Starch, its Growth and Properties--Surface Cells of Petals--Pollen
and its Functions--Seeds.
The white substance so dear to the laundries under the name of starch
is found in a vast variety of plants, being distributed more widely
than most of the products which are found in the interior of vegetable
cells.
The starch grains are of very variable size even in the same plant,
and their form is as variable as their size, though there is a general
resemblance in those of the same plant which allows of their being
fairly easily identified after a moderate amount of practice. Sometimes
the grains are found loosely packed in the interior of the cells, and
are then easily recognised as starch grains by their peculiar form
and the delicate lines with which they are marked; but in many places
they are pressed so closely together that they assume an hexagonal
shape under the microscope, and bear a close resemblance to ordinary
twelve-sided cells. In other plants, again, the grains never advance
beyond the very minute form in which they seem to commence their
existence; and in some, such as the common oat, a great number of very
little granules are compacted together so as to resemble one large
grain.
There are several methods of detecting starch in those cases where
its presence is doubtful; and the two modes that are usually employed
are polarised light and the iodide of potassium. When polarised
light is employed--a subject on which we shall have something to say
presently--the starch grains assume the characteristic “black-cross,”
and when a plate of selenite is placed immediately beneath the slide
containing the starch grains, they glow with all the colours of the
rainbow. The second plan is to treat them with a very weak solution
of iodine and iodide of potassium, and in this case the iodine has
the effect on the starch granules of staining them blue. They are so
susceptible of this reaction that when the liquid is too strong the
grains actually become black from the amount of iodine which they
imbibe.
Nothing is easier than to procure starch granules in the highest
perfection. Take a raw potato, and with a razor cut a very thin
slice from its interior, the direction of the cut not being of the
slightest importance. Put this delicate slice upon a slide, drop a
little water upon it, cover it with a piece of thin glass, give it a
good squeeze, and place it under a power of a hundred or a hundred and
fifty diameters. Any part of the slice, provided that it be very thin,
will then present the appearance shown in Plate III. Fig. 9, where an
ordinary cell of potato is seen filled loosely with starch grains of
different sizes. Around the edges of the slice a vast number of starch
granules will be seen, which have been squeezed out of their cells by
pressure, and are now floating freely in the water. As cold water has
no perceptible effect upon starch, the grains are not altered in form
by the moisture, and can be examined at leisure.
III.
FIG.
1. Laurel leaf, transverse section
2. Starch, Wheat
3. Do. from Pudding
4. Do. Potato
5. Outer Skin, Capsicum pod
6. Starch, Parsnip
7. Do. Arrow Root, West Indian
8. Do. “Tousles Mois”
9. Do. in cell of Potato
10. Do. Indian Corn
11. Do. Sago
12. Do. Tapioca
13. Root, Yellow Water-Lily
14. Starch, Rice
15. Do. Horsebean
16. Do. Oat
17. Pollen, Snowdrop
18. Do. Wallflower
19. Do. Willow Herb, a pollen tube
20. Do. Violet
21. Do. Musk Plant
22. Do. Apple
23. Do. Dandelion
24. Do. Sowthistle
25. Do. Lily
26. Do. Heath
27. Do. Heath, another species
28. Pollen, Furze
29. Do. Tulip
30. Petal, Pelargonium
31. Do. Periwinkle
32. Do. Golden Balsam
33. Do. Snapdragon
34. Do. Primrose
35. Do. Scarlet Geranium
36. Pollen, Crocus
37. Do. Hollyhock
38. Fruit, Galium, Goosegrass
39. A hook of ditto more magnified
40. Seed, Red Valerian
41. Portion of Parachute of same, more magnified
42. Seed, Foxglove
43. Do. Sunspurge
44. Parachute, Dandelion seed
45. Seed, Dandelion
46. Do. Hair of Parachute
47. Do. Yellow Snapdragon
48. Do. Mullein
49. Do. Robin Hood
50. Do. Bur-reed
51. Do. Willow Herb
52. Do. Musk Mallow
[Illustration: III.]
On focusing with great care, the surface of each granule will be seen
to be covered with very minute dark lines, arranged in a manner which
can be readily comprehended from Fig. 4, which represents two granules
of potato starch as they appear when removed from the cell in which
they took their origin. All the lines evidently refer to the little
dark spots at the end of the granule, called technically the “hilum,”
and represent the limits of successive layers of material deposited
one after another. The lines in question are very much better seen if
the substage condenser be used with a small central stop, so as to
obtain partial dark-field illumination. Otherwise they are often very
difficult of detection.
In the earliest stages of their growth the starch granules appear to be
destitute of these markings, or at all events they are so few and so
delicate as not to be visible even with the most perfect instruments,
and it is not until the granules assume a comparatively large size that
the external markings become distinctly perceptible.
We will now glance at the examples of starch which are given in the
Plate, and which are a very few out of the many that might be figured.
Fig. 2 represents the starch of wheat, the upper grain being seen in
front, the one immediately below it in profile, and the two others
being examples of smaller grains. Fig. 6 is a specimen of a very minute
form of starch, where the granules do not seem to advance beyond
their earliest stage. This specimen is obtained from the parsnip; and
although the magnifying power is very great, the dimensions of the
granules are exceedingly small, and except by a very practised eye they
would not be recognisable as starch grains.
Fig. 3 is a good example of a starch grain of wheat, exemplifying the
change that takes place by the combined effects of heat and moisture.
It has already been observed that cold water exercises little, if
any, perceptible influence upon starch; but it will be seen from
the illustration that hot water has a very powerful effect. When
subjected to the action of water at a temperature over 140° Fahr.,
the granule swells rapidly, and at last bursts, the contents escaping
in a gelatinous mass, and the external membrane collapsing into the
form which is shown in Fig. 3, which was taken out of a piece of hot
pudding. A similar form of wheat starch may also be detected in bread,
accompanied, unfortunately, by several other substances not generally
presumed to be component parts of the “staff of life.”
In Fig. 7 are represented some grains of starch from West Indian
arrowroot, and Fig. 8 exhibits the largest kind of starch grain known,
obtained from the tuber of a species of canna, supposed to be _C.
edúlis_, a plant similar in characteristics to the arrowroot. The
popular name of this starch is “Tous les Mois,” and under that title
it may be obtained from the opticians, or chemists.
Fig. 10 shows the starch granules from Indian corn, as they appear
before they are compressed into the honeycomb-like structure which
has already been mentioned. Even in that state, however, if they are
treated with iodine, they exhibit the characteristics of starch in a
very perfect manner. Fig. 11 is starch from sago, and Fig. 12 from
tapioca, and in both these instances the several grains have been
injured by the heat employed in preparing the respective substances for
the market.
Fig. 13 exhibits the granules obtained from the root of the water-lily,
and Fig. 14 is a good example of the manner in which the starch
granules of rice are pressed together so as to alter the shape and
puzzle a novice. Fig. 16 is the compound granule of the oat, which
has already been mentioned, together with some of the simple granules
separated from the mass; and Fig. 15 is an example of the starch grains
obtained from the underground stem of the horse-bean. It is worthy of
mention that the close adhesion of the rice starch into those masses is
the cause of the peculiar grittiness which distinguishes rice flour to
the touch.
Whilst very easily acted on by heat, starch-granules are very resistent
to certain other reagents. Weak alkalies, in watery solution, readily
attack them, but by treating portions of plants with caustic potash
dissolved in strong spirit, the woody and other parts may be dissolved
away; and after repeated washing with spirit the starch may be mounted.
This, however, must never be in any glycerine medium, except that given
on p. <a href=\"#Page_172">172</a>.
* * * * *
In Plate III. Fig. 1, may be seen a curious little drawing, which is
a sketch of the laurel-leaf cut transversely, and showing the entire
thickness of the leaf. Along the top may be seen the delicate layer
of “varnish” with which the surface of the leaf is covered, and which
serves to give to the foliage its peculiar polish. This varnish is
nothing more than the translucent matter which binds all the cells
together, and which is poured out very liberally upon the surface of
the leaf. The lower part of this section exhibits the cells of which
the leaf is built, and towards the left hand may be seen a cut end of
one of the veins of the leaf, more rightly called a wood-cell.
We will now examine a few examples of surface cells.
Fig. 5 is a portion of epidermis stripped from a Capsicum pod,
exhibiting the remains of the nuclei in the centre of each cell,
together with the great thickening of the wall-cells and the numerous
pores for the transmission of fluid. This is a very pretty specimen for
the microscope, as it retains its bright red colour, and even in old
and dried pods exhibits the characteristic markings.
In the centre of the Plate may be seen a wheel-like arrangement of
the peculiar cells found on the petals of six different flowers, all
easily obtainable, and mounted without difficulty.
Fig. 30 is the petal of a geranium (Pelargonium), a very common object
on purchased slides. It is a most lovely subject for the microscope,
whether it be examined with a low or a high power,--in the former
instance exhibiting a most beautiful “stippling” of pink, white, and
black, and in the latter showing the six-sided cells with their curious
markings.
In the centre of each cell is seen a radiating arrangement of dark
lines with a light spot in the middle, looking very like the mountains
on a map. These lines were long thought to be hairs; but Mr. Tuffen
West, in an interesting and elaborate paper on the subject, has shown
their true nature. From his observations it seems that the beautiful
velvety aspect of flower petals is owing to these arrangements of the
surface cells, and that their rich brilliancy of colour is due to the
same cause. The centre of each cell-wall is elevated as if pushed up by
a pointed instrument from the under side of the wall, and in different
flowers this elevation assumes different forms. Sometimes it is merely
a slight wart on the surface, sometimes it becomes a dome, while in
other instances it is so developed as to resemble a hair. Indeed,
Mr. West has concluded that these elevations are nothing more than
rudimentary hairs.
The dark radiating lines are shown by the same authority to be formed
by wrinkling of the membrane forming the walls of the elevated centre,
and not to be composed of “secondary deposit,” as has generally been
supposed.
Fig. 31 represents the petal of the common periwinkle, differing from
that of the geranium by the straight sides of the cell-walls, which do
not present the toothed appearance so conspicuous in the former flower.
A number of little tooth-like projections may be seen on the interior
of the cells, their bases affixed to the walls and their points tending
toward the centre, and these teeth are, according to Mr. West, formed
of secondary deposit.
In Fig. 32 is shown the petal of the common garden balsam, where the
cells are elegantly waved on their outlines, and have plain walls.
The petal of the primrose is seen in Fig. 34, and that of the yellow
snapdragon in Fig. 33; in the latter instance the surface cells assume
a most remarkable shape, running out into a variety of zigzag outlines
that quite bewilders the eye when the object is first placed under the
microscope. Fig. 35 is the petal of the common scarlet geranium.
In several instances these petals are too thick to be examined without
some preparation, and glycerine will be found well adapted for that
purpose. The young microscopist must, however, beware of forming his
ideas from preparations of dried leaves, petals, or hairs, and should
always procure them in their fresh state whenever he desires to make
out their structure. Even a fading petal should not be used, and if the
flowers are gathered for the occasion, their stalks should be placed
in water, so as to give a series of leaves and petals as fresh as
possible.
* * * * *
We now pass from the petal of the flower to the pollen, that coloured
dust, generally yellow or white, which is found upon the stamens, and
which is very plentiful in many flowers, such as the lily and the
hollyhock.
This substance is found only upon the stamens or anthers of full-blown
flowers (the anthers being the male organs), and is intended for the
purpose of enabling the female portion of the flower to produce fertile
seeds. In form the pollen grains are wonderfully diverse, affording
an endless variety of beautiful shapes. In some cases the exterior is
smooth and marked only with minute dots, but in many instances the
outer wall of the pollen grain is covered with spikes, or decorated
with stripes or belts. A few examples of the commonest forms of pollen
will be found on Plate III.
Fig. 17 is the pollen of the snowdrop, which, as will be seen, is
covered with dots and marked with a definite slit along its length.
The dots are simply tubercles in the outer coat of the grain, and
are presumed to be formed for the purpose of strengthening the
membrane, otherwise too delicate, upon the same principle which gives
to “corrugated” iron such strength in proportion to the amount of
material. Fig. 18 is the pollen of the wall-flower, shown in two views,
and having many of the same characteristics as that of the snowdrop.
Fig. 19 is the pollen of the willow-herb, and is here given as an
illustration of the manner in which the pollen aids in the germination
of plants.
In order to understand its action, we must first examine its structure.
All pollen-grains are furnished with some means by which their contents
when thoroughly ripened can be expelled. In some cases this end is
accomplished by sundry little holes called pores; in others, certain
tiny lids are pushed up by the contained matter; and in some, as in
the present instance, the walls are thinned in certain places so as to
yield to the internal pressure.
When a ripe pollen-grain falls upon the stigma of a flower, it
immediately begins to swell, and seems to “sprout” like a potato in
a damp cellar, sending out a slender “pollen-tube” from one or other
of the apertures already mentioned. In Fig. 19 a pollen-tube is seen
issuing from one of the projections, and illustrates the process better
than can be achieved by mere verbal description. The pollen-tubes
insinuate themselves between the cells of the stigmas, and, continually
elongating, worm their way down the “style” until they come in contact
with the “ovules.” By very careful dissection of a fertilised stigma,
the beautiful sight of the pollen-tubes winding along the tissues of
the style may be observed under a high power of the microscope.
The pollen-tube is nothing more than the interior coat of the grain,
very much developed, and filled with a substance technically named
“fovilla,” composed of “protoplasm” (the semi-liquid substance which
is found in the interior of cells), very minute starch grains, and some
apparently oily globules.
In order to examine the structure of the pollen-grains properly, they
should be examined under various circumstances--some dry, others placed
in water to which a little sugar has been added, others in oil, and it
will often be found useful to try the effect of different acids upon
them.
Fig. 20 is the pollen of the common violet, and is easily recognisable
by its peculiar shape and markings. Fig. 21 is the pollen of the
musk-plant, and is notable for the curious mode in which its surface
is belted with wide and deep bands, running spirally round the
circumference. Fig. 22 exhibits the pollen of the apple, and Fig. 23
affords a very curious example of the raised markings upon the surface
of the dandelion pollen. In Fig. 24 there are also some very wonderful
markings, but they are disposed after a different fashion, forming a
sort of network upon the surface, and leaving several large free spaces
between the meshes. The pollen of the lily is shown in Fig. 25, and is
a good example of a pollen-grain covered with the minute dottings which
have already been described.
Figs. 26 and 27 show two varieties of compound pollen, found in two
species of heath. These compound pollen-grains are not of unfrequent
occurrence, and are accounted for in the following manner.
The pollen is formed in certain cavities within the anthers, by means
of the continual subdivision of the “parent-cells” from which it is
developed. In many cases the form of the grain is clearly owing to the
direction in which these cells have divided, but there is no great
certainty on this subject. It will be seen, therefore, that if the
process of subdivision be suddenly arrested, the grains will be found
adhering to each other in groups of greater or smaller size, according
to the character of the species and the amount of subdivision that has
taken place. The reader must, however, bear in mind that the whole
subject is as yet rather obscure, and that further discovery may throw
doubt on many theories which at present are accepted as established.
Fig. 28 shows the pollen of the furze, in which are seen the
longitudinal slits and the numerous dots on the surface; and Fig. 29
is the curiously shaped pollen of the tulip. The two large yellow
globular figures at each side of the Plate represent the pollen of
two common flowers; Fig. 36 being that of the crocus, and Fig. 37 a
pollen-grain of the hollyhock. As may be seen from the illustration,
the latter is of considerable size, and is covered with very numerous
projections. These serve to raise the grain from a level surface, over
which it rolls with a surprising ease of motion, so much so indeed
that if a little of this substance be placed on a slide and a piece
of thin glass laid over it, the glass slips off as soon as it is in
the least inclined, and forces the observer to fix it with paper or
cement before he can place it on the inclined stage of the microscope.
The little projections have a very curious effect under a high
power, and require careful focusing to observe them properly; for the
diameter of the grain is so large that the focus must be altered to
suit each individual projection. Their office is, probably, to aid in
fertilisation.
* * * * *
The seeds of plants are even easier of examination than the pollen, and
in most cases require nothing but a pocket lens and a needle for making
out their general structure. The smaller seeds, however, must be placed
under the microscope, many of them exhibiting very curious forms. The
external coat of seeds is often of great interest, and needs to be
dissected off before it can be rightly examined. The simplest plan in
such a case is to boil the seed well, press it while still warm into
a plate of wax, and then dissect with a pair of needles, forceps, and
scissors under water. Many seeds may also be mounted in cells as dry
objects, after being thoroughly dried themselves.
A few examples of the seeds of common plants are given at the bottom of
Plate III.
Fig. 38 exhibits the fruit, popularly called the seed, of the common
goosegrass, or Galium, which is remarkable for the array of hooklets
with which it is covered. Immediately above the figure may be seen a
drawing of one of the hooks much magnified, showing its sharp curve
(Fig. 39). It is worthy of remark that the hook is not a simple curved
hair, but a structure composed of a number of cells terminating in a
hook.
Fig. 40 shows the seed, or rather the fruit, of the common red
valerian, and is introduced for the purpose of showing its plumed
extremity, which acts as a parachute, and causes it to be carried
about by the wind until it meets with a proper resting-place. It is
also notable for the series of strong longitudinal ribs which support
its external structure. On Fig. 41 is shown a portion of one of the
parachute hairs much more magnified.
The seed of the common dandelion, so dear to children in their
play-hours, when they amuse themselves by puffing at the white
plumy globes which tip the ripe dandelion flower-stalks, is a very
interesting object even to their parents, on account of its beautiful
structure, and the wonderful way in which it is adapted to the place
which it fills. Fig. 45 represents the seed portion of one of these
objects, together with a part of the parachute stem, the remainder of
that appendage being shown lying across the broken stem.
The shape of the seed is not unlike that of the valerian, but it is
easily distinguished from that object by the series of sharp spikes
which fringe its upper end, and which serve to anchor the seed firmly
as soon as it touches the ground. From this end of the seed proceeds
a long slender shaft, crowned at its summit by a radiating plume of
delicate hairs, each of which is plentifully jagged on its surface, as
may be seen in Fig. 46, which shows a small portion of one of these
hairs greatly magnified. These jagged points are evidently intended to
serve the same purpose as the spikes below, and to arrest the progress
of the seed as soon as it has found a convenient spot.
Fig. 42 is the seed of the foxglove, and Fig. 43 the seed of the
sunspurge, or milkwort. Fig. 47 shows the seed of the yellow
snapdragon; remarkable for the membranous wing with which the seed
is surrounded, and which is composed of cells with partially spiral
markings. When viewed edgewise, it looks something like Saturn with
his ring, or, to use a more homely but perhaps a more intelligible
simile, like a marble set in the middle of a penny. Fig. 48 is a seed
of mullein, covered with net-like markings on its external surface.
These are probably to increase the strength of the external coat, and
are generally found in the more minute seeds.
On Fig. 50 is shown a seed of the burr-reed; a structure which is
remarkable for the extraordinary projection of the four outer ribs, and
their powerful armature of reverted barbs. Fig. 51 shows another form
of parachute seed, found in the willow-herb, where the parachute is
not expanded nearly so widely as that of the valerian; neither is it
set upon a long slender stem like that of the dandelion, but proceeds
at once from the top of the seed, widening towards the extremity,
and having a very comet-like appearance. Two more seeds only remain,
Fig. 49 being the seed of Robin Hood, and the other, Fig. 52, that of
the muskmallow, being given in consequence of the thick coat of hairs
with which it is covered.
Many seeds can be well examined when mounted in Canada balsam.