing through
the lens, converge to a point thus:--
[Illustration]
In the diagram O is the object and P the principal focus of the lens.
The third case occurs where the object is nearer to the lens than its
principal focus, then the rays after passing through the lens, diverge
and never meet.
[Illustration]
We have already stated that when white light passes through a prism
it is broken up into different coloured rays varying from violet to
red. The reason for this is that all the light rays composing white
light are not bent equally as they pass from one medium to another.
The violet rays are bent the most, the indigo next, blue next, down to
red, which is least bent. Once more, considering the double convex lens
as made up of a number of prisms, let us represent, by a diagram, the
course of parallel rays of white light through it.
[Illustration]
A A′ represent the parallel rays of white light falling on the lens, L
L′. The blue rays are bent more than the red, so the principal focus
of the former is at C and of the latter at D. The consequence of this
difference in bending of the various coloured light rays would be
most serious in microscopic work were not means devised to overcome
it. Objects for instance at C in our diagram would appear blue, at D
they would appear red, whilst at E E′ though no single colour would
predominate they would be illumined with many coloured rays, though
less strongly than at C or D.
This chromatic aberration, as it is called, depends amongst other
things on the nature of the glass used in lens construction. It has
been found, however, that a combination of flint and of crown glass
will overcome the difficulty. In a later chapter we shall explain
the difference between these two kinds of glass. In practice, a
plano-concave lens of flint glass is combined with a double convex lens
of crown glass and, if the nature of the glass is satisfactory, as
also the shapes of the lenses, there is full correction for chromatic
aberration, and objects viewed through such a lens will not appear with
coloured margins.
There is one further trouble likely to occur in such, or any lens. We
write of the rays meeting at a point. In our diagrams we represent the
rays by straight lines, really they are much more complicated than they
appear in a diagram. It is quite easy to take a ruler and make our
imaginary light rays meet at a point, as a matter of fact, where real
lenses and real light rays are concerned, it is very difficult, if not
impossible, to make the latter meet at a single point. One more diagram
may make the matter clear.
[Illustration]
Parallel rays A pass through our lens and, as we know, they should all
meet at a point P, the principal focus of the lens; the majority do so,
but some meet at other points, such as P′. In consequence of this it
is difficult to obtain a clear image of an object at P, and the lens is
said to suffer from spherical aberration. The perfect simple lens would
be one fully corrected for chromatic and spherical aberration.
CHAPTER IV
THE COMPOUND MICROSCOPE
In our chapters, dealing with, the history of the Microscope, we
attempted to trace the gradual development of the compound instrument
from the simple lens; we stated that the latter, in a crude form, had
been known and used from very early times and that the former developed
side by side with the telescope. We have also said a few words in
Chapter III. concerning light for the reason that the microscope can
be better understood and used more efficiently when we are acquainted
with the phenomena due to light. The simple lens, sold under the name
of pocket magnifier, in its cheapest form consists of a double convex
lens, that is to say, a lens with two outwardly curved surfaces. Better
quality pocket magnifiers consist of two or more lenses, which may be
either double convex; plano-convex, _i.e._, with one surface perfectly
flat and the other outwardly curved, or they may be constructed of a
combination of double convex and plano-concave lenses, such as were
described on p. .
The object of both the simple and compound microscope is to make
objects appear larger than they do to the naked eye. When we buy our
pocket lens we shall find that these little instruments are constructed
to give different degrees of enlargement, some make objects appear
five times larger than they do to the naked eye, some ten, some
fifteen and some twenty times larger. Twenty times is about the limit
of magnification for the ordinary pocket lens. If we are observant we
shall notice something else--the greater the magnification the nearer
we must hold the lens to our object. Within certain limits, this is
not a very serious matter, but a point is reached where we must hold
our lens so near to the object that we cannot see it, and that is why
we cannot obtain very great enlargement with a pocket lens. Despite
this fact, as we read in our opening chapter, some very wonderful
discoveries have been made with these simple microscopes.
Now we wish to show how a compound microscope works and, having done
so, to explain the uses of its various parts. We shall consider the
lenses of the instrument to be double convex; we do this for the sake
of simplicity. Even in the cheapest compound microscopes of to-day
simple convex lenses are never used, for the reason we explained in
our last chapter. To understand the course of the light rays passing
through our microscope, however, we may look upon the lenses as being
merely double convex.
Let us try a simple experiment first of all. For the purpose we require
two double convex lenses, one capable of magnifying more than the
other, a sheet of paper and a candle. We must darken the room in which
we make the experiment and, having lighted the candle, we may proceed
to make a compound microscope, for that is really what we are about to
do. Taking the lens which gives the greatest magnification, we look
through it till we can see a clearly defined image of the lighted
candle, then we fix the lens at that spot, so that, during the rest of
our experiment, the candle and lens remain at the same distance from
one another. Now we put the piece of paper as nearly as we can in the
position of our eye, moving it nearer or further from the lens till we
have a perfectly clear image of the candle thrown upon it. The first
thing to strike us is that the image is upside down; it is known as a
real, inverted image. Real because it can be thrown upon a screen and
inverted--well because it is upside down. There are some images, as we
shall learn in a moment, which can be seen but which cannot be thrown
upon a screen: they are called virtual images.
Having fixed our sheet of paper in position, we take our second lens,
focus it sharply upon the back of the sheet of paper, being careful to
keep the centres of the two lenses as far as possible in a straight
line with one another. Having obtained a sharp image we remove the
paper and gradually advance our second lens towards the first. We soon
reach a point where we have a very much larger image of the candle than
the first lens gave us; we must fix our second lens at this point
for we now have a compound microscope, a very crude one certainly and
without the trimmings which make the microscope so useful. Before we
proceed to explain what has happened to the light rays we must take our
paper screen once more and place it as near as possible to the spot
where our eye was situated when we saw the second image. We shall find
that, however much we may move our screen to or from the second lens we
can never manage to obtain an image upon it for the reason that this
second image is virtual, but unlike the first image it is not inverted.
[Illustration]
One or two diagrams will help to explain our experiment and, instead of
the lighted candle, we will suppose that our object is an arrow--it is
easier to draw and serves just as well. The magnification of the object
by our first lens may be represented by the diagram below, where AA is
the lens, CD the object and D′C′ its image.
The arrow C′D′ shows the point at which we placed our screen, and as
our diagram shows, the image is magnified and inverted.
Our second lens, we remember, was focussed on the back of the paper,
placed at C′D′; for practical purposes we may ignore the thickness of
the paper and say that it was focussed on the image C′D′. Had we left
it at that, the further course of the rays through the second lens
would be represented by a replica of the diagram we have just given.
But, in our experiment, we moved the second lens nearer and nearer to
C′D′ till we obtained a clear much magnified erect image of C′D′, let
us call this second image C″D″, and represent the course of the light
rays by a diagram.
[Illustration]
We may well ask, why did the lens AA, our first lens, form a real
image whilst the second lens BB, which is precisely similar to AA,
except that its magnifying power is not so great, form a virtual
image? The formation of a real or a virtual image is nothing to do
with magnification, so we repeat--why do two similar lenses form
different kinds of images? Let us refresh our memories with the
remarks concerning the principal focus of lenses in the last chapter,
then we may try another experiment. The principle focus of a double
convex lens, we remember, is the point to which parallel rays of
light converge, after passing through the lens. If now our object is
further away from the lens than its principal focus, a state of affairs
that existed in the case of our lens AA and the object CD, we obtain
a magnified, real but inverted image; if, on the other hand, using
the same lens if we wish, the object is nearer to the lens than its
principal focus, we obtain a magnified virtual and erect image. The
form of image then depends on the relative positions of lens and object
and not on the magnifying powers of the former.
After this digression, we will see what happens when we combine the
diagram showing the real, inverted image, formed by the lens AA with
the virtual erect image, formed by the lens BB. In reality we will
draw a diagram showing the path of the light rays through our compound
microscope.
[Illustration]
We have used the same lettering as in our previous diagrams and we
see that, also as before, a real, inverted image C′D′ of the object
CD is formed by the lens AA and a virtual, erect image C″D″ of the
image C′D′ is formed by the lens BB, the object CD being further from
the lens AA than its principal focus and the image C′D′ being nearer
to the lens BB than its principal focus. One very important point we
must notice before we leave the diagram. We have mentioned several
times that the image formed by BB is erect and so it is, but it is an
erect image of an already inverted image, so that the final image of
CD, as seen by the eye E is inverted. The fact that objects viewed
through the microscope appear upside down is puzzling at first. To all
intents our two double convex lenses represent a compound microscope;
actually, they should be fixed at either end of a tube, blackened on
the inside. The lens AA, nearest to the object, would then be known
as the objective and the lens BB nearest to the observer’s eye would
be known as the ocular or, more commonly the eyepiece. There are, of
course, very many refinements, designed to make the instrument capable
of performing the most accurate work, and needless to say these simple
lenses would neither give very great magnification nor any clear
images. Let us describe a more refined compound microscope than the
one we constructed in our darkened room. The optical parts, that is
to say the lenses, are the most important parts of every microscope,
upon their qualities depend the degree of efficiency of the instrument;
the metal portions, known collectively as the stand, contribute to the
easier, smoother working of the microscope.
[Illustration:
_By the courtesy of Messrs. F. Davidson & Co._
THE HEAD OF A DOG FLEA
No wonder the flea is an annoying creature. As the plate shows, it is
armed with knives, lances and saws, all designed to injure the skin
of its victim.
]
The stand must claim our attention first. The base of the instrument,
called the foot, is usually either three-legged or horse-shoe shaped;
whatever its form it should be heavy, for only thus can the microscope
be steady, and steadiness is essential in all microscopic work. At
the top of the foot there is a joint, in order that all the other parts
of the stand may be inclined at any angle, from the vertical to the
horizontal. Just above the joint is a bent arm of brass, to the forward
end of which a brass tube is affixed. This tube is designed to hold
the lenses, the objective at its lower end, the eyepiece at its upper
end. The tube is always blackened inside; were this not the case, light
passing through the objective would be reflected in all directions from
the sides of the tube and a clear image of the object could never be
obtained. The tubes of microscopes vary in length according to their
country of origin; English and American tubes are ten inches long,
those of continental make vary from a little more than six inches to
rather more than seven inches in length.
Affixed to the lower end of the bent arm of brass, mentioned above,
is a flat metal plate, known as the stage; at its centre, there is a
circular hole through which rays of light pass to illuminate objects
placed upon it. Below the stage, at the edge nearest to the foot, there
is a metal peg, over which fits a tube to which a mirror is attached by
a moveable joint. The mirror reflects light rays through the opening
in the stage. The tube, holding it, can be slipped up and down the peg
under the stage, thereby bringing it nearer to or further from the
object and so altering the intensity of the reflected light, as we
shall explain in a moment. Owing to its moveable joint, it is possible
to swing the mirror to the right or left, so that the reflected light
rays do not pass directly through the object on the stage, but strike
it on one side or the other, thereby giving what is known as oblique
illumination.
The cheapest forms of compound microscopes have all the parts we have
mentioned, and focussing is carried out by sliding the tube, with its
objective and eyepiece, up and down within its holder, in order to
bring the objective further from or nearer to the object.
In more expensive instruments there are further refinements, in fact,
on some of the very costly present-day instruments, there are so many
appendages and appurtenances that it is doubtful whether some of them
are not more of a hindrance than a help, at any rate they increase the
possibility of trouble by their liability to get out of order. Such
microscopes are only of use to very expert workers; there are, however,
a good many additional features to be found on quite moderate-priced
instruments, features which are a great help to the microscopist.
It is obvious that we cannot attain any degree of accuracy in
focussing, especially with high magnifications, when we must perforce
raise or lower the tube by hand. To obviate this difficulty, most
microscopes are provided with mechanism known as a coarse adjustment;
it consists of milled screws at either end of a metal rod; in the
centre of the rod there is a little cog-wheel which engages with a row
of notches on the tube. By turning the milled screws slightly in either
direction, we can impart a considerable upward or downward movement
to the tube carrying the objective and focussing at once becomes a
more simple matter. The coarse adjustment is only useful for examining
objects with a low magnification; if we use it when objects are being
highly magnified we run the risk of screwing our objective down upon
our object, to the certain destruction of the latter and the probable
injury of the former. To obviate such a catastrophe, most of the
better class microscopes are also provided with a fine adjustment. By
means of this adjustment, which externally takes the form of a single
milled screw, a considerable turn of the screw in either direction only
imparts a very slight upward or downward movement to the microscope
tube. In the best instruments, movements of as little as one hundredth
part of a millimetre may be imparted to the tube by the fine adjustment
and, seeing that there are about twenty-five and a half millimetres to
the inch, it is obvious that a good fine adjustment is very delicate
and, being so, must be treated with care. The fine adjustment is used
to supplement coarse adjustment in the final focussing, when using high
magnifications.
A few words may be devoted to the mirror, for on its intelligent use
much depends. Usually we shall find that it is plano-concave, that is
to say, flat on one side and hollowed out on the other. The use of
the mirror, as we have mentioned already, is to reflect rays of light
through the opening of the stage on to the object we desire to examine.
Both mirrors will reflect parallel rays of light to a point, just as a
double convex lens will so direct them from their course that they meet
at a point. The concave mirror gives the more powerful illumination,
because it reflects more light rays than a flat mirror of the same
diameter.
We have mentioned that, to obtain full advantage from the mirror it
should be capable of movement to and from the stage. When we desire
strong illumination we arrange the mirror so that its reflected rays
meet at a point coinciding with our object. Should less intense
illumination be required, we slide the mirror nearer to the stage, and
of course nearer to our object, so that the reflected rays meet at a
point above the object.
[Illustration]
The two diagrams, given below, show the path of the rays of light,
where O is the object, and a trial with our microscope will soon show
which position gives the more powerful illumination.
For high-power work, such as bacteriology or even the examination of
sections of plants, etc., even the best concave mirror will not give a
sufficiently powerful illumination; accordingly an instrument, known
as the condenser, is fixed below the stage, between the mirror and the
object. The condenser, as its name implies, condenses the rays of light
reflected to it by the mirror. It consists of a series of lenses so
arranged that they will throw a very powerful cone of light. Provision
is made for focussing the rays from the condenser on to the object.
Sometimes, for special forms of illumination, it is necessary to cut
off some of the rays of light passing through the condenser. It may be
that we desire to dispense with the outer rays of the cone of light
or, when delicate details are being studied, we may wish to impede the
central rays. In either case diaphragms, popularly called “stops” are
used. Our diagrams show A the outer rays of a cone of light cut off and
B the central rays similarly treated.
[Illustration]
In old pattern microscopes and in many instruments not provided with
condensers, the diaphragm used for the purpose of cutting off the outer
rays of the cone of light, consists of a blackened circular metal
plate, perforated with a number of different sized circular holes.
This plate is fixed below the stage in such a manner that, as it is
revolved, holes of various diameters are brought one by one within the
cone of light. It need hardy be remarked that the smaller the hole
in the diaphragm the more light is cut off and the less reaches the
object. In more modern instruments and in practically all which are
fitted with a condenser, an Iris diaphragm is fitted. A diaphragm of
this nature consists of a number of thin, blackened, metal leaves,
fastened to a metal ring in such a manner that, when the ring is
revolved, the leaves close together, making the opening in the centre
smaller and smaller. The Iris diaphragm has many advantages over the
old perforated metal plate. At will, we can have any opening from full
to the merest pin-point or we can cut off the light rays altogether,
should we wish to do so; we are not confined to a definite number of
stops. As we cut off these outer rays of light we shall find that, up
to a certain point, though the illumination becomes less and less the
object becomes more and more clear, or, to use the correct expression,
its definition is improved.
When it is necessary to cut off some of the central rays of the light
cone, either a circle of glass with an opaque centre is dropped into a
metal holder below the stage, or a circular metal plate, held in the
centre of a metal ring by three arms, is used in the same manner.
The effect of cutting off the central rays of the light cone is, of
course, to reduce the illumination and to show up delicate detail to
advantage. No direct rays of light reach the objective, such as do
pass into the microscope are all diffused from the edges of the object.
We have already mentioned that the optical parts of the compound
microscope are of greater importance than what may be termed the
mechanical portions and the objectives are more important than
the eyepieces. Better results can always be obtained with a good,
high-power objective and a low-power eyepiece, than with an inferior
objective and a good quality eyepiece. The merits of the eyepiece,
however great, will not be adequate compensation for the failings of
the objective. Modern objectives are composed of several lenses and of
a combination of flint and crown glass, as we explained in our last
chapter. They are so designed that they can be screwed into the lower
part of the microscope tube. The focal length of each objective is,
or should be, marked upon it; as a general rule, however, it may be
taken that the smaller the lower lens, the shorter its focal length and
therefore the greater its magnifying power.
The form of eyepiece most usually met with is known as Huyghen’s.
It consists of two plano-convex lenses, with their flat or plane
surfaces directed away from the objective. The smaller of the two
lenses is situated nearer to the eye of the observer and is known
as the eyeglass; its function is to magnify the image formed by
the objective. The larger, lower lens is known as the field or
collecting glass; it renders the image clearer though, in so doing,
it reduces the magnification of the eyeglass. In instruments provided
with more than one eyepiece we shall wish to know which gives the
greater magnification; this is or should be marked upon the metal rim
surrounding the eyeglass but, in general, it may be stated that the
shorter the eyepiece the greater its magnification. We repeat again,
increase your magnification always, when possible, by using higher
power objectives rather than eyepieces with greater magnifying powers.
Sometimes it is necessary to use a greater magnification than our most
powerful objective will give us; then we must fit our most powerful
eyepiece and draw out the upper part of the microscope tube--in the
best instruments they are made to pull out, after the manner of the
telescope. The effect of so doing will be to increase the magnification
considerably but, at the same time, the definition or clearness is
seriously impaired.
For the examination of practically all our microscopic objects we
require a number of slides, little glass slips of good, thin, clear
glass. They may be used over and over again unless we make permanent
preparations, but we are hardly likely to do so in our early days. The
slides are held in place on the microscope stage, either by a pair of
clips attached thereto or by resting against a bar running across the
stage. We may here remark that it is essential always to keep one’s
microscope slides absolutely clean. Dirty slides denote the careless
worker; moreover, dirt when magnified is misleading. Objects which
are being examined in water or any other liquid should be covered
with a cover-slip, an exceedingly thin circle or square of glass. The
cover-slip is as much a protection for the objective as for the object
and its cleanliness also, is all important.
We have not mentioned any refinements such as the mechanical stage, by
means of which slides on the stage may be rotated, moved to the front
and to the back of the stage or from side to side. We have omitted
these because they are not essential even for the very best work; they
lend additional comfort to the use of the microscope but, again, they
are not essential. The microscopist who requires such luxuries may
learn about them in the larger text-books on the microscope.
CHAPTER V
ANIMAL LIFE IN THE PONDS AND STREAMS
The enthusiastic microscopist will probably never lack material for his
instrument, whatever branch of microscopical work he may decide to make
his own. To the student of Pond Life, either animal or vegetable, there
is granted a never-ending store of beautiful and interesting objects.
Because one pond has been thoroughly searched and all that it can offer
has been carefully examined, we must not conclude that no other pond
will be worth our attention. Though indeed many little animals occur
over and over again in practically every pond, there are other equally
interesting animals which only occur in certain localities and for
these we must keep a sharp look-out.
The apparatus needed for the collection of the denizens of ponds and
streams, need consist of no more than a net with a very fine mesh and a
jar in which to bring our captures home; for, of course, animals which
dwell in water need not be dried on the journey home. Various useful
accessories for the student of pond life are sold at very reasonable
rates by most scientific instrument makers.
We shall find many representatives of the animal world in our pond
and exceedingly interesting most of them will prove. From the mud we
may obtain the “protean animalcule,” known to scientists as _Amœba
Proteus_, the most lowly of all animals. Though this creature is
plentiful and just visible to the naked eye, he is not easy to separate
from his surroundings. He is almost colourless and therefore paler than
the mud. Having secured him on the end of a glass rod, let us examine
him in a drop of water on a slide. At first he will remain motionless,
as a protest against being disturbed; we shall not have to wait long,
however, for soon one part of his body will be seen to protrude and
then grow larger and larger till it forms a false foot; other parts may
follow suit, till he is more elongate than oval, and he moves in the
direction of his false foot with a curious gliding motion. His pace is
not great and has been calculated at a twenty-fifth of an inch in an
hour. Really the “protean animalcule” is little more than an animated
drop of jelly, a fact we can substantiate by watching him feed. His
food consists of minute water plants such as diatoms, and when one of
these plants comes within his line of march he simply surrounds it with
his false feet and, as it were, flows around it. When he has digested
all he can he flows away from the undigested portions; he has no mouth
or any of the organs usually associated with animal anatomy.
While hunting for our _Amœba_, it is highly probable that a very
active little slipper-shaped organism may have forced himself upon our
attention. From his shape he has earned the popular name of the slipper
animalcule. He is rather more highly organised than the _Amœba_ for he
possesses a mouth, as we shall see when we are able to examine him. So
rapidly does he swim, however, that something must be done to curb his
activity; he may either be killed with a drop of weak acid or we may
put a little tuft of cotton wool on our drop of water and a coverslip
lightly over that. The threads of cotton wool will form a network, in
the meshes of which the active little animalcule will be confined.
Careful observation will show that he is, like a slipper, more pointed
at one end than at the other; that there is a funnel-shaped orifice,
his mouth, at one side of his body; and that he is covered with little
threads which lash the water with rhythmic movement and propel him with
considerable rapidity. These little threads also send currents of water
to his mouth, and in the water is his food.
Having examined these two free swimming denizens of the pond, we may
advantageously turn our attention to some of the weeds growing therein.
Careful examination with our pocket lens will almost certainly reveal a
number of minute living creatures attached to the submerged stems and
leaves. It is impossible to describe all the interesting creatures we
might find: we must content ourselves with two, because they are common
inhabitants of many ponds. One interesting little creature we are
almost certain to find, the _Hydra_. He may be green or he may be brown
but in structure he will remind us of a small sea anemone. When we
put him under the microscope he has the appearance of a mass of jelly
attached to the water plant on which we found him. He will soon open
himself out, however, and we shall see that his free end is provided
with a number of tentacles; these he waves about in the water to draw
small swimming creatures to his mouth which is situated in the centre
of the group of tentacles. Any luckless creature, coming within reach
of the _Hydra_, is at once stung with one of the barb-shaped stings
which stud his tentacles and then passed to his mouth. The _Hydra_
is wonderfully tenacious of life; it is said that he has been turned
inside out, like a glove finger, without suffering any inconvenience.
Probably the specimen we are examining will have a swelling on the side
of its body, which might be mistaken for the result of some injury; it
is nothing of the kind; it is merely a bud which will grow into a young
_Hydra_ and, when old enough, become detached from its parent and float
away to another plant. Under a fairly high magnification, we shall
almost certainly see something gliding rapidly over the body of the
_Hydra_; its movements are too quick to allow of careful examination;
the creature which is almost as elusive as the slipper animalcule is a
parasite of the _Hydra_.
Not quite so common as our last object but still common enough to be
mentioned here is the beautiful “bell animalcule.” Like the _Hydra_,
this creature, except in its very young stages, remains affixed to a
water plant. In shape the “bell animalcule” resembles a wineglass on
a long delicate stem; round the part corresponding to the rim of the
glass, there is a fringe of the hair-like, water-lashing structures
with which so many of these lowly creatures are provided, and these
structures also surround the entrance to the funnel-shaped mouth. When
undisturbed, the bell animalcule has its slender stalk fully extended
and its little threads lash the water vigorously, causing currents,
containing food material, to travel towards its mouth. A sharp tap on
the microscope slide will cause the creature to contract, the threads
cease their lashing and the stalk contracts spirally, so that the body
of the animalcule is drawn close to the object to which it is attached.
By degrees the spiral uncoils and the little threads resume their
lashing.
Sometimes, as we examine our bell animalcule, we may be fortunate
enough to see it splitting into two parts to form two separate
individuals. This curious process should be watched carefully. The
upper part of the bell splits first and, by degrees the whole bell
divides into two equal parts so that we have a pair of bells on a
single stalk. The next stage consists of the formation of a ring of
whip-like structures round the base of one of the bells; both bells,
by the way, have the circlet of whips round their upper edges. Soon
after these additional little whips are formed, the owner of them
breaks away from the stalk and swims about in the water for a time,
finally coming to rest on a suitable water weed. Then the lower ring
of whips has served its purpose and in its place a long stalk grows;
from this time forward the new bell animalcule will never move from the
position it has chosen. This form of increase, this simple splitting
takes place over and over again but by degrees the little animal
appears to become exhausted and the process slows down or stops.
The partially exhausted _Vorticella_ may gain increased vitality by
fusion with another individual and this process also we may have the
luck to see though it is less frequent than the simple splitting.
Sometimes a bell may be observed to divide, not into halves, but
into two unequal parts. The smaller of these parts may divide again
into from two to eight parts, each one of which, having developed a
fringe of little whips, swims off on its own account. These little
barrel-shaped swimming forms, instead of settling down and forming
stalked bells, seek an exhausted creature, fuse with it near the base
of its bell and finally become absorbed by it. The result of this
fusion is that the bell animalcule takes on a new lease of life and
once more begins to divide actively.
In our search for specimens for our microscope we may come across a
very common pond dweller, closely related to the bell animalcule,
known by the name of _Carchesium Spectabile_. We cannot fail to
recognise its family likeness to the form we have already studied, for
it consists of a large number of stalked bells growing on a single
parent stem. It is really a little colony of bells. When one of the
young individuals of _Carchesium_ settles down in the spot it has
selected for its dwelling-place, it grows a stalk just as _Vorticella_
did, and it divides later into two individuals. Now in _Vorticella_
only the bell divides, in _Carchesium_ part of the stalk divides also
and, instead of swimming away to find a new home it remains attached to
the parent stalk. When this has happened several times a goodly colony
is formed.
There are few fresh-water animals more commonplace and apparently
uninteresting when observed casually than the pond sponge and the river
sponge. Yet, if we take either of them home and examine them with the
aid of our microscope, we shall be delighted with our specimens. In
reality they are of absorbing interest and, at certain times of the
year we may easily obtain young sponges, and capital objects they make
for the microscope.
[Illustration:
_Photos by Flatters & Garnett_
1. STARCH GRAINS OF POTATO
Starch is largely used as an adulterant of various foods. The
Potato starch grain resembles a miniature oyster shell.
2. PHOSPHORESCENCE ANIMALCULAE
These minute animals, occurring in millions, render sea water
beautifully phosphorescent.
3. PROTEUS ANIMALCULE
The lowliest of all animals. It is a jelly-like animal which
changes its shape from hour to hour.
4. CYCLOPS
A common one-eyed water animal. The female, illustrated, carries
her eggs in a pair of relatively large bladder-like sacs.
]
Before we describe our two specimens let us try to explain what manner
of creature a sponge really is. If we examine any bath sponge, we
notice that it is perforated with many small holes and some larger
ones. Some sponges show this better than others. The small holes
are pores, the large ones mouths, but, as we shall see in a moment
we must not run away with the idea that they in any way resemble
our familiar idea of a mouth. These large holes are called oscula by
scientists, but we wish to avoid scientific words as much as possible.
The simplest sponge of all, consists of a little bag, which remains
affixed by its base to a seaweed. All over its sides there are many
pores and at its tip there is a single osculum; it is known as the
Purse Sponge and is common round our coasts. The inside of the purse
sponge is lined with cells, each one of which is tipped with a little
whip which waves about unceasingly. The waving of the whip causes water
to flow through all the pores into the hollow bag and out again by way
of the osculum. Although most sponges, including our fresh-water forms,
are much more complicated than the purse sponge, the same thing happens
in them all, water is drawn in by way of the pores and forced out by
way of the oscula.
The best place to seek for the fresh water sponges is on the
under sides of floating wood, broken tree branches and the like.
Their appearance depends upon whether they have been growing in a
well-lighted spot or in darkness; they contain chlorophyl as do the
higher plants, and sponges grown in the light are green, those which
the light has not reached are buff-coloured or corn-yellow. The pond
sponge is brighter green than its river frequenting relative and is
a coarser creature altogether. It often forms little finger-like
outgrowths, whereas the river sponge is more leaflike.
If we examine one of these sponges in a watch glass of water to which
we have added a very little carmine powder, we can easily see the water
currents entering the pores and coming from the oscula. Towards autumn,
if we open a river sponge we shall see little yellow bodies about the
size of a pin’s head; they are the buds from which the young sponges
arise. One of these must be removed very carefully and placed in a
watch glass of water; if our specimen be placed in the sun, we shall
not have many days to wait before we find that it has given rise to
an active transparent little creature, to a young sponge in fact. By
repeating our experiment with carmine, by the aid of the microscope, we
can see the water currents passing through its body.
Wherever we collect our pond water we are certain to find some of the
wheel animalculæ or rotifers. There are such a number of different
kinds that we might describe several, and yet not mention the one that
any of our readers had happened upon. They may be recognised, because
they are always so transparent that all their internal organs may be
plainly seen and they always have two or more discs or lobes, at their
forward end, fringed with fine whip-like threads. These little threads
are constantly in motion, so that they appear like little revolving
wheels hence the name, wheel animalcule. Nearly all these creatures
have a sucker or false foot at their tail end by means of which they
attach themselves to some support or they may swim freely in the water.
The study of rotifers has been the life-work of some scientists and
they will afford any microscopist, who is interested in them with
abundant occupation.
Amongst the water weeds we shall probably find another small but
striking creature, known as the sun animalcule, _Actinophrys Sol_. Why
exactly it is called the sun animalcule we cannot say, probably it has
earned its title from the fact that it resembles the conventional idea
of the sun, with light rays radiating all round its circumference.
The creature is whitish-grey in colour, spherical in shape and just
visible to the naked eye. All over the surface of its body there are
a number of apparently empty spaces, known as vacuoles--they probably
account for its peculiar colour. Everywhere, it is studded with long,
slender-pointed rod-like outgrowths. But rarely, the sun animalcule
exhibits any movement and for long periods the only signs that it is
living occur at feeding time. Its food consists of water animals and
plants, of varying size. When a small animal comes into contact with
one of the pointed rods, which radiate from the animalcule, it appears
to be held there in some unaccountable manner and, after a pause, it
begins a fateful journey by sliding down the rod to the spherical
body of its captor. Then it is passed into one of the vacuoles and
digestion very soon takes place. That it does not do so immediately
is shown by the fact that the wheels of a wheel animalcule, which has
been passed to the vacuole of our subject, continue their movements
for a considerable period. When larger animal food is partaken of,
a different method is pursued by the sun animalcule. A water-flea,
for instance, coming into contact with one of the rods will struggle
violently in its efforts to escape. Then the sun animalcule shows
real signs of life, for some of the other rods bend over and hold the
captive so that it, eventually, is passed to a vacuole.
In our chapter on agriculture we mention a peculiar flat worm known as
the liver fluke. This unpleasant creature has one or two relatives who
make their home in ponds and, though they are not more beautiful than
the liver fluke to look upon, they are quite harmless. One of these
flat worms is about 3/4-inch long and not unlike an indian club in
shape. It moves about, with some speed either by the aid of a sucker on
its head or by a curious gliding movement. The other pond-frequenting
flatworm is more like the liver fluke, its leaflike oval body, about
1/2-inch long, is pointed fore and aft. It is a common sight to see it
gliding here and there in search of still smaller animals off which it
may make a meal.
The sea mat and bird’s head, two common animal colonies of the
sea-shore, possess pond-dwelling relatives of the greatest interest
to the microscopist. Like the seaside forms they dwell in colonies.
One of the commonest is known as _Lophopus Crystallinus_ and it may
be found attached to duckweed, the curious little plant whose tiny
leaves float upon the surface of the water. _Lophopus_, when at rest,
resembles a little piece of jelly. If we are patient and watch it
under our microscope we shall see it expand, sending forth a number of
stalks, each one tipped with a horse-shoe shaped feathery tentacle.
Each of these tentacles belongs to a separate animal which with its
fellows forms a colony. Not so interesting are the branched, threadlike
colonies of _Plumatella Repens_ which may be sought upon the leaves of
water plants. Our ponds can furnish no more extraordinary object for
our microscope than a colony of _Cristatella Mucedo_; it is curious in
appearance and still more curious from the fact that though a colony
of animals it acts like a single individual in crawling over the weeds
and stones in shallow, sun-kissed water. The _Cristatella_ colony is
jelly-like and greenish in colour, in length it may grow to a couple
of inches. Its under surface is flat, whilst from its upper, convex
surface the little animals forming the colony wave their brush-like
tentacles in the water.
In searching the various pond weeds for specimens we are sure to
meet with various jelly-like masses; these must always be examined
carefully. They may be the egg-masses of interesting water creatures;
various water-snails, for instance, protect their eggs with a
jelly-like covering.
If we meet with any large fresh-water mussels, sometimes called swan
mussels, we shall probably find the fleshy parts of the molluscs
swarming with minute creatures, which we may conclude are parasites.
Mussels, like nearly all living creatures, have their parasites it is
true, but what we have discovered will almost certainly prove to be
young mussels. We must examine them under the microscope to make sure.
Then we shall see, if young mussels they be, very minute and very
thin-shelled little creatures; the edges of their shells are armed with
teeth and are quite unlike the highly polished and smooth shells of the
parent mussel. We shall also see a long thread issuing from the animal
within the shell. If we are examining the young mussel in water we
shall notice that it is constantly snapping its two shells together.
Had we left these young mussels undisturbed, a very curious life they
would have led. They would have remained attached to their parent for
some little time probably, or some of them might have fallen to the
muddy bed of the pond. Their behaviour in either case would be the
same. The long threads that we have already examined would have floated
in the water and, directly they were touched by a passing fish, the
little shells would begin to snap violently. The lucky ones would not
snap in vain for they would close upon the fin or tail of the fish and
then their snapping would cease. Like the bulldog, these little mussels
may take a long time in getting hold but when once they have managed
to fasten their teeth into anything it is well-nigh impossible to make
them let go. Usually they never let go, but are carried away by the
fish. In most cases, the irritation they set up in the flesh of their
new-found foster parent causes a swelling to occur with the result that
the little mussels are engulfed. Within the flesh of the fish they go
through a series of changes; they lose their teeth and their tell-tale
thread, they become in fact miniatures of the adult mussels, then they
manage to escape from the fish, settle down in the mud and fend for
themselves.
The crabs and lobsters which we know so well have fresh water relatives
in nearly every pond. Many of the creatures we have examined have
needed careful search to discover their whereabouts; not so the fresh
water crustacea, as they are called. Their activity, their curious
movements in the water compel attention.
The fresh water shrimp is a curious little creature, sometimes he
paddles his three pairs of hind legs and sometimes he jerks his body in
a ludicrous manner, in either case he manages to propel himself rapidly
through the water. He is about half an inch long, brown in colour and
with a curved body not unlike a shrimp. If we examine him under the
microscope we notice that his front legs are bent forwards, whilst his
hind legs are bent backwards. The male water flea is much larger than
the female, a fact which probably accounts for the fact that these
little animals often carry their wives about with them by seizing them
with their fore legs.
The water louse we may also encounter, he is not nearly so interesting
as the fresh water shrimp. He is closely related to the wood louse
which we all know, and has a similar flattened body.
Very much smaller though even more interesting is the common water
flea. By day these animals retire to the mud at the bottom of the pond
but, morning and evening, they swim actively with a curious jerky
motion. We must examine our specimen carefully for he is of more than
ordinary interest. We cannot fail to observe how transparent he is,
so much so that all his internal organs can be plainly seen, but let
us deal with his exterior first of all. His large eyes are plainly
visible, but his most conspicuous feature is the pair of large branched
feelers, by means of which he swims. If we examine several specimens,
one or more is certain to be a female, then we may probably observe
an egg in process of formation in the brood pouch, a large, elongated
cavity, just below the back of the animal. Immediately above the brood
pouch, the heart is situated and, if we can induce its owner to keep
still for a moment or two, the heart-beats may be plainly seen.
Not unlike the water flea as it swims about in the water of our
collecting jar is the curious, transparent little creature known as
_Cypris_. Although so transparent its body is contained in a pair of
shells, very similar to those of the mussel; a fact which formerly
led to its being classed with the shell-fish. We may well examine
this little fellow under the microscope for much of his structure may
be made out through his shell. His very conspicuous eye is sure to
attract our attention; he possesses but a single eye and seems to make
up for the lack of a second by having a very large one. Two pairs of
feelers project beyond the shell in front. Of his two pairs of legs the
foremost, or at least their tips, hang down below the shell, but the
last pair, as we can see through the shell, are turned upwards. At the
hinder end of the body, there are two long bristles, they may best be
seen when _Cypris_ is swimming. Of what use exactly these bristles may
be to their owner is not definitely known, but it is thought that they
are of service in keeping his shell clean.
Another active little animal, quite as common as the water flea, is
sure to attract our attention. It is no larger than the water flea
but much more elongated; some specimens are bigger than others, and
the bigger ones are the females. Its name is _Cyclops_ and, though so
common, it has no popular name.
_Cyclops_ is so named on account of the fact that it possesses but a
single eye; it is, however, rather an interesting creature in other
respects, so we will study it more closely. Looking down upon the
creature, we see that the front part of its body is composed of an
undivided shield, behind which there are four plates and behind these
again there are in the male five and in the female four segments or
rings, at the extreme tip there is a forked tail, each fork being
furnished with a number of bristles. On the head are two pairs of
organs, one pair long the other pair short and, if we observe _Cyclops_
in the act of swimming, we shall see that the long pair of organs
play the chief part. In the centre of the front of the head there
is a black or red patch--the eye. Very frequently we may meet with a
specimen carrying a relatively large bladder-like body on each side of
its abdomen. These bladders which are each about one-third as long as
the creature which carries them, are egg-sacs.
If we are able to secure one or two specimens with egg-sacs attached,
we can study the young _Cyclops_ without much difficulty. Take
a small glass tube--a test-tube as used by chemists will serve
admirably--partly fill it with pond water and add a little water weed,
then introduce the egg-bearing females and place in the light. We must
watch the tube from day to day, and before long it will be evident
that the young ones have arrived in the world, for we shall have no
difficulty in seeing dozens of little white specks swimming about in
the water and settling on the sides of the tube. We must remove one
carefully on the end of a glass rod or on a paint brush and examine
it in a drop of water under the microscope. This young creature is
totally unlike its parent, it is oval and possesses three pairs of
stiff bristles, of which the first pair are simple and the other two
pairs are branched. Although the bristles are used solely for swimming
at this stage, it may be of interest to mention that in the adult
_Cyclops_ they become transformed into jaws and the two pairs of organs
we have already examined. At the front end of the oval body we can
closely distinguish the single eye, which persists throughout life.
CHAPTER VI
PLANT LIFE IN PONDS AND STREAMS
In this chapter we shall confine ourselves to the true water-dwelling
plants, as distinct from those, such as the water lilies, which though
never found growing on dry land, appear undecided whether they will
be water plants or land plants. Looking at the matter from a more
scientific point of view, all our pond plants will be much lower in the
scale of development than the water lilies and other flowering plants.
Pond life is rich in subjects for the microscopist. Any stagnant
pool may contain organisms which will delight the naturalist who has
always depended upon his unaided vision. Curiously enough, amongst
the most wonderful of all these pond-dwelling plants are the Diatoms,
which consist of but a single cell. They are so numerous, they exist
in so many different forms and in so many different situations that
were we able to describe them all, we should require the whole of a
large volume, much larger than this. In colour, Diatoms are usually
brown or brownish, although they contain chlorophyll, the green
colouring matter of higher plants. In shape they may be rod-shaped,
crescent-shaped, circular, wedge-shaped, oblong or oval. Some float
about freely in the water, some are attached to supports by means
of stalks. Some lead a solitary life and others dwell together in
colonies. One feature they have in common, a curious flinty cell wall,
and this is their most interesting point to the microscopist. This
natural armour is in two parts which fit one within the other like the
two halves of a Japanese basket. All manner of beautiful sculpturing
marks these beautiful frustules as they are called; in some cases they
are perforated and the living matter from within passes through the
pores and forms a jelly-like covering for the little plant.
We must make a point of collecting all the Diatoms we can find, for
they are always interesting; moreover, they are easily preserved and
made into permanent slides, for the little plants may be boiled in
acid to destroy their living parts and the frustules will survive the
boiling undamaged.
One might wonder how such humble plants, surrounded as they are with
flinty walls, could increase. They frequently do so in a simple manner.
The living matter of which the plant is composed pushes the frustules
apart and divides across the middle. The result of this event is the
formation of two plants, each with a single frustule. In a very short
time each plant grows a new frustule, but it is always much smaller
than the one with which it started.
The movements of some of the free swimming, that is to say non-attached
Diatoms, are worthy of study. The scientific name of one kind,
translated into everyday language, means little boats, and indeed they
are well named for their beautiful aquatic manœuvres rival those of any
ship.
Somewhat similar in habit to the brown Diatoms are the green Desmids,
but, whereas, the former also occur in the sea, the latter are all
confined to fresh water. Sometimes Desmids are so numerous that
they make the pond water as green as green-pea soup. It would be as
impossible to describe all these plants as was the case with the
Diatoms, but generally they may be recognised by the fact that they are
composed of two similar halves, separated by more or less of a waist.
Although some of the Desmids exhibit a certain amount of movement they
are not active like the Diatoms.
Late spring and autumn are the best seasons to hunt the ponds
for our next object, which rejoices in the name _Chlamydomonas
Angulosa_, a good example of the extraordinary fact that some of
the smallest animals and plants have the longest names. This little
plant is interesting in itself and doubly so, because it was for
long thought to be an animal; it is wonderfully animal-like in its
movements. _Chlamydomonas_ is very minute, so we must use our highest
magnification when we examine it. It is an oval, one-celled plant
enclosed in a clear membrane. The green colouring matter is arranged in
the form of a cup, within the hollow of which is a mass of granular
living substance. At the forward end of the plant, there is a clear
space and near the tip a brownish dot, known as the eye spot; which,
though incapable of seeing as we understand it, is sensitive to light,
as shown by the fact that the little plant will swim towards moderately
intense light and away from strong light. If we stain the plant with
iodine we can plainly see a pair of little whips arising from the clear
portion at the forward end; it is by the lashing of these that the
plant is enabled to swim.
In the mud of our pond we may find a little colony of plants which
might forgivably be mistaken for a collection of the individuals we
have just studied. Each member of the colony is very much smaller
than _Chlamydomonas_ to be sure, but each one has the outer membrane,
the brownish eye spot and the pair of little whips. On the other hand
the chlorophyll fills the whole of each cell and is not arranged in
the form of a cup. Sixteen cells form a colony, and the whole mass is
a flat plate; the little whips move in unison and the whole colony
revolves after the manner of a wheel.
Another little colony we may encounter also, consists likewise of
sixteen cells very like the ones we have described, but somewhat
wedge-shaped instead of oval. A jelly-like mantle encloses the colony
and, in outline, it is spherical, so that when the little whips, which
project through the mantle, lash the water the whole colony revolves.
The most remarkable of these colonies of cells is known as _Volvox
Globator_, it may be recognised by its perfectly spherical shape and
its characteristic movements in water. _Volvox_ is about 1/25 inch in
diameter, and although to the uninitiated it appears to be a single
minute plant, in reality it is a colony of upwards of twenty thousand
cells. The colony may be considered as being made up of thousands upon
thousands of cells, very similar to those of _Chlamydomonas_, and each
one arranged with its pair of little whips directed outwards.
Within the _Volvox_ sphere we may observe a number, usually one to
eight, of smaller spheres. These are so-called daughter colonies which
have arisen from the continued division of special cells. They develop,
fairly rapidly, into young _Volvox_ colonies, then they burst through
the cells of the parent colony, swim out into the water and quickly
grow to the size of the _Volvox_ from which they were formed.
There is another kind of _Volvox_ of a yellowish colour and much
smaller than _Globator_, the big one is the one we must procure; it is
much more easily studied.
Certain of the plants we shall find in our pond are so animal-like in
their movements, that the microscopist who sees them for the first
time may wonder whether we are not mistaken in calling them plants. We
have already described the common _Chlamydomonas_, with its curious
jerky method of propelling itself through the water. There is,
however, an equally common one-celled, pond-frequenting plant which
has puzzled naturalists even more, for it certainly possesses many
very animal-like characteristics. Its name is _Euglena Viridis_ and
we require our highest magnification to examine it for it does not
exceed one-two hundred and fiftieth part of an inch in length. Usually,
_Euglena_ is cigar-shaped but, as it possesses the very unplant-like
characteristic of not having a firm cell wall it can change its shape
to a considerable extent and it often assumes curious forms. At the
forward end of this minute plant there is a single whip-like thread, by
means of which it swims; a little below the base of the whip, there is
a red eye spot. Elsewhere we have described how the protean animalcule
feeds by flowing round its food-material, _Euglena_ feeds in a similar
manner, but it also feeds after the manner of a plant. When this active
little plant is about to increase it either divides into two lengthways
or becomes surrounded with a firm wall within which it breaks up into a
number of young forms which are released later by the bursting of the
wall.
[Illustration:
_Photos by Flatters & Garnett_
1. BLADDERWORT
A British water plant which entraps small animals in its bladders
and digests its captives.
2. SPORES OF HORSE TAIL
These spores with their thread-like outgrowths vary in appearance
according to the moisture in the air.
3. HAIRS ON A POTATO LEAF
The star-shaped hairs on a potato leaf make beautiful objects for
the microscope. Leaves of many plants are clothed with curious
hairs.
4. SPIROGYRA
A green thread-like water weed. Two threads are shown fusing
together; from each part of these fused threads a new plant will
arise.
]
Small wonder that scientists were in doubt concerning the true nature
of _Euglena_, its non-walled cell and peculiar mode of feeding are
indeed puzzling. There are many similar cases amongst these lowly
organisms, in fact there are some creatures which may best be described
as partly plant and partly animal. Those of our readers who are
interested in such problems may read an excellent account given much
more fully than we could give it here, in Professor F. W. Keeble’s
_Plant Animals_, one of the excellent Cambridge Manuals of Science.
The fact is that the demarcation between plants and animals, low in
the scale of development, is not nearly so pronounced as it is amongst
the higher forms of life. Amongst the plants of our pond we shall find
that, like the land plants, most of them are green, and many of them
are thread-like, so that the task of distinguishing one from another
may appear difficult. Examined with the naked eye, many of them appear
remarkably similar to one another; under the microscope the differences
are obvious.
One of the most remarkable of the commoner pond plants is known as
_Oscillatoria_; it does not boast of a popular name but its scientific
name is not very difficult to remember after we have witnessed its
oscillations. _Oscillatoria_ is a plant with particularly animal-like
movements. It is merely a thread, usually green-blue in colour, but
sometimes red or violet. The threads are never branched and, except
when in motion, are straight. Under a moderately high magnification we
can see that this thread-like plant is not composed of a single cell
but that it consists of a number of cells, placed end to end. Sometimes
a few of these cells will break away and start life on their own
account.
Whatever interest the structure of _Oscillatoria_ may have for us, we
cannot help being struck with its movements, and we must make a point
of observing them. The movement is peculiar and not easy to describe
nor, so far as we know, has it ever been explained. A thread will be
seen to glide backwards and forwards, becoming somewhat curved and,
at the same time, revolving on its axis. Eel-like is perhaps a good
description of the movement. It is possible to distinguish this plant
from other pond dwellers by its slimy “feel” which arises from the fact
that each thread is enclosed in a jelly-like sheath.
The silk weeds, _Cladophora Glomerata_ are somewhat similar in
appearance to the plant we have just described, but they are denizens
of running streams rather than of ponds. They are the green thread-like
plants we so often see attached by one end to rocks and stones beneath
running water. Each plant consists of a long, cylindrical structure
composed of several cells. We mention the silk weeds here, because they
are best of all plants for showing cell growth. This growth takes place
at or near the unattached end of the plant and is easily observed. The
end cell may be watched for the process. Its green contents will be
observed to contract in the middle so that it assumes an hour-glass
shape. Then, where the contraction has taken place, we can watch the
formation of a wall right across the cell, so that when the process is
completed we have two cells where formerly there was one. More rarely,
the contents of a cell will be observed to bulge out a side wall, then
a new wall is formed to divide it off from the main cell and thus the
beginning of a branch is formed. The silk weeds exhibit no movements,
except such as are imparted to them by the running water.
A very beautiful little pond plant is known to science as _Draparnalda
Glomerata_. We shall probably forget its name, but we can never forget
the plant itself when once we have been fortunate enough to see it.
A single row of large, transparent cells, containing very little
green colouring matter, forms the main part of the plant. At regular
intervals from these transparent cells, there arise rings of deep green
branches, each one tipped with an extraordinarily long, colourless
hair. _Draparnalda_ is indeed a plant worth looking for.
Two common little green plants grow so near to the edges of ponds that
they may well be included amongst our pond plants. The simpler of the
two, known as _Vaucheria Sessilis_ thrives on almost any damp soil and
may even form a covering on soil in pots. In structure, _Vaucheria_ is
very simple for it consists of a single, frequently branched, tubular
cell. The little attaching organ, by means of which the plant fixes
itself to some firm support, is colourless, so too is the tip of the
cell where growth takes place.
We must examine this little plant when we come across it and we must
not fail to notice that it is composed of but one cell. The only time
at which we can find any cross walls, is when _Vaucheria_ is about to
increase. Then the tip of the cell swells, like a little club, and a
cross wall separates it from the rest of the plant. The contents of
this cell rounds itself off, becomes fringed with innumerable little
lashing whips and escapes from a pore at the tip of the cell in which
it was formed. This little organism swims about for a time in the
water, for _Vaucheria_ only increases in this manner when it is under
water, at length it comes to rest and forms a new plant.
Closely related to _Vaucheria_, but not quite so common, is the curious
little plant known as _Botrydium Granulatum_. Like our previous
example, it is a one-celled plant and herein lies its interest. When
find a specimen growing by a pond, we shall notice a green bladderlike
portion, not more than 1/6 inch in diameter, which projects above the
ground. Below ground, if we pick the soil away carefully we may observe
a number of colourless, branched structures which do duty for roots.
The bladder and underground portions are hollow, being lined in the
case of the former, with a network of chlorophyll beautiful to observe
under the microscope.
_Botrydium_ usually increases by buds which form on the green bladder,
break away and grow into new plants. Should the level of the water in
the pond rise, and the little plant become submerged, it splits up into
a number of small bodies, each provided with a minute whip-like organ,
with which they swim rapidly ashore where they develop into new plants.
In dry weather the plant forms a number of little cells each one
surrounded by a firm cell wall. When moisture comes again these little
cells give rise to structures which will grow into new plants.
Surpassing even _Drapernalda Glomerata_ in point of beauty, is the
“water-net,” _Hydrodictyon Reticulatum_, but it is not nearly so
common, being confined to ponds in the South and Midlands. When full
grown it hardly comes under the heading of a microscopic object, for it
may measure as much as six inches in length. This remarkable pond plant
consists of an open network of green filaments. Apart from its striking
appearance the most remarkable thing about the “water net” is its rapid
growth. Carpenter, a noted microscopist says:--“The original cells of
which the net is composed measure one-two thousand five hundredth part
of an inch in length but in a few hours they grow to one-twelfth of an
inch or 1/3 of an inch in length. We often hear people remark that they
can see plants grow, but their statements are not literally true; in
the case of the ‘water net’ it is actually possible to see the growth.”
By the side of our pond we shall probably observe some masses of a
bluish-green jelly-like substance. It is uninteresting-looking material
for the microscope, but we must not pass it by. The blue-green jelly
encloses a plant called _Nostoc_, which resembles nothing so much as a
necklace of beads; this we can plainly see under a low magnification.
We shall observe that the plant is twisted spirally within its covering
and also that most of the cells, which we have compared to beads, are
similar to one another in size. At intervals there are larger cells
and as the plant increases in length they are emptied periodically, so
they are evidently used as food stores. From time to time portions of
the plant break away, worm their way out of the jelly and move about,
rather after the manner of a worm. Eventually, these wanderers come
to rest and become surrounded with more of the blue-green jelly, thus
forming a new _Nostoc_ colony.
These plants sometimes appear, apparently from nowhere, on garden
paths, walls and similar situations, during damp autumn weather.
On this account the plants have been called “fallen stars.” Their
appearance is not so mysterious as it might seem for the _Nostoc_
colony has probably been where it is found, all through the summer,
in a dried up, contracted state. Only when rain comes, does the jelly
envelope absorb water, swell up and assume its normal appearance. The
blue-green scum which floats on stagnant water is a closely related
plant.
Another plant which looks like scum on the water is known as
_Spirogyra_ and a very beautiful object it makes for the microscope.
It is bright green, without a bluish tinge, so it need not be confused
with the plant we have just mentioned. There are seventy or so
different kinds of _Spirogyra_, therefore our description must be the
one that will apply to all. The plant is thread-like and, even in the
larger kinds, the threads are not more than one-hundredth part of an
inch in diameter. Like _Oscillatoria_ the plants are not attached to
any support. Each thread is composed of many cells, arranged end to
end; we can distinguish the cell walls clearly, but what will chiefly
attract our attention are the beautiful bands of green colouring
matter, running spirally, round each cell. If we are fortunate, we
shall see the plant in the act of increasing; this is not the simple
operation we witnessed in the silk weeds. From two threads lying
parallel to one another, we shall see swellings arise on the adjacent
cell walls; the beautiful spiral bands will begin to break up at the
same time and to collect in a mass towards the centre of each cell.
The swellings of adjacent cells touch, their end walls break down so
that the two cells become connected, our figure shows the fusion taking
place, then the contents of one cell passes into the adjacent cell and
fuses with the contents of the latter. The new cell, which now contains
not only its original contents but that from a cell in another plant,
becomes detached from the thread, assumes an oval shape and sinks to
the bottom of the pond, where it rests awhile. At a later period the
cell wall bursts and a new thread of _Spirogyra_ develops from it.
When we are examining _Spirogyra_ we may notice some very minute
brick-red, spherical bodies adhering to the green threads. These are
little pond animals, known by the name of _Vampyrella Spirogyræ_. This
little creature passes through an interesting and easily observed
series of changes. Its life is very uneventful, consisting of a good
meal of _Spirogyra_, a period of rest followed by an increase, and
this is repeated over and over again. If we watch our little sphere
through the microscope, we may be lucky enough to see the contents
divide into four parts. Now we must watch carefully, for very
interesting events are about to take place. Each of the four parts into
which the contents of the sphere has split, escapes into the water
and swims about for a time; it then becomes spherical, but instead of
having a smooth outer surface, as it had when we first observed it,
we can see that it is now studded all over with very fine threads. It
then wanders along a _Spirogyra_ plant, attaches itself to one of the
cells, perforates its wall and sucks out the contents. This performance
it repeats several times then, evidently satiated, it loses its threads
and resumes the appearance it had when first observed and thus it
rests, till division of its contents into four parts takes place once
more and the little comedy is repeated.
Now we must tear ourselves away from our pond; there are very many
interesting objects for our microscope on the sea shore and if we delay
too long here, we shall not be able to give them the attention they
deserve.
CHAPTER VII
THE MICROSCOPE AND PLANT LIFE
The science of botany consists of many branches and, in most of
them, the microscope is the scientist’s constant aid. The study of
bacteria, really a branch of botany, we have dealt with in another
chapter, so here we will omit these interesting though lowly plants.
By far the number of botanical objects for the microscope consist
of sections--exceedingly thin slices of whatever portion of the
plant is being examined, cut either with a sharp razor or a special
instrument called a microtome. Section cutting, though not a difficult
accomplishment, requires a considerable amount of practice and cannot
be learned from a book; all our descriptions, therefore, will be
confined to objects from the plant world which may be studied without
the assistance of razor or microtome.
One cannot help being struck with the fact that green is the prevailing
colour among plants and the reason is not far to seek. If we take a
cabbage leaf and carefully tear off the skin, we shall find green
spongy matter below. A little of this green material may be examined
under the microscope and will show us rounded green bodies composed
of a substance called chlorophyll. Now chlorophyll is absolutely
necessary to all plants, except the fungi and to one or two parasitic
plants. It is necessary because, by its aid, plants can build up raw
food material into food which will be useful to them. It is not formed
in darkness; that is why a board, a roller or any similar object left
on a lawn, causes the grass below to turn yellow; it is the reason
also why certain parts of plants, not usually green, turn that colour
when exposed to the light. Chlorophyll does not always occur in round
globules, sometimes it is found in bands.
One of the most interesting botanical studies for the microscope is
furnished by the leaf of the American water-weed. This plant, which was
introduced into the country from North America some years ago, has now
spread far and wide and is easily obtained. A leaf which is slightly
yellowed with age is the best to take for the experiment. It should
be cut from the plant, placed at once in a small bottle of water and
kept warm for a few hours; this may be accomplished by keeping the
bottle in one’s pocket. After a sufficient interval, put a drop of
water in a clean slide, put the portion of leaf in the drop of water,
cover with a coverslip and examine with a moderately high power. If the
experiment has been properly carried out a wonderful sight will reward
us. We shall see that the leaf is divided into a number of divisions
called cells; this name has been handed down from the very early days
of the microscope, because of a supposed resemblance to the cells in
a bee’s honey comb. In each cell we shall see signs of activity, the
little round grains of chlorophyll are there, but instead of being
stationary, as in the cabbage leaf, they are slowly moving round the
walls of each cell. In reality they are carried along in the stream of
living matter within the cell. It is a wonderful sight and brings home
to the observer very forcibly a fact which is liable to be forgotten,
that the plant is just as much a living being as an animal. Perhaps
our experiment will not succeed at the first attempt, then we must try
again; maybe we have been too rough in detaching the leaf or we have
not kept it sufficiently warm. Sometimes the movement may be started by
slightly warming the slide over a flame; too much heat, of course, will
kill the leaf.
We shall see this green colouring matter over and over again in our
botanical studies, in fact it is found in all manner of situations,
in leaf and stem. Very often its colour is hidden by sap of another
colour, as for instance in copper beech leaves or in the brown
seaweeds. Chlorophyll dissolves in alcohol, however, and this affords
us a ready means of detecting its presence though we cannot see its
green colour. If we boil any leaf, suspected of containing chlorophyll,
in alcohol we shall obtain a solution with rather peculiar properties
because, when held up to the light it appears green, but when light is
reflected from it, it appears reddish.
From the under side of the cabbage leaf whence we obtained our first
specimen of chlorophyll, we must now take another piece of skin. If
we perform the operation properly the skin will be colourless, like
a piece of thin parchment; any green colour will show that we have
torn off more than the skin and we must make another attempt. Having
secured our piece of skin we place it in a drop of water on a clean
slide and examine it under the microscope. We first notice that the
skin is divided up into a number of small areas called cells and dotted
here and there amongst the cells are several oval bodies, containing
chlorophyll. These oval bodies are the pores through which the leaf
breathes, amongst other things. In the centre of each pore there is
a hole, at least there is if the pore is open, for the two cells
comprising the pore have the power of opening and closing.
It is interesting to try the same experiment with a fern leaf and to
notice that there are pores, very similar to those of the cabbage, but
that the walls bounding the cells of the leaf are irregular and that
they contain chlorophyll. We may try several other leaves and also the
upper and lower surfaces of leaves, then we shall soon learn that, in
leaves with distinct upper and lower surfaces, there are far more pores
on the lower than the upper surface; leaves like those of the iris have
almost the same number on each side, and floating leaves, like those of
water lily have all their pores on the upper side. There is a reason
for this; the pores are likely to become filled with dust, being on the
lower side they are protected somewhat; flat leaves, by their shape,
afford no protection and floating leaves must have their pores on the
upper surface to obtain air.
There are many other interesting things we may learn about leaves, with
the help of our microscope. The cabbage leaf is quite smooth, but if we
are observant we shall have noticed that sometimes each leaf appears
as though it had been powdered, it has a decided bloom. The bloom does
not appear on the leaf for ornament but for a purpose. It is a waxy
substance and it prevents the leaf from losing moisture too quickly
in dry weather. This is very important for the plant; if the moisture
taken up from the soil were lost in the air too quickly by the leaves,
the plants would wither and eventually die. It is not all plants which
can wear a protective covering when danger threatens, most plants have
either no protection or are permanently protected. There is a large
class of plants with folded or rolled leaves; heather and marram grass
belong to this class. We must examine some of these leaves and we shall
find that all the pores are on the inside of the leaf whether it be
folded or rolled. The reason for this is that moisture also escapes
through the pores and, when they are thus protected, it is not carried
off too quickly by drying winds.
Many plants are protected, as far as their leaves are concerned, at
anyrate, by hairs. They take the place of the bloom in such plants as
the cabbage. There are thousands of plants with hairy leaves and they
will provide as many interesting objects for our microscope. Let us
examine as many as we can for the hairy covering of each plant will
be a little different to the one we examined previously. There are
simple hairs, quite ordinary affairs, forked hairs, branched hairs,
T-shaped, star-shaped and club-shaped hairs. If we are clever with our
microscope we shall notice that, however complex each hair may be it is
really nothing but one cell of the skin of the leaf which has assumed a
peculiar shape.
The leaves of the nettle are armed with ordinary and stinging hairs;
the latter are worth examining and we shall notice that there is one
great difference between all the other hairs we have examined and
the stinging hairs of the nettle. The former are of one cell only,
the latter of several cells. A high magnification will show that the
stinging hair of the nettle is not quite so simple as it appears at
first sight.
There are plants whose leaves are protected by very thick skins and
others whose leaves become armed with hard flinty matter, so that they
resemble stones rather than leaves.
If we can find some quite young seedlings we must manage to secure one
or more for examination under our microscope. We must take one up very
carefully and wash the earth from its roots--if we pull the earth away
our specimen will be ruined. Near the tips of the root branches we
shall see something which might be mistaken for mould. Our microscope
will show us that they belong to the root; they are, in fact, root
hairs. We shall very likely be able to make out that, like the leaf
hairs, each root hair is made up of a single cell. The root hairs are
interesting because it is through them that water is taken up by the
plant from the soil.
In many plants a considerable space separates the root from the leaves.
When we have learned how to cut sections, we can make slides for our
microscope which will show us the whole course along which the water
travels, from its point of entry at a root hair to its exit at a leaf
pore. Although we have not yet reached that stage, it need not prevent
us from seeing some of the minute tubes through which the water passes.
Any fleshy stemmed plant will serve our purpose. We must tear it to
pieces lengthways with a needle and we shall find many threads--this is
not their correct name but it expresses our meaning--running the whole
length of the stem. They run, in fact, from the tips of the root to the
leaf, and may be seen as leaf veins. If we remove one of these threads
and tease it with needles on a slide we shall reduce it to still
finer threads. By the way, teasing in the sense we have used it here,
means separating the various parts. Let us examine some of these fine
threads, we shall see that some of them appear like coiled springs at
first glance. A more careful examination will show us long tubes with
spiral thickenings. We all know the garden hose-pipe with stout wire
coiled round it as a protection; these tubes may clearly be compared
with the familiar hose-pipe, where the rubber portion represents the
cell wall and the stout wire the thickened parts of the wall. There is,
however, this great difference, the wire is outside the hose-pipe, the
thickened portion of the plant tube is inside the wall. These tubes are
the ducts for water passing from root to leaf.
If the agricultural side of botany attracts us we shall not have much
difficulty in finding many more objects from the fungus world than are
mentioned in our chapter on Agriculture, whilst the study of bacteria
may truthfully be termed never ending. Ponds and rivers teem with
vegetation suitable for microscopic study. The testing of foods for
impurities is largely botanical work. The botanist, of all men, need
never allow his microscope to be idle.
Our British insectivorous plants are of great interest and they will
supply us with some objects for our microscope. We only possess
three different kinds of these curious plants in this country, the
Bladderworts which live in water and the Sundews and Butterwort, which
frequent moist, peaty land.
The pond-dwelling Bladderworts are not rare, indeed they occur in
plenty in certain localities, but they are not very evenly distributed
over the country and in some districts one may search for them in vain.
It is worth while making a special effort to obtain a specimen. Each
plant bears a number of hollow structures, the bladders. There is an
entrance to each bladder, edged with stiff hairs and closed by a trap
door, which opens inwards but will not open outwards. All these parts
may be seen under the microscope as may the interior of a bladder; its
walls are studded with short hairs. When small water animals enter a
bladder, it is said that they do so to escape from their enemies; they
are entrapped forever, they die and eventually decay. The juices which
arise from their decaying bodies are absorbed by the hairs lining the
bladder.
The Sundews are pretty plants, with rosettes of reddish leaves and
minute white flowers. With the naked eye we can usually see many drops
of clear liquid on the leaves, a number of substantial-looking hairs
and a few insects adhering thereto. If we examine one of these hairs
under the microscope, we shall see that it is club-shaped; it is, in
fact, a hair which gives off a sticky liquid with the power of holding
any luckless insect that settles thereon and absorbing its softer
parts for the nourishment of the Sundew. An examination of a complete
leaf with our pocket lens will show that where an insect has settled,
several of these hairs have curled over so that they touch their
victim. Because the hairs possess this power of movement they are often
wrongly called tentacles.
Butterwort, like Sundew has a rosette of leaves but they are greasy
looking and pale green. Their flowers are a pretty blue. Most probably
we shall notice that the edges of the leaves are curled inwards,
and if we look below the curled portion we shall surely find some
captured insects undergoing digestion. The leaf of this plant makes an
interesting object for the microscope. There are two kinds of hairs
on its surface, short stout ones and longer knobbed ones; the former
give off a sticky liquid which holds any small insects that touch it,
the latter give off digestive juices. While the hairs are used in
digestion, after the manner of those of Sundew, the leaf itself curls
so that more of the hairs are brought into contact with the victim and
thereby its digestion is hastened.
Many small flowers may be examined with low magnifications. When we
examine them thus, we shall probably realise for the first time how
beautiful are many of these seemingly inconspicuous blossoms. Grass
flowers are always interesting; they are not ornamental it is true, but
that does not detract from their interest. There is one part of each
flower known as the stigma; it is the part on which the pollen grain
must be placed in order that seeds may be formed. The pollen grains
are taken to the stigmas in many ways, but the most usual agencies
are insects and wind. In the case of grasses, wind is the agency and
for that reason the stigmas of grass flowers are feathery, so that
they can easily hold the pollen grains carried to them by the lightest
breeze. We shall probably see many pollen grains entangled in the
feathery stigma of the flower we are examining. In the flowers of other
plants we shall find, when we magnify them, that there are all manner
of contrivances on the stigmas, all designed for holding the pollen
grains; hairs, knobs, hooks and the like.
An interesting collection could be made of various pollen grains,
which are easily obtained by merely dusting the anthers of flowers
on to a clean, dry slide. They are varied in shape, colour and size;
some are smooth, some studded with spines, others again, those of the
_Mallow_ for example, have little lids which open when the pollen grain
germinates. The germination of pollen grains is easily observed under
the microscope, by putting a few of the grains in an exceedingly weak
solution of sugar and water. The vigil may be a long one, but if the
pollen grains are ripe and fresh, and the sugary solution sufficiently
weak, the patience of the microscopist will be rewarded by the
observance of the bursting of the pollen grain’s coat and the outgrowth
of the pollen tube.
Other pollen grains worthy of examination, are those of various lilies,
of _Eschscholtzia_ and of _Scotch Fir_; the last named have curious
little air-bladders, for the purpose of rendering them more buoyant.
Many lowly plants thrive in weak sugar solutions, after the manner of
pollen grains. The yeast plant is one of them. A very small portion of
yeast, in a drop of sugar solution, will show us one of the simplest
methods of vegetable reproduction. Yeast is a fungus and it is also a
plant composed of only one cell. Under the microscope, it appears as
a colourless oval body. The sugar solution causes it to multiply and,
after the lapse of a little time, most of the yeast plants will be seen
to bear outgrowths, called buds, which grow larger and larger till, at
length, they break away from the parent plants and start a separate
existence. Sometimes, when these plants are increasing very rapidly,
the buds will bear smaller buds and these again still smaller ones till
a fairly long chain of yeast plants is formed.
It is always interesting and also instructive to make comparisons as
we progress with our work. To illustrate our meaning let us compare
the budding of the yeast plant with the budding of the hydra, which is
described in our chapter on pond life. In the same chapter we described
the division of a proteus animalcule into two separate organisms, a
process which is also undergone by bacteria when circumstances are
favourable to their increase. We shall find many points of similarity
if we make careful comparisons, and several important differences.
Objects for the microscope we can find in plenty, without going far
afield. The white mould which we can probably find in the larder, on a
pot of jam or other food that has been allowed to stand for some time,
will provide a good subject to start upon. A little of this plant, for
such it is, carefully lifted on to a dry slide will show the threads of
the mould, terminated by round black knobs. Breathe on the specimen and
the moisture of your breath will cause these little balls to break and
set free a quantity of fine dust-like bodies called spores. The spores
will be carried about in the air, they are so light, and if they settle
on a suitable medium they will germinate and start another growth of
mould. The blue-green mould of cheese is constructed quite differently;
its spores are not contained in any hollow structures like those of our
first object, but grow in chains radiating from a central point, like
the outstretched fingers of the hand. It is this fungus, by the way,
which imparts the colour and flavour to gorgonzola cheese.
For some reason living organisms, possessing the power of movement,
be it ever so slight, are always more attractive than those which are
apparently motionless. Let us study two common objects from the plant
world which may easily be obtained by any nature student, objects
which owe their power of movement--not to be confused with locomotion,
by the way--to the presence or absence of moisture in the air. On
the under side of the fronds of many ferns there will be found more
or less rounded reddish-brown spots. These outgrowths, for such they
are, vary in position and shape according to the species of fern. An
examination, with a pocket lens, will show that these brownish spots
consist of minute tufts of knobbed structures, growing from the tissues
of the frond. Sometimes the structures are naked, sometimes covered
with a membrane. In either case, one or more of the knobbed structures
is worth examining under the microscope; we shall then see that it
consists of a stalk terminated by a thin walled portion, shaped like a
bi-convex lens. Round the edge of the greater part of this lens-shaped
portion there is a much more substantial-looking rim. Within the
lens-shaped part we can easily see brown spores. If we have chosen our
object at an opportune moment, any excessive moisture in the air will
cause the thick-walled rim to straighten itself out, tearing away the
thin-walled, lens-shaped part in so doing and setting free the spores.
Closely related to the ferns, the horsetails provide another
interesting object for the microscope. The fertile shoots of these
plants, somewhat resembling asparagus, though in reality belonging
to an entirely different family, will, when gently tapped on a clean
dry microscope slide, leave behind a pale yellow powder. The powder
consists of spores, and most interesting they are. When dry, each spore
will be seen to have four somewhat thread-like outgrowths, flattened at
the end; breathe on the spores and each of the outgrowths will coil up
so as to form a complete covering for the body of the spore. As drying
takes place, these outgrowths gradually uncoil again.