I. THEIR PLACE IN NATURE
=331. Order of development.=—All the forms that have hitherto claimed
our attention belong to the great division of Spermatophytes, or
seed-bearing plants, designated also as _Phanerogams_, or flowering
plants. They comprise the higher forms of vegetable life, and because
they are more conspicuous and better known than the other groups,
they have been taken up first, since it is more convenient, for
ordinary purposes, to work our way backward from the familiar to the
less known, rather than in the reverse order.
But it must be understood that this is not the order of nature. The
geological record shows that the simplest forms of life were the
first to appear, and from these all the higher forms were gradually
evolved. There is no sharp line of division between any of the orders
and groups of plants, but the line of development can be traced
through a succession of almost imperceptible changes from the lowest
forms to the highest, and it is only by a study of the former that
botanists have come to understand the true nature and structure of
the latter.
=332. Basis of distinction.=—_Cryptogams_, or seedless plants as
a whole, are distinguished from the phanerogams by their simpler
structure and by their mode of propagation, which in the former
is by means of spores, while in the phanerogams it is by seeds. A
spore is a simple organic body, consisting usually of a single cell
which separates from the parent plant at maturity and gives rise to
a new individual. A seed is a complicated, many-celled structure,
containing within itself the rudimentary structure of a new plant
already organized.
Beginning with the simplest forms, cryptogams are grouped in three
great orders:—
=333. I. Thallophytes=, or thallus plants.—This group takes its name
from the _thallus_ structure that characterizes its vegetation. In
its typical form, a thallus is a more or less flat, expanded body, of
which the lichens and liverworts offer familiar examples among land
plants, and the kelps and laminarias among seaweeds. It may be of any
size and shape, however, and sometimes consists of a mere filament,
as in the common brook silk, or even of a single cell (Fig. 429).
The term is applied in general to the simplest kinds of vegetable
structure, in which there is no differentiation of tissues, and no
true distinction of root, stem, and leaves. While it is not peculiar
to the thallophytes, it has attained its most typical development
among them, and the name is therefore retained as distinctive of
that group. It embraces two great divisions, the Algæ and Fungi. The
first includes seaweeds and the common freshwater brook silks and
pond scums, besides numerous microscopic forms whose presence escapes
the eye altogether, or is made known only by the discolorations and
other changes caused by them in the water. To the fungi belong the
mushrooms and puffballs, the molds, rusts, mildews, and the vast
tribe of microscopic organisms called _bacteria_, which are so active
in the production of fermentation, putrefaction, and disease.
[Illustration: FIG. 425.—A seaweed with broad, expanded thallus.]
[Illustration: FIG. 426.—Anthoceros, a liverwort with flat, spreading
thallus.]
=334. II. Bryophytes=, or moss plants.—This group likewise contains
two main divisions, Mosses and Liverworts. Familiar examples of the
latter are the flat, spreading green plants, bearing somewhat the
aspect of lichens, met with everywhere on wet rocks and banks around
shady watercourses. The name is a reminiscence of their former use in
medicine as a specific for diseases of the liver, and not, as in the
case of the liver leaf, of a fancied resemblance to that organ.
Mosses are one of the best defined of botanical orders, and are
easily recognized by their slender, leafy fruiting stalks, growing
usually in dense, spreading mats, and presenting every appearance of
a highly organized structure, well differentiated into root, stem,
and leaves.
The liverworts represent the more primitive division of the group,
and in some of their forms approach so near the thallophytes that it
is not difficult to recognize them as connecting links in the same
chain of life. Their relationship to the next higher group is not
clear, but while they represent a more primitive stage of evolution
than the mosses, the development of the latter has followed a course
divergent from the main line of evolutionary progress.
[Illustration: FIG. 427.—A shoot of peat moss with ripe spore fruits,
_f_, _f_.]
[Illustration: FIG. 428.—A common fern (_Polypodium vulgare_).]
=335. III. Pteridophytes=, or fern plants, are classed roughly in
the three divisions of ferns, horsetails, and club mosses. They
differ greatly in structure, but all possess a vascular system, and
a well-organized structure of root, stem, and leaves. They rank next
to the spermatophytes in the order of development, and the group is
of especial interest on account of its relationship to the higher
plants. One of its divisions, the club mosses, has probably given
rise to at least one section of the gymnosperms, while the ferns are
regarded as the ancestors of the true flowering plants, which make
up the great class of angiosperms, and represent the highest type of
evolution yet attained in the vegetable kingdom.
II. THE ALGÆ
MATERIAL.—Simple forms of green algæ can be found on the shady side
of tree trunks, damp walls, old fence palings, and the outside of
flowerpots. _Pleurococcus_, one of the commonest kinds, occurs as a
green, powdery mat or felt in damp places, and is often accompanied
by _protococcus_, another good specimen for study. _Spirogyra_ and
other filamentous algæ can be found in stagnant pools and ditches
and in old rain barrels.
APPLIANCES.—Eosin solution, nitric acid, alcohol, iodine solution;
a white china plate; a hand lens; a compound microscope, and slides.
=336. Variety of forms.=—This group embraces plants of the greatest
diversity of form and structure, from the minute volvox and desmids
that hover near the uncertain boundaries dividing the vegetable from
the animal world, to the giant kelps of the ocean, which sometimes
attain a length of from six hundred to one thousand feet. They are
usually classed according to their color, as green, brown, and red
algæ, including various subdivisions of each group. They all contain
chlorophyll, by means of which they manufacture their own food,
though in the red and brown divisions it is masked by the presence of
other pigments—an adaptation to the modified light that reaches them
at various depths under water. With few exceptions they can live only
in the water, and unlike any other form of plant life, attain their
highest development in the salty depths of the ocean. The freshwater
forms are small and inconspicuous, and generally of a more simple
type than the seaweeds. The great majority of them belong to the two
classes of green and blue-green algæ. The former is believed to have
furnished the type from which the higher plants have been evolved.
[Illustration: FIG. 429.—Three stages in the division of a one-celled
alga (_Glœocaspa polydermatica_): _A_, division of a cell just
beginning; _B_, division further advanced; _C_, four cells after
division, remaining in contact.]
=337. Study of a one-celled alga.=—Put a little of the green algæ
in water on a glass slide. Hold up to the light, or over a sheet
of white paper, and examine with a hand lens; then place under
the microscope. It will probably be found to contain a number of
minute organisms, but the pleurococci can be recognized as small
round bodies of a bright green color, some of them separate, others
adhering together in groups of two, four, or more, with the sides
that are in contact slightly flattened. Each of these bodies is an
individual plant consisting of a single cell, whence they are said
to be _unicellular_. Draw one of the single cells and one of the
groups, or colonies, as they appear under the microscope. Try to
make out the cell wall and the nucleus, and label all the parts (see
7). If you have any difficulty in distinguishing the cell wall, drop
a little glycerine or salt water on the slide. This will cause the
cell contents to shrink by osmosis (56, 59). Can you make out the
structure of the cell colonies? They have resulted from the peculiar
mode of multiplication that prevails among this class of plants. A
cell elongates, contracts in the middle, and divides into two parts,
each of which becomes an independent plant like the mother cell. See
if you can find one in the process of division. The daughter cells
repeat the process, each one giving rise to two new individuals, and
so on indefinitely. The new cells do not always separate immediately
on their formation, but frequently adhere together for a time, in
colonies, before falling away and beginning an independent existence.
=338. Reproduction by fission.=—This kind of reproduction is called
_fission_, or cell division, and marks a very primitive stage of
development. Under stress of adverse conditions the cells formed
by division may remain inactive for a time. They are then called
_resting spores_, and when more favorable circumstances arise, they
begin again their work of reproduction and growth as actively as ever.
=339. Meaning of the name.=—The suffix _coccus_ is a Latin noun
(plural _cocci_) meaning a grain or berry, and is a general term
applied to any small, round organism consisting of a single cell;
hence, _micrococcus_, a minute round body; _protococcus_, a primitive
form, or prototype of one-celled bodies; and _pleurococcus_, which
may be freely translated “a one-sided little round body,” from the
flattening of the adjacent sides during fission—_pleuro_ meaning
lateral, or pertaining to the side.
It is important to remember this definition, as the term _coccus_
is of very frequent occurrence in works of biology, as a suffix for
designating small round bodies of various kinds.
=340. Examination of a filamentous alga.=—Place on a white dish a
few drops of water containing some of the green pond scum common in
stagnant pools and ditches. Examine with a hand lens; of what does
it appear to consist? Are the filaments all alike, or are they of
different lengths and thickness? Soak a number of them in alcohol for
half an hour and examine again; where has the green matter gone? Do
these algæ contain chlorophyll? (336; Exp. 65.) This class are called
filamentous algæ on account of their slender, threadlike thalli,
which look like bits of fine floss floating about in the water. The
bubbles of oxygen which they sometimes give off in great abundance
cause the frothy appearance that has given rise to their popular
name, “frog spit.”
[Illustration: FIGS. 430, 431.—_Spirogyra_ (magnified): 430, two
filaments beginning to conjugate; 431, formation of spores.]
=341. Spirogyra.=—The filamentous algæ are very numerous, and a
drop of pond scum will probably contain several kinds. At least
one of these, it is likely, will be a _Spirogyra_, as this is one
of the commonest and most widely distributed of them all. Place a
filament under the microscope and notice the spiral bands in which
the chlorophyll is disposed within the cells. It is from this spiral
arrangement that the species takes its name. Do you notice any
roundish particles inclosed in the chlorophyll bands? Test with a
little iodine solution and see what they contain. Each filament will
be seen, when sufficiently magnified, to consist of a number of more
or less cylindrical cells joined together in a vertical row, and thus
forming the simple threadlike thallus which characterizes this class
of algæ. Physiologically, each cell is an independent individual, and
often exists as such. Can you see the cell nucleus? If not, place a
few filaments in a solution of eosin and add a drop of acetic acid to
give the solution a pale rose color. After twenty to thirty minutes,
examine again; the nucleus will be stained a deep red. If you can
find an unbroken filament, examine both ends to see whether there is
any differentiation of base and apex.
=342. Conjugation.=—See if you can find two filaments sending out
lateral protuberances toward each other. Watch and notice that after
a time these projections come together and unite by breaking down
the cell walls dividing them, the protoplasm in each contracts, the
contents of one pass over into the other, and the two coalesce,
forming a new cell but little, if any, larger than the original
conjugating bodies. This cell germinates under favorable conditions
and produces a new individual. This method of reproduction is known
as _conjugation_. The cells thus produced by the union of the
contents of two separate cells may either germinate at once, and give
rise to new individuals, or remain quiescent for a time, as resting
spores.
Practical Questions
1. Are any of the green algæ parasitic? How do you know? (186, 336.)
2. Why is their presence in water regarded as denoting unhygienic
conditions?
3. Mention some of the ways in which their presence may contribute
to the contamination of drinking water.
4. Refer to Exp. 66, and account for the bubbles and froth that
usually accompany these plants in the water.
5. Can you suggest any other causes than the evolution of oxygen
that might produce the same effect?
6. Is the presence of these gas bubbles of any use to floating
plants?
III. FUNGI
[Illustration: FIG. 432.—A common form of mold, magnified, showing
thallus modified into a fibrous mycelium: _a_, _a_, spore cases; _b_,
mycelium. (_After_ KOPF, in part.)]
=343. Classification.=—In the fungi the thallus structure is greatly
modified, appearing usually as a network of fine threads called the
_mycelium_ (pl., _mycelia_), from a Greek word meaning “fungus”
(369). These plants are all, with a few doubtful exceptions,
parasites or saprophytes which contain no chlorophyll and are
incapable of supporting an independent existence. Biologists are
divided as to their position in the genealogical tree of life.
The weight of authority at present inclines to the view that they
are degenerate forms derived from the algæ, but they have been so
modified by their parasitic habits as to render their position in the
general scheme of life a doubtful one. They represent an offshoot,
or side branch, as it were, of the great evolutionary line, and so
may be considered for the present as standing apart in a class by
themselves.
=344. Numbers and variety.=—Fungi exceed every other class of
living organisms both in the number of species and of individuals
composing them. They include such diverse forms as bacteria, molds,
rusts, mildews, mushrooms, and the like, ranging in size all the way
from the giant puffball, a foot or more in diameter, to the almost
inconceivably minute influenza bacillus, of which nearly two thousand
million can inhabit a single drop of water without inconvenient
crowding!
[Illustration: FIG. 433.—Cephalothecium, a fungus parasitic on
rosehips—greatly magnified. (_From_ Mo. Botanical Garden Rep’t.
Photographed by Hedgcock.)]
=345. The parasitic habit.=—But while their life history is obscure
and hard to trace, the fungi are, as a class, well differentiated by
their parasitic habit. They contain no chlorophyll, can manufacture
no food, and consequently have to obtain it ready-made from the
tissues of living or dead animals and plants. On this account they
are active agents in the production of disease and decay, especially
certain of those manifold forms that have been grouped together
under the general head of bacteria. While not responsible for all
the disease known to be caused by living organisms,—some very
serious ones, such as malaria and cattle fever, being due to animal
parasites,—the majority of those that have been most carefully
investigated are traced to the bacteria, or other fungi. After any
of these parasites have found a lodgment in the body of an organism
whose tissues furnish them a congenial habitat, they multiply with
enormous rapidity, and through the action of certain poisons called
_toxins_, which they excrete, give rise to the most destructive
diseases in both animals and plants; and no rational sanitary science
is possible without a knowledge of their habits and life history. Add
to the vast amount of human suffering that is to be laid at their
door the economic damage done by rust and smut fungi, by molds and
blights and mildews, and we shall be tempted to conclude that the
“battle of life” is largely a struggle against these invisible foes.
[Illustration: FIGS. 434-437.—Disease-producing bacteria: 434,
bacteria of consumption (_Bacillus tuberculosis_); 435, cholera
bacillus; 436, bacilli of anthrax, showing spores; 437, typhoid
bacillus.]
=346. Useful fungi.=—Not all fungi, however, are injurious. On the
contrary, the great majority of them are harmless, and very many
kinds are positively beneficial to man. Without the yeasts and
bacteria of fermentation we could not have our bread and cheese.
Other forms are active agents in the fertilization of soils, it
having been estimated that there are 100,000 or more of these
infinitesimal laborers at work in every cubic centimeter (about ¹⁄₁₆
of a cubic inch) of virgin soil! Even the bacteria of putrefaction,
which we are accustomed to regard as the embodiment of all that is
foul and loathesome, are engaged in an unceasing work as scavengers,
without which life would no longer be possible on our globe, as will
be shown in the following section.
A. BACTERIA
MATERIAL.—A vessel of water in which hay has been left to soak for
several hours; a freshly boiled potato.
APPLIANCES.—A double boiler for sterilizing; a number of clean
glass jars and bottles; cotton wool for stoppers; a compound
microscope.
CULTURE MEDIUMS.—A freshly boiled potato answers very well for
ordinary purposes. “Bread mash” can be made by drying some bread
crumbs in an oven, then mashing and mixing them to a paste with
boiling water; sterilize by three successive heatings in a double
boiler. A sterilized preparation of gelatine solution is the medium
most commonly used.
=347. How to obtain specimens for observation.=—While bacteria are
plentiful almost everywhere, it is not always easy to capture them
just when and where you want them. For this purpose, put some hay
in water and leave in a warm place away from the light until the
liquid becomes cloudy or a film forms on the surface. This will show
that bacteria are present. If it is desired to study any particular
kind of bacterium, inoculate one of the culture mediums described
under “material,” or a few drops of sterilized extract of beef, with
a small quantity of the substance to be examined, or with dust or
scrapings from the locality under consideration.
EXPERIMENT 93. BY WHAT MEANS ARE BACTERIA COMMONLY DISTRIBUTED?—Put
a slice of freshly boiled potato into each of three glass tumblers
and cover with a filter of cotton wool held in place by tying
tightly with a cord, or by an elastic band. Set them all in a
vessel of water, bring it to a boil, and keep at that temperature
for half an hour, to sterilize the air in the tumblers. When they
have cooled, lift the cotton from (1) for a minute or two and then
replace. Carefully pass the tip of a medicine dropper through the
filter of (2) so as to prevent the entrance of unsterilized air,
and put on the slice of potato a small quantity of the bacterial
liquid prepared as directed in the last paragraph. Leave (3)
unopened. Keep all together in a warm, dark place and observe at
intervals of from 12 to 24 hours. Do any bacteria appear in (3)?
Do any appear on the potato in (2), where the liquid was dropped?
Are they more, or less abundant than in (1)? Since cotton wool is
entirely impervious to the smallest microörganisms known, would you
judge from this experiment that bacteria can get into any place
unless carried there by the air, or by some other means?
EXPERIMENT 94. CAN BACTERIA BE CARRIED BY PURE AIR?—On a warm (and
preferably cloudy) day, put a slice of potato on a plate, and leave
uncovered in an unused room or closet, free from dust, and kept
carefully closed. Put another slice arranged in exactly the same
way in an open window on a dusty street, or in a room that is used
and daily swept and dusted. Do bacteria appear in the first plate?
In the second? Is air free from dust a good conveyor of bacteria?
EXPERIMENT 95. WHAT CONDITIONS ARE FAVORABLE TO BACTERIAL
GROWTH?—Strain some of your culture liquid into half a dozen small
bottles of the same size, filling each about half full. Put (1) in
a dark, cool place—on ice, if the weather is warm; (2) in a dark,
warm place; (3) in a warm, well-lighted place; into (4) put a drop
of carbolic acid, formalin, corrosive sublimate, or boracic acid,
and keep in a dark, warm place. Keep (5) in boiling water for half
an hour or more, and then place beside (2). Keep (6) in a freezing
mixture of salt and ice for several hours, then place with (2) and
(5). Examine all at intervals of from 12 to 24 hours. In which
bottles is the presence of bacteria indicated by cloudiness of the
contained liquid, or the formation of a surface film? In which do
they appear first? In which most abundantly? In which last, or not
at all? What is the effect of light and darkness on their growth?
Of heat and cold? Of disinfectants? Name the circumstances that
tend to hinder their growth, in the order of their efficacy.
=348. Microscopic study of bacteria.=—Put a drop of hay infusion on a
slide and examine with the highest power of the microscope. You will
see a multitude of very small glistening bodies including different
kinds of bacteria, a majority of which are probably the hay bacillus,
_B. subtilis_, shown in Figs. 443, 444. Notice that some forms move
about freely, while others are non-motile. Which kind are the more
numerous? The motion may be either mechanical, resembling that of
the small dust particles we see dancing about in the sunshine, or
apparently voluntary, and caused by the vibration of little whiplike
cilia. Can you distinguish the two kinds? Try to make out clearly
the different shapes you see. Some appear as slender chains or
filaments, but this is due to the individual cells’ adhering together
for a time before breaking up and beginning an independent existence.
The small, rounded bodies, like a period (Fig. 438), are _cocci_; the
slender, rod-shaped ones—sometimes slightly curved (Fig. 440)—are
_bacilli_ (sing., _bacillus_); the comma-shaped ones, and those
generally showing a slight spiral curvature, are _vibrios_ (Fig.
441); the spirally twisted ones, like a corkscrew (Fig. 442), are
_spirilli_ (sing., _spirillum_). These are the principal forms which
it is important to distinguish and remember. The names are applied
very loosely, however, in practice, bacillus being often used as a
general term applicable to almost any kind,—the spirillum of cholera,
for instance, being commonly known as the cholera bacillus, while by
some authors vibrios are ranked as a variety of spirillum.
[Illustration: FIGS. 438-442.—Typical forms of bacteria: 438, coccus
type; 439, the same, hanging together in chains; 440, rod-shaped
bacteria (bacillus type), the clear areas in some of these are
spores; 441, forms of vibrio; 442, forms of spirillum.]
=349. Life history of a typical bacterium.=—A pure culture of the
_Bacillus subtilis_ can easily be obtained by boiling some of the
hay infusion for half an hour and then leaving in a warm place till
the usual indications of the presence of bacteria appear (347).
The spores of this micro-organism are so resistant that they can
withstand the temperature of boiling water for several hours, while
those of most other forms of bacteria are killed by a few minutes’
exposure to it; hence, the crop that develops after boiling will
consist of a pure culture of the hay bacillus.
[Illustration: FIGS. 443, 444.—Hay bacillus (_B. subtilis_): 443, a
portion of the film from the culture liquid, the black lines, _e_,
being bacteria in the vegetative state; 444, spore formation; _a_,
_d_, motile cells and chain of cells: _b_, non-motile cells; _c_,
spores and chain of spores from the film _e_.]
In their active state these organisms will be seen to consist of
single-celled, rod-shaped bodies, about three or four times as long
as broad, and generally cohering in bands or filaments, as shown in
Fig. 444, _c_. The black dots within the cells are the spores. Each
individual bacterium produces but a single spore, or rather becomes
a spore itself, by the contraction of its contents and the formation
around them of a strong inclosing membrane. On germinating, the
spores give rise to little ciliated, one-celled organisms called
“swarm spores,” that swim about freely in the containing medium and
multiply rapidly for a time by cell division. After this they pass
again into the quiescent state, ready, whenever favorable conditions
arise, to begin anew the repetition of their life cycle, which is an
irregular alternation of cell division and spore formation.
=350. Resistance of spores.=—Bacteriologists are not fully agreed as
to the cause of spore formation, some holding that it takes place
only when conditions are most favorable for bacterial growth, others
claiming the reverse. The consensus of opinion at present is toward
the view that the spores are a provision for tiding over periods of
stress and difficulty. They are capable of retaining their vitality
for a long time, and are much harder to kill than the bacterial
cells in their ordinary vegetative state, as was seen in the case of
the hay bacillus. The spores of one species of potato bacillus have
retained their vitality after four hours of boiling, and those of the
typhoid bacillus after continuous exposure to a freezing temperature
for more than three months. The majority of bacteria, in their
vegetative state, are, however, either killed or rendered inert by
temperatures ranging below 10° or above 50° centigrade—equivalent to
about 50° and 122° Fahrenheit, respectively. It is easy to see what
important bearing these facts have on the process of disinfection.
=351. Reproduction and multiplication.=—The ordinary mode of
reproduction in bacteria, as in other unicellular organisms, is by
fission (337, 338). As each individual forms but a single spore, no
increase in numbers could take place by this means alone. Hence,
while the spores are an important factor in the preservation of
the species by continuing its existence under conditions which the
active organisms could not survive, their successful propagation
depends on their power of rapid multiplication by division. If
this process were to go on unchecked, every hour, in an unbroken
geometrical progression, the progeny of a single bacterium would,
in 24 hours, number nearly 17 million; in 25 hours, 34 million; in
26 hours, 68 million, and in five days they would cover the entire
surface of the globe, land and sea, to a depth of 3 feet! In ordinary
standard milk sold by dairymen, and containing, when examined, less
than 10,000 microbes to the cubic centimeter,—about 20 drops,—the
number was found to have increased after 24 hours to 600 million.
It is comforting to know, however, that the majority of these are
of the harmless kinds which are the active agents in the making of
buttermilk and cheese.
The effects of their rapid multiplication will be better appreciated
when we consider that bacteria are the smallest of known living
creatures. If 1000 of the influenza bacilli were spread out in
a single layer with their sides touching, but not overlapping,
they would not take up more room than one of the periods used in
punctuating this book; and a coccus concerned in a tubercular disease
prevalent among cattle in South America has recently been discovered,
of which double that number could be accommodated in the same space.
[Illustration: FIGS. 445, 446.—Milk (highly magnified): 445, pure,
fresh milk, showing fat globules; 446, milk that has stood for hours
in a warm room in a dirty dish, showing fat globules and many forms
of bacteria.]
=352. Distribution of bacteria.=—Ordinary air, when free from
dust, contains, on the average, not more than five germs to the
liter—equal to about 1 for every 12 cubic inches. Pathogenic, or
disease-producing, germs seldom occur in ordinary fresh air, and
even when present are, under ordinary circumstances, harmful only to
people whose bodies, by reason of physical weakness or unhygienic
habits, offer a congenial soil for their multiplication. Numerous
instances are known in which perfectly healthy persons have carried
about infectious disease germs in their bodies and even transmitted
them to others without experiencing any inconvenience, or even being
aware of their presence. Among others, the germs of pneumonia,
diphtheria, and tuberculosis are often found in the mouth, nose,
and sputum of perfectly healthy persons. There are also a number
of bacteria that are regular inhabitants of the mouth, some of
which are the cause of decayed teeth and foul breath. One form of
bacterium, concerned in the production of inflammation and abscesses
(_Staphylococcus_) is so constantly present on the human epidermis
that one authority has declared it impossible to sterilize the
skin so thoroughly as to free it entirely of this microbe. It is
ordinarily not harmful unless it comes in contact with open wounds
and abrasions.
=353. The economic importance of bacteria.=—It is hard to say
whether these organisms concern us most on account of the damages
attributable to them on the one hand, or the benefits we owe them
on the other. If they were all as harmful as the pathogenic kinds,
life would hardly be possible on the globe, while without their
presence life as we know it would have ceased to be possible long
ago. They are nature’s great army of scavengers, the sole agents of
decomposition, without which dead organic matter would be subject
only to the slow changes by which the rocks and mineral matter of the
earth’s crust are disintegrated, and the undecomposed bodies of the
vast procession of plants and animals that have existed since life
first began on our globe would long ago have cumbered its surface to
such an extent as to render impossible the continued development of
life such as we know.
=354. Sterilization= is the process of ridding a substance of living
microörganisms. To do this effectively, the process must be repeated
several times at intervals, so as to give any spores that may have
survived previous applications time to pass into the vegetative
state, when their power of resistance is diminished and they are more
easily destroyed. The incubation period, as the time required for
the germination of the spores is called, is different for different
kinds of bacteria; hence the importance, from a sanitary point of
view, of a thorough knowledge of their life history.
=355. Disinfection= is sterilization on a large scale, and the same
principles apply to both. Heat is the safest disinfectant for objects
that will bear it, if continued long enough and repeated often enough
at a sufficiently high temperature. Freezing will destroy some kinds
of germs and check or retard the development of nearly all, but
is not to be relied on as a permanent germicide, since even among
flowering plants there are many kinds, not only of seeds, but of
perennial vegetative forms that are capable of enduring an arctic
temperature of many degrees below freezing for long continued periods.
Chemical disinfectants act usually as microbe poisons, and are
unsuitable as sterilizers for food, though valuable in the
purification of houses, clothing, and utensils—especially the
instruments employed in surgical operations.
The prevention of the growth of bacteria, especially in wounds
and surgical incisions, whether by means of chemical or physical
agencies, is known as _antisepsis_.
Practical Questions
1. Why should a person recovering from an ague continue for some
time after to take quinine every third or every seventh day? (350,
354.)
2. Name some of the principal diseases produced by bacteria.
3. What is the principle to be acted on in the avoidance of such
diseases? (Exps. 94, 95.)
4. Are the same means equally effective for prevention and for
cure? (354, 355; Exps. 93-95.)
5. Why is “fresh air” beneficial in a sick room? (352; Exp. 94.)
6. Does it act as a disinfectant, or as a mere diluent of the
infected air of the room? (352.)
7. Why ought preserved fruits and vegetables to be scalding hot
when put into the can? (355.)
8. Why is it necessary to exclude the air from them? (Exps. 93, 94.)
9. Reconcile question 8 with question 5.
10. Why does the use, for drinking purposes, of water that has been
boiled render a person less liable to infectious diseases? (355.)
11. Was the old-fashioned practice of handing the baby round to be
promiscuously kissed by friends and neighbors a good one for the
baby? (352.)
12. Why is the spitting habit to be condemned? The use of common
drinking cups in schoolrooms and other public places? (352.)
13. Is it proper from a sanitary point of view that roommates at
a boarding school, or even members of the same family, should use
soap, towels, and other articles of the toilet in common? (352.)
B. YEASTS
MATERIAL.—A piece of fresh baker’s yeast, some warm water, and a
little honey or sugar solution; a pipette, or a medicine dropper;
three or four clean pint bottles or preserve jars.
To raise a crop of yeast fungi for observation, rub one fourth of a
fresh yeast cake in water enough to make a paste; add one pint of
water, with a tablespoonful of honey or sugar, and stir well.
EXPERIMENT 96. WHAT CONDITIONS FAVOR THE GROWTH OF YEAST?—Pour
equal parts of the liquid made as directed (see Material) into each
of three pint bottles, stopper lightly, and label. Put (1) in a
warm, dark place; (2) in a cool, dark place; and (3) in a bright
light in a warm place. Observe at intervals of a few hours the
changes that occur in each. Notice the bubbles that rise from the
liquid. In which bottle do they form most rapidly? Lower a lighted
match into it, or transfer some of the gas with a pipette into a
vessel containing limewater, and tell what it is. Taste some of the
fermenting liquid. Is it sweet? What has become of the sugar that
was put into it?
=356. Yeasts and ferments.=—Yeasts belong to a very different order
of fungi from the bacteria, but on account of their simplicity of
structure and the similarity of their action to that of some of the
latter, it is usual to consider them together. They are the active
agents of fermentation, and include a large number of species. The
kind used for household purposes is the same as that employed in
making beer. Of this species there are many varieties, each one of
which gives a characteristic taste to the beer made from it; and
brewers, by paying attention to the cultivation of yeasts, give their
product the special flavors peculiar to the different brands. This
kind of yeast is not known to exist except in a state of cultivation,
and probably owes its survival and present condition of development
to a symbiosis with man, on account of its usefulness in bread
making, and still more, perhaps, to its part in the gratification of
his bibulous propensities, for among savage tribes the manufacture of
fermented liquors is practiced long before the wholesome art of bread
making.
[Illustration: FIGS. 447-449.—Forms of common yeast (_Saccharomyces
cerevisiæ_): 447, brewers’ yeast; 448, household yeast (the large
grains are starch); 449, yeast from beer sediment, showing budding.
(Figs. 447, 448 × 250; Fig. 449 × 1270.)]
There are other yeasts existing in a state of nature, such as those
on the surface of fruits, which cause the latter, under certain
circumstances, to ferment and decay. For this reason artificial
ferments are not needed in making wine and other alcoholic liquors
from fruits. Fermentation is also caused by certain forms of
bacteria, as in the formation of vinegar and the souring of milk.
Such bacteria often contaminate the yeast ferments.
=357. Microscopic examination.=—Place a drop of the cultural
liquid on a slide and examine under the highest power of the
microscope. What do you see? These egg-shaped bodies are yeast
plants, unicellular organisms like the pleurococcus. Do you see any
chlorophyll? Are the yeasts parasitic? How do you know? What do they
live on? (Suggestion: What food substance that has disappeared was
put into the culture liquid?) In getting their nourishment from the
sugar, these fungi disintegrate it into alcohol and carbon dioxide,
which is a process of fermentation. It is the bubbles of gas that
were seen rising in the liquid which cause beer to effervesce and
bread to rise. They permeate the dough and by their expansion produce
the sponginess peculiar to leavened bread. Look for a cell with a
bud forming on it; from what part does it appear to grow? Where a
number of buds remain for some time attached to the mother cell (Fig.
449), they form a _colony_. Make a sketch of a single cell and of a
colony of two or more adherent ones, labeling all the parts. If the
cell wall cannot be made out clearly, run a little glycerine, or salt
water, under the cover glass with a medicine dropper. What causes the
contents of the cell to contract and leave the wall? (56, 59.)
=358. Reproduction.=—From time to time buds break away from the
mother cell and form new individuals or colonies of their own. This
process is called multiplication by budding, and is only another form
of cell division.
Whenever reproduction takes place by other means than seeds or
spores, it is said to be _vegetative_. This sort of reproduction
is not confined to unicellular plants, but exists also among the
phanerogams, the propagation of species by means of buds, tubers,
rootstocks, runners, grafting, and the like being variations of
the same process. On the other hand, yeasts and bacteria and the
unicellular algæ have the power, under extreme conditions, to form
resting spores, which sometimes lie dormant for years and resume
their activity when favorable conditions return.
Practical Questions
1. When is fermentation useful to man?
2. What is the effect on canned fruits and vegetables if yeast
cells get into them?
3. Why does milk turn sour in warm weather? (350, 351; Exp. 96.)
4. Why do buttermilk and clabber spoil if left standing too long?
(345, 356.)
5. What causes bread to be “heavy”? (356, 357.)
6. Why will dough not rise unless kept in a warm place? (Exp. 96.)
7. Why is beer kept cold during fermentation? (350, 356.)
C. RUSTS
MATERIAL.—A leaf of wheat affected with red rust; a leaf or a stalk
with black rust. Some barberry leaves with yellowish pustules on
the under side, which under the lens look like clusters of minute
white corollas. These are popularly known as “cluster cups.” As the
spots on barberry occur in spring, the red rust in summer, and the
black rust in autumn, gather the specimens as they can be found,
and preserve for use.
[Illustration: FIGS. 450, 451.—Leaf of wheat affected with orange
leaf-rust (_Puccinia rubigo-vera_), uredo stage: 450, upper side of
leaf; 451, under side.]
The orange leaf, or brown, rust (_Puccinia rubigo-vera_) is more
common in some parts of the country than the ordinary wheat rust
(_Puccinia graminis_), but the two are so much alike that the
directions given will do for either. If the orange leaf-rust (so
named from its color, and not from any connection with orange
leaves, the logical relation of the words being orange leaf-rust,
and not orange-leaf rust) is used, the cups and pustules should be
looked for on plants of the borage family—comfrey, hound’s-tongue,
etc. The orange leaf-rust of apple is caused by a fungus which will
serve to illustrate the same class of parasites. The “teleuto”
stage of this will be found on cedar trees, in the excrescences
commonly known as “cedar apples”; the “cluster cups” on the leaves
of apple and haw trees affected with the disease.
=359. Red rust.=—Uredo stage. Examine a leaf of “red rusted” wheat
under the lens, and notice the little oblong brown dots that cover
it. These are clusters of spore cases, and are the only part that
appears above the surface. Viewed under the microscope, the red rust
will be seen to consist of a mycelium (see Fig. 452), which ramifies
through the tissues of the leaf and bears clusters of single-celled
reddish spores that break through the epidermis and form the reddish
brown spots and streaks from which the disease takes its name. These
spores, falling upon other leaves, germinate in a few hours and form
new mycelia, from which, in six to ten days, fresh spores arise.
Formerly this was thought to complete the life history of the fungus,
to which the name of _Uredo_ was given. It is now known, however,
that the red rust is merely a stage in the life cycle of the plant,
and to this stage the old name uredo is applied, the spores being
called _uredospores_.
=360. Black rust.=—Teleuto stage. Next examine with a lens a part
of the plant attacked by black rust. Do you observe any difference
except in the color? Do the two kinds of rust attack all parts
of the plant equally? If not, what part does each seem to affect
more particularly? At what season does the black rust appear most
abundantly? Place a section of the diseased part under the microscope
and notice that the difference in color is due to a preponderance
of long, two-celled bodies with very thick, black walls (Fig. 453).
These are called _teleutospores_, a word meaning “final spores,”
because they are formed only toward the end of the season. They are
developed from the same mycelium with the uredospores, and are not
a product of the latter, but collateral with them and belong to the
same stage in the life history of the fungus. After they appear,
the uredospores cease to be developed at all, and only the dark
teleutospores are produced. These remain on the culms in the stubble
fields over winter, ready to begin the work of reproduction in
spring. For this reason the teleutos are popularly known as “winter
spores” in contradistinction to the uredos, or “summer spores,” whose
activity is confined to the warm months.
[Illustration: FIG. 452.—Uredo spores of wheat rust (_Puccinia
graminis_), magnified. (_From_ COULTER’S “Plant Structures.”)]
[Illustration: FIG. 453.—Teleutospores of wheat rust, magnified.
(_From_ COULTER’S “Plant Structures.”)]
[Illustration: FIG. 454.—Teleutospore germinating and forming
sporidia, _s_, _s_. (_From_ COULTER’S “Plant Structures.”)]
It was formerly supposed that black rust was caused by a different
fungus from that producing red rust, and to it the name _Puccinia_
was given. This has been retained as a general designation for all
fungi undergoing these two phases, and the particular form of fungus
that we are now considering is known in all its stages as _Puccinia
graminis_.
=361. The nonparasitic stage.=—The formation of teleutospores
completes that portion of the life history of the fungus during which
it is parasitic on wheat and grasses of different kinds. In spring
they begin to germinate on the ground, each cell producing a small
filament, from which arise in turn several small branches. Upon the
tip of each of these branches is developed a tiny sporelike body
called a _sporidium_ (Fig. 454), which continues the generation of
the rust fungus through the next stage of its existence. The filament
which bears these sporidia is not parasitic, but when the sporidia
ripen and the spores contained in them are scattered by the wind,
there begins a second parasitic phase, which forms the most curious
part of this strange life history.
=362. The æcidium.=—Examine next the under side of some barberry
leaves (or comfrey, etc., if orange leaf-rust is used) for clusters
of small whitish bodies that appear under the lens like little white
corollas with yellow anthers in the center. Examine a section of one
of these under the microscope and notice that the yellow substance
is composed of regular layers of colored spores. The corolla-like
receptacles containing them, popularly known as “cluster cups,”
are borne on a mycelium produced from the spores described in the
last paragraph. This mycelium is parasitic on barberry or other
leaves, according to the kind of fungus, and was long believed to be
a distinct plant, to which the name _Æcidium_ (pl., _Æcidia_) was
given. This term is now applied to the cluster cups, and those fungi
which at any period of their life history produce them are called
æcidium fungi.
[Illustration: FIG. 455.—Section through a barberry leaf, showing on
the upper side two spermogonia, _s_, _s_; and on the under side, an
æcidium, _æ_.]
=363. Spermogonia.=—On the upper surface of the leaves that bear
the æcidia, notice some small black dots hardly larger than pin
points, but which, when sufficiently magnified, appear as little
flask-shaped bodies (Fig. 455) under the epidermis. These are known
as _spermogonia_, or _pycnidia_. When mature, they break through
the epidermis so that the necks protrude, and discharge a quantity
of minute cells or spores, very like some that, later on, we shall
find playing an important part in the reproductive processes of
certain other fungi, and of mosses and liverworts. In the rust fungi,
however, their function is not understood. They may possibly be
survivals of organs which were once active in the life processes of
the plant, but have become useless under changed conditions. Do such
organs throw any light on the evolutionary history of plants?
[Illustration: FIG. 456.—A species of “cedar apple”
(_Gymnosporangum_), showing the uredo-teleuto stage of the apple rust
fungus. (_From_ a photograph by Prof. F. E. Lloyd.)]
=364. Connection between barberry and wheat rust.=—With the discharge
of the æcidium spores, the part of the life cycle of the fungus
spent on the barberry comes to an end, and it is ready to begin the
uredo-teleuto stage over again as soon as it finds a suitable host.
Where there are no barberries, it is capable of propagating without
them, either by adapting itself to some other host plant, or by
omitting the æcidium stage altogether. The parasitic habit being an
acquired one, the fungus, like some animal organisms that we know of,
can often be “educated” by force of circumstances into tolerating,
and even thriving upon, foods which under other circumstances it
would reject. The wheat rust is known to be capable of propagating
year after year in the uredo stage, the spores surviving through
the winter on volunteer grains and grasses; and in no other country
in the world does rust do greater damage to the wheat crop than in
Australia, where the barberry is practically unknown. This power of
accommodation possessed by many parasites is one of the difficulties
the agriculturist has to contend with in the development of rustproof
varieties.
=365. Polymorphism.=—Plants that pass through different stages in
their life history are said to be _polymorphic_, that is, of many
forms. The habit is very common among the lower forms of vegetation.
The fact that one or more of the phases are sometimes omitted, as
the æcidium phase of wheat rust in warm climates, suggests the idea
that it may be of use in helping the plant to tide over difficult
conditions. Besides giving better chances of obtaining nourishment,
it probably has the same effect as cross fertilization among
flowering plants, in imparting increased strength and vitality to the
succeeding generation. Wheat rust produced from barberry æcidia is
said to be much more vigorous—and consequently more destructive—than
when derived from a uredo that has reproduced itself for several
generations.
=366. The damage done by rust= to the host is through the destruction
of its tissues by the mycelium. The chlorophyll is destroyed so that
the plant can no longer manufacture food, and is too starved to
produce good grain. There are many varieties of wheat rust, which
have been found on twenty-seven different kinds of grain. Most
of them are specialized to a particular host plant and will not,
ordinarily (364), infest any other. Has this fact any bearing upon
the production of rustproof varieties?
Practical Questions
1. Is a farmer wise to leave scabby and mildewed weeds and bushes
in the neighborhood of his grain fields? (364, 365.)
2. Are there any objections to the presence of volunteer grain
stalks along roadsides and in fence corners during winter? (364.)
3. Should cedar trees be allowed to grow near an apple orchard?
Give a reason for your answer. (p. 317, Material.)
4. Should diseased plants be plowed under? (361.)
5. What disposition should be made of them?
6. Ought diseased fruits to be left hanging on the tree?
7. Why is it necessary to pick over and discard from a crate or bin
all decaying fruits and vegetables?
8. Does a rotation of crops tend to prevent the spread of disease
in plants? Give reasons for your answer.
9. Are rustproof varieties to be relied on indefinitely? (364.)
D. MUSHROOMS
MATERIAL.—Any kind of gilled mushroom in different stages of
development, with a portion of the substratum on which it grows,
containing some of the so-called spawn. The common mushroom sold in
the markets (_Agaricus campestris_) can usually be obtained without
difficulty. Full directions for cultivating this fungus are given
in Bulletin 53 of the U. S. Department of Agriculture. From 6 to 12
hours before the lesson is to begin, cut the stem from the cap of a
mature specimen, close up to the gills, lay it, gills downward, on
a piece of clean paper, cover with a bowl or pan to keep the spores
from being blown about by the wind, and leave until a print (Fig.
466) has been formed.
=367. Mushrooms and toadstools.=—The popular distinction which limits
the term “mushroom” to a single species, the _Agaricus campestris_,
and classes all others as toadstools, has no sanction in botany. All
mushrooms are toadstools and all toadstools are mushrooms, whether
poisonous or edible. The real distinction is between mushrooms and
puffballs, the former term being more properly applied to fungi which
have the spore-bearing surface exposed.
=368. Examination of a typical specimen.=—The most highly specialized
of the fungi, and the easiest to observe on account of their size and
abundance, are the mushrooms that are such familiar objects after
every summer shower. The _gilled_ kind—those with the rayed laminæ
under the cap—are usually the most easily obtained. Specimens should
be examined as soon after gathering as possible, since they decay
very quickly.
=369. The mycelium.=—Examine some of the white fibrous substance
usually called spawn through a lens. Notice that it is made up of
fine white threads interlacing with each other, and often forming
webby mats that ramify to a considerable distance through the
substratum of rotten wood or other material upon which the fungus
grows. This webby structure, often mistaken for root fibers, is
the thallus or true vegetative body of the plant, the part rising
above ground, and usually regarded as the mushroom, being only the
fruit, or reproductive organ. Place some of the mycelium under the
microscope and notice that it is composed of delicate filaments made
up of single cells placed end to end, as in Spirogyra (341). These
filaments are called _hyphæ_.
=370. The button.=—Look on the mycelium for one of the small round
bodies called buttons (Fig. 457). These are the beginning of the
fruiting body popularly known as the mushroom, and are of various
sizes, some of the youngest being barely visible to the naked eye.
After a time they begin to elongate and make their way out of the
substratum.
[Illustration: FIG. 457.—Mycelium of a mushroom (_Agaricus
campestris_), with young buttons (fruiting organs) in different
stages: 1, 2, 3, 4, 5, sections of fructification at successive
periods of development; _m_, mycelium; _st_, stipe; _p_, pileus; _l_,
gill, or lamina; _v_, veil.]
[Illustration: FIG. 458.—Diagram of unexpanded _Amanita_, showing
parts: _a_, volva; _b_, pileus; _c_, gills; _d_, veil; _e_, stipe;
_m_, mycelium.]
=371. The veil and the volva.=—Make a vertical section through the
center of one of the larger buttons after it is well above ground,
and sketch. Notice whether it is entirely enveloped from root to cap
in a covering membrane—the _volva_ (Fig. 458, _a_)—or whether the
enveloping membrane extends only from the upper part of the stem to
the margin of the cap—the _veil_ (Fig. 458, _d_); whether it has both
veil and volva, or finally, whether it is naked, that is, devoid of
both.
=372. The stipe, or stalk.=—Notice this as to length, thickness,
color, and position; that is, whether it is inserted in the center
of the cap or to one side (excentric), or on one edge (lateral).
Observe the base, whether bulbous, tapering, or straight, and whether
surrounded by a cup, or merely by concentric rings or ragged bits
of membrane (the remains of the volva). Look for the _annulus_ or
ring (remains of the veil) near the insertion of the stipe into the
cap, and if there is one, notice whether it adheres to the stipe, or
moves freely up and down (Fig. 459, _a_); whether it is thick and
firm, or broad and membranous so that it hangs like a sort of curtain
round the upper part of the stipe (Fig. 467, _a_). Break the stem
and notice whether it is hollow or solid; observe also the texture,
whether brittle, cartilaginous, fibrous, or fleshy.
[Illustration: FIG. 459.—Parasol mushroom (_Lepiota procera_),
showing movable annulus: _st_, stipe; _a_, annulus, or ring; _u_,
umbo; _p_, _p_, floccose patches left by volva.]
[Illustration: FIG. 460.—Chanterelle (_Cantharellus cibarius_), with
infundibuliform pileus and decurrent gills.]
=373. The pileus, or cap.=—Observe this as to color and surface,
whether dry, or moist and sticky; smooth, or covered with scurf or
scales left by the remains of the volva, as it was stretched and
broken up by the expanding cap (Fig. 459, _p_, _p_). Note also the
size and shape, whether conical, expanded, funnel-shaped (Fig. 460),
or _umbonate_—having a protuberance at the apex (Fig. 459)—or whether
the margin is turned up at the edge (revolute, Fig. 467), or under
(involute, Fig. 459).
=374. The gills, or laminæ.=—Look at the under surface and notice
whether the gills are broad or narrow, whether they extend straight
from stem to margin, or are rounded at the ends, or curved, toothed,
or lobed in any way. Notice their attachment to the stipe, whether
_free_, not touching it at all; _adnate_, attached squarely to the
stem at their anterior ends; or _decurrent_, running down on the stem
for a greater or less distance (Fig. 460).
[Illustration: FIGS. 461-463.—Section of a gilled mushroom: 461,
through one side, showing sections of the pendent gills, _g_, _g_
(slightly magnified); 462, one of the gills more enlarged, showing
the central tissue of the trama, _tr_, and the broad border formed by
the hymenium, _h_; 463, a small section of one side of a gill very
much enlarged, showing the club-shaped basidia, _b_, _b_, standing
at right angles to the surface, bearing each two small branches with
a spore, _s_, _s_, at the end. The sterile paraphyses, _p_, are seen
mixed with the basidia.]
[Illustration: FIGS. 464, 465.—A tube fungus (_Boletus edulis_): 464,
entire; 465, section, showing position of the tubes.]
=375. The hymenium.=—Cut a tangential section through one side of
the pileus and sketch the section of the gills as they appear under
a lens, or a low power of the microscope. Notice that the blade
consists of a central portion called the _trama_ (_tr_, Fig. 462)
and a somewhat thickened portion, _h_, constituting the _hymenium_,
or spore-bearing surface. Now examine, under a high power, a small
section from the edge of a gill, including a bit of the trama.
Notice that this last consists of a tissue of mycelial cells (Fig.
463) covered by the hymenium, or spore-bearing membrane, which is
thickly clothed with a layer of elongated, club-shaped cells (_b_,
_b_ and _p_, _p_, Fig. 463) set upon it at right angles to the
surface. Some of these put out from two to four, or in some species
as many as eight, little prongs, each bearing a spore (_s_, _s_,
Fig. 463), while others remain sterile. The spore-bearing cells
are called _basidia_; the sterile ones, _paraphyses_; and the
whole spore-bearing surface together, the _hymenium_, from a Greek
word meaning a membrane. It is from the presence of this expanded
fruiting membrane that the class of mushrooms we are considering gets
its botanical name, _Hymenomycetes_, membrane fungi. The hymenium
is not always borne on gills, but is arranged in various ways which
serve as a convenient basis for distinguishing the different orders.
In the tube fungi, to which the edible boletus belongs (Figs. 464,
465), the basidia are placed along the inside of little tubes that
line the under side of the pileus, giving it the appearance of a
honeycomb. In another order, the porcupine fungi, they are arranged
on the outside of projecting spines or teeth, while in the morelles
they are held in little cups or basins.
[Illustration: FIG. 466.—Spore print of a gilled mushroom.]
[Illustration: FIG. 467.—Deadly agaric (_Amanita phalloides_),
showing the broad pendent annulus, _a_, formed by the ruptured veil;
the cup at the base, _c_, and floccose patches on the pileus, left by
the breaking up of the volva.]
=376. Spore prints.=—When the gills are ripe, they shed their spores
in great abundance. Take up the pileus that was laid on paper,
as directed under Material, on page 323, and examine the print
made by the discharged spores; it will be found to give an exact
representation of the under side of the pileus.
=377. The spores.=—Notice the color of the spores as shown in the
print. This is a matter of importance in distinguishing gill-bearing
fungi, which are divided into five sections according to the color of
the spores. One source of danger, at least, to mushroom eaters would
be avoided if this difference was always attended to, for the deadly
amanita (_Amanita phalloides_) and the almost equally dangerous fly
mushroom (_A. muscaria_) both have white spores, while the favorite
edible kind (_Agaricus campestris_), though white-gilled when young,
produces dark, purple-brown spores that cannot fail to distinguish it
clearly for any one who will take the trouble to make a print.
=378. Economic properties.=—Most of the wood-destroying fungi belong
to this and allied orders. They are among the worst enemies the
forester has to deal with (140), and millions of feet of lumber are
destroyed every year by them.
[Illustration: FIG. 468.—Portion of the root of a maple affected with
rot caused by the mycelium of a fungus that has penetrated to its
interior.]
Over seven hundred kinds of fungi growing in the United States have
been described as edible, but the evil repute into which the whole
class has been brought by the poisonous qualities of a few species,
and the difficulty, to any but an expert, of distinguishing between
these and the harmless kinds, has caused them to be generally
neglected as articles of diet. While they are pleasant relishes and
furnish an agreeable variety to our daily fare, their food value has
been greatly exaggerated. They contain a large proportion of water,
often over 90 per cent, and the most valued of them, the _Agaricus
campestris_, is about equivalent to cabbage in nutrient properties.
Practical Questions
1. Why are mushrooms generally grown in cellars? (186, 343.)
2. Name any fungi you know of that are good for food or medicine or
any other purpose.
3. Name the most dangerous ones you know of.
4. Do you find fungi most abundant on young and healthy trees, or
on old, decrepit ones? Account for the difference. (141, 343, 378.)
5. Do you ever find them growing on perfectly sound wood anywhere?
6. Are they ever beneficial to a tree? (86.)
7. Is it wise to leave old, unhealthy trees and decaying trunks in
a timber lot?
IV. LICHENS
MATERIAL.—Specimens can be found almost everywhere, growing on
rocks, walls, logs, stumps, and trees. Some of the more common
kind are: _Parmelia_, recognizable by the shallow spore cups borne
on the upper surface of the thallus; _Cladonia_, by the little
stalked receptacles, like goblets, in which its spores are held;
_Physcia_, by its bright orange color. Where practicable, it is
well to have several different kinds for comparison. Iceland moss
(_Cetraria islandica_) can generally be obtained from the grocers,
and is a good example of an intermediate form between foliaceous
and fruticose lichens.
If the specimens are very dry, they will be too brittle to handle
conveniently, and should be moistened by soaking a short time in
water. This will render them quite flexible and also bring out the
green color more clearly.
[Illustration: FIG. 469.—Foliaceous lichens: _A_, _Xanthoria
(Physcia) parietina_; _B_, _Parmelia conspersa_; _a_, spore cups.]
=379. Examination of a typical specimen.=—The commonest kind of
lichens, and generally the most easily obtained, are those that
grow on rocks and tree trunks in flat, spreading patches. Their
margins are much dented and curled, giving them a somewhat leaflike
appearance, whence they are called “foliaceous” lichens. This broad,
expanded body is the thallus, or vegetative part, as distinguished
from its reproductive part. Examine carefully the thallus of your
specimen. Note the size and shape of the indentations. Is there any
order or regularity about them, such as was observed in the lobing of
leaves? Is there any difference in color between the upper and under
sides? What other differences do you notice? Do you see anything
like hairs, or rootlets, on the under side? Mount one of them in
water and place under the microscope. What does it look like? Compare
with one of the hairs from a leaf of mullein, gromwell, blueweed, or
other hairy plant, with the hypha of a fungus mycelium, and with your
study of the root hair in 67 (_a_). Is it a hair or a root? These
rootlike hairs are called _rhizoids_, and serve to anchor the lichen
to its substratum. Look on the upper side for little cup-shaped or
saucer-shaped receptacles. On what part of the thallus are they
situated? Examine with a lens and see if you can make out what they
contain. These cups are the spore cases. The lichen fungus belongs to
the division of sac fungi, which produce their spores in closed sacs,
or cups.
[Illustration: FIG. 470.—Portion of the thallus of a lichen,
magnified, showing imprisoned algæ.]
=380. Structure of the thallus.=—Make a thin section through a
thallus and place under the microscope. Notice the small green bodies
enveloped in the hyphæ of the fungus. Are they most abundant near the
upper or the lower epidermis? Has their green color anything to do
with this, and with the difference in color between the two surfaces
of the thallus? (184.) Do they look like chlorophyll granules? Can
you tell what they are? Compare with your study of the unicellular
algæ (337) and with Fig. 429. Does this throw any light on their real
nature?
[Illustration: FIG. 471.—Artificial lichen mycelium, _m_, made by
sowing spores of a fungus, _sp_, among alga cells, _a_.]
=381. The lichen thallus a composite body.=—You will probably have no
difficulty in making out that these small round bodies are green algæ
of some kind, but of what species will depend upon the kind of lichen
with which it is associated. In Cladonia and the bearded lichen
(Fig. 473), it is a protococcus; in other forms, a pleurococcus or a
nostoc—and so on, each species of lichen fungus being specialized to
a certain form of alga. The great botanist, De Bary, showed that it
is even possible to produce a lichen thallus artificially by sowing
the spores of a fungus among the cells of the particular alga with
which it is able to unite. The spores will germinate without the
alga, but soon perish unless they come in contact with the right
one. It is thus made clear that the lichen plant as a whole is a
combination of elements belonging to two distinct orders, the algæ
and fungi, but so closely associated as to constitute practically a
single individual.
=382. Slavery, or partnership?=—Now, what can be the object of this
peculiar association? Is it a symbiosis, or a case of enslavement?
The fungi, as we know, are all parasites, unable to manufacture
their own food or to exist at all except at the expense of other
organisms, living or dead. But the lichens have refined upon the
gross rapacity of their order, and instead of indiscriminately
destroying the hosts that furnish their nourishment, have used their
victims to better purpose by converting them into contented, well-fed
slaves! The imprisoned algæ perform for them the same service that
the chlorophyll bodies do for the higher plants, and so the lichen
fungi have the advantage of other parasites in getting their food
manufactured at home, so to speak. And while the algæ have to do
double work in order to feed both themselves and their masters,
the fungus, in return, shelters them against cold and drought, and
prolongs their growing period by giving them a more continuous supply
of moisture and food materials, which it draws from the substratum by
means of its rhizoids. In this way both plants are enabled to live in
situations that neither could occupy without the other.
[Illustration: FIG. 472.—A crustaceous lichen (_Graphis elegans_)
growing on holly: _A_, natural size; _B_, slightly magnified.]
=383. Reproduction.=—The multiplication of the lichen algæ is
exclusively vegetative. The fungus, on the other hand, reproduces
normally by spores, and the fruiting bodies found on the thallus
originate from the fungus mycelium.
=384. Classification.=—To be strictly accurate, the two kinds of
vegetable bodies that make up the lichen thallus would probably have
to be classified separately, as algæ or fungi, respectively, but as
fructification is the generally accepted basis of classification,
and the plant body is too intimately permeated with both kinds of
tissue to be divided, each lichen body as a whole is classed with
its particular kind of fungus. The entire group, on account of the
distinctive characters that mark it, is placed in a separate order
of its own. This includes three principal divisions, distributed
according to the shape of the thallus, and its habit of growth:
(1) _Crustaceous_, those that adhere closely to the substratum, as
if glued or inscribed on it; (2) _Foliaceous_, with a broad, more
or less lobed and leaflike thallus that adheres loosely to the
substratum by means of rhizoids springing from its under surface; (3)
_Fruticose_, with branching, stemlike thallus attached at the base
like a regularly rooting plant (Figs. 473, 474). Among these are the
Iceland moss, used as an article of food by man, and the reindeer
moss (_Cladonia rangiferina_), which is the chief sustenance of the
reindeer.
[Illustration: FIGS. 473, 474.—Fruticose lichens: 473, _Usnea
barbata_, bearded lichen; 474, _Cladonia rangiferina_, reindeer moss:
_A_, sterile; _B_, fruiting portion.]
Practical Questions
1. Have lichens any economic value? (384.)
2. In what way are they most useful? (320.)
3. Do you find them, as a general thing, on healthy young trees and
boughs, or on old ones, and those showing signs of decay?
4. Do you ever find them growing on trees or other objects in
densely inhabited areas,—cities, large towns, and manufacturing
centers?
5. Do they grow more thickly on the shady (northern) side of
rocks, walls, and trees growing in the open, than on the sunny and
(presumably) warmer sides?
6. Mention some ways in which a growth of lichens might be
beneficial to a tree.
7. In what ways could it be harmful?
V. LIVERWORTS
MATERIAL.—Liverworts can generally be found growing with mosses
in damp, shady places, and are easily recognized by their flat,
spreading habit, which gives them the appearance of green lichens.
_Marchantia polymorpha_ (Fig. 475), one of the largest and best
specimens for study, is common in shady, damp ground throughout
the states. It is diœcious, and specimens bearing both male and
female organs should be provided. _Lunularia_, a smaller species
that can be recognized by the little crescent-shaped receptacles on
some of the divisions of the thallus, is abundant in greenhouses on
the floor, or on the sides of pots and boxes kept in damp places;
but the spore-bearing receptacles are seldom or never present, the
species being an introduced one and possibly rendered sterile by
changed conditions. _Anthoceros_ (Fig. 426) and leafy liverworts,
such as that shown in Fig. 484, also make good examples for study.
EXPERIMENT 97. WHY ARE THE UPPER AND UNDER SIDES OF A LIVERWORT
DIFFERENT?—Plant a growing branch of marchantia, or of any flat,
spreading liverwort, in moist earth so that the upper side will lie
next the soil, and watch for a week or two, noting the changes that
take place. What would you infer from these as to the cause of any
differences that may have been observed between the two surfaces?
=385. Examination of a typical liverwort=—The thallus.—The broad,
flat, branching organ that forms the body of the plant is the
thallus. Examine the end of each branch; what do you find there? Are
the two forks into which the apex of the branches divides equal or
unequal? Compare the growing end with the distal one; does it proceed
from a true root? Notice that as the lower end dies, the growing
branches go on increasing and reproducing the thallus.
[Illustration: FIGS. 475, 476.—Umbrella liverwort (_Marchantia
polymorpha_): 475, portion of a female thallus about natural size,
showing dichotomous branching; _f_, _f_, archegonial or female
receptacles; _r_, rhizoids; 476, portion of a male thallus bearing an
antheridial disk or receptacle, _d_, and gemmæ, _g_, _g_.]
[Illustration: FIG. 477.—A portion of the upper epidermis of
marchantia, magnified, showing rhomboidal plates with a stoma in
each.]
Do you find anything like a midrib? If so, trace it through the
branches and body of the thallus; where does it end? Does it seem
to be formed like the midrib of a leaf? Hold a piece of the thallus
up to the light and see if you can detect any veins. Is it of the
same color in all parts, and if there is a difference, can you give
a reason for it? Examine the upper surface with a lens. Peel off a
piece of the epidermis, place it under a low power of the microscope,
or between two moistened bits of glass, and hold up to the light,
keeping the upper surface toward you; what is its appearance?
Observe a tiny dot near the center of the rhomboidal areas into which
the epidermis is divided and compare it with your drawings of stomata
(181, 183). What would you judge that these dots are for? While
differing in structure from the stomata of leaves, they serve the
same purposes and may be regarded a more rudimentary form of the same
organ.
=386. Rhizoids.=—Wash the dirt from the under side of a thallus and
examine with a lens; how does it differ from the upper surface? Do
you see anything like roots? Place one in a drop of water under the
microscope. Compare with similar organs found on the lichen (379).
What are they? Would rhizoids be of any use on the upper side?
stomata on the under side?
=387. Gemmæ.=—Look along the upper surface for little saucer-shaped
(in lunularia, crescent-shaped) cupules (_g_, _g_, Fig. 476). Notice
their shape and position, whether on a midrib or near the margin.
Examine the contents with a lens and see if you can tell what they
are. These little bodies, called _gemmæ_, are of the nature of
buds, by which the plant propagates itself vegetatively somewhat
as the onion and the tiger lily do by means of bulblets. Sow some
of the gemmæ on moist sand, cover them with a tumbler to prevent
evaporation, and watch them develop the thalloid structure.
=388. The fruiting receptacles.=—Procure, if possible, thalli with
upright pedicels bearing flattened enlargements at the top (Figs.
475, 476). These are thallus branches modified into receptacles
containing the reproductive organs, which, in marchantia, are
diœcious, the two kinds growing on separate thalli. Notice their
difference in shape, one kind being slightly lobed or scalloped, the
other rayed like the spokes of a wheel. The first kind are known as
_antheridial_, or male, receptacles; the second as _archegonial_, or
female.
=389. The antheridia.=—Examine one of the male receptacles on
both surfaces and in vertical section. Notice the tiny egg-shaped
bodies sunk in little cavities between the lobes just under the
upper epidermis (Fig. 478). These are antheridia. When mature, they
rupture at the apex, and multitudes of extremely small bodies, called
_antherozoids_, or _spermatozoids_, are discharged from them.
[Illustration: FIG. 478.—Longitudinal section of a male receptacle of
marchantia polymorpha, magnified: _a_, antheridia; _t_, thallus; _s_,
ventral scales; _r_, rhizoids.]
[Illustration: FIG. 479.—Under side of an archegonial receptacle
enlarged. The archegonia are borne among the hairs on the under
surface, which is presented to view in the figure; _f_, a spore case.]
=390. Archegonia.=—Next examine one of the female receptacles. Look
on the under surface, between the narrow divisions of the receptacle,
for radiating rows of flask-shaped bodies with their necks turned
downward, and all surrounded by a toothed sheath or involucre
(Fig. 479). These bodies are the archegonia, or female organs, and
correspond, loosely speaking, to the ovaries of flowering plants. If
the receptacle is a mature one, the archegonia will be replaced by
the ripe spore cases (_sporangia_), as at _f_, Fig. 479.
Make enlarged drawings of the upper surface of a male and a female
receptacle, and of a vertical section of each, passing through
an antheridium in the male, and an archegonial row in the female
receptacle. Label the parts observed in each.
=391. Minute study of an archegonium.=—Place under the microscope
a very thin, longitudinal section through a ray of a receptacle
containing a young archegonium, and observe that the latter consists
of a lower portion, the _venter_, _v_, Fig. 480, and an upper part,
the neck, which is perforated by the _neck canal_, _ca_. The venter
contains the _egg cell_, _o_, and the ventral canal cell, _vc_. The
neck canal is filled with small cells which, at maturity, dissolve
into a mucilaginous substance that swells on being wet and discharges
itself through the top of the neck, leaving an open passage to the
venter, where the egg cell is ready to be fertilized.
[Illustration: FIGS. 480, 481.—480, young archegonium of M.
polymorpha; _v_, ventral portion; _o_, egg cell; _vc_, ventral canal
and cells; _ca_, neck canal with cells; 481, the same ready for
fertilization after discharge of the mucilaginous fluid.]
Make a drawing of the section as seen under the microscope, labeling
all the parts.
=392. Fertilization.=—In the liverworts, and in cryptogams generally,
this process has to take place under water, as the antherozoids are
motile only in a liquid, but the amount required is so small that a
few drops of rain or dew will enable them to make their journey to
the archegonium. The mucilaginous substances discharged from the neck
canal attract them to the mouth of the opening, one or more of them
penetrates to the egg cell, and fertilization is accomplished. Do you
see any analogies between this and the same function among flowering
plants? (250, 251.)
=393. The spore case.=—After fertilization the egg becomes an
_oöspore_, capable of producing a new plant. Instead, however, of
separating from the mother plant and giving rise to an independent
growth, it germinates within the archegonium and produces there a
small, stalked body, called a _sporogonium_, or _sporophyte_, which
at length ripens into a spore case, as shown at _f_, Fig. 479. At
maturity this capsule-like sporophyte ruptures at the apex, and
discharges a mass of spores, mingled with elongated filaments
called _elators_, which, by their elastic movements, assist in
disseminating the spores. These latter, on germinating, produce,
not a simple sporophyte like that which bore them, but the thallus
of the liverwort with all its complicated arrangement of antheridia
and archegonia and vegetative organs that seem to foreshadow, by the
analogies they suggest, the coming of the higher plants.
=394. Sexual and asexual reproduction.=—We find here a very marked
change from the simple reproductive processes observed in the algæ
and fungi. In the forms thus far considered, this function was
carried on mainly by simple vegetative fission or budding, with a
more or less irregular intervention of resting spores. If only one
kind of spore is concerned, reproduction is said to be _asexual_.
When two different kinds of cells, the egg and sperm cell, unite to
form an oöspore, as in the liverworts, reproduction is said to be
_sexual_. While sexual reproduction takes place to some extent among
both algæ and fungi, the prevailing method among thallophytes is
asexual, and may be carried on in three different ways: by fission
(and budding), by resting spores, and by conjugation.
Representing the plant body by _A_ and the resting spores by _a_,
the primitive asexual processes may be expressed to the eye by the
accompanying formulas:—
(1) Fission and budding: _A_ → _A_ → _A_ → _A_ →
(2) Resting spores: _A a_ → _A a_ → _A a_ →
(3) Conjugation: _A_ + _A_ → _a_ → _A_ + _A_ → _a_ →
In (3), as was seen in the conjugating cells of the spirogyra (342),
the method is a little more complicated, showing an approach toward
the sexual process. In each of these cases, however, there is only
one kind of cell concerned, while in the liverworts there are not
only different kinds, technically known as _gametes_, but specialized
organs, archegonia and antheridia, for producing them. The thallus
body bearing these organs is termed the _gametophyte_, because it
bears the gametes, or sexual organs,—the suffix _phyte_ meaning a
plant; for example, _epiphyte_, on or upon plants; _spermophyte_,
or _spermatophyte_, seed plant; _sporophyte_, spore plant. The
_sporophyte_, produced within the archegonium, bears simple nonsexual
spores that are capable of germinating independently. Structurally it
is a separate, individual organism, though it does not appear as such
in this class, but lives inclosed in the archegonium, as a parasite
on the mother plant.
=395. Alternation of generations.=—If we represent the sporophyte
by _S_, the thallus, or gametophyte, by _G_, the female gamete, or
egg cell, by _fg_, the antherozoids (male gametes) by _mg_, the
fertilized egg cell, or oöspore, resulting from their union by _oös_,
and the asexual spores discharged from the sporophyte by _o_, this
complicated mode of reproduction may be expressed diagrammatically as
follows:—
_fg_ _fg_
╱ ╲ ╱ ╲
_G_ _oös_→_s_→_o_→_G_ _oös_→_s_→_o_→_G_→etc.
╲ ╱ ╲ ╱
_mg_ _mg_
A glance at the diagram will show a continual interchange of the
sexual and asexual modes of reproduction, in which each generation
gives rise to its _opposite_, the asexual sporophyte producing
the sexual gametophyte, and this in turn, through its gametes,
giving rise to the asexual sporophyte. This regular recurrence
in genealogical succession of two differing forms is what is
meant by the expression “alternation of generations.” Analogous
processes occur also among some of the thallophytes, but as there
is no well-defined differentiation of sporophyte and gametophyte,
alternation proper may be regarded as beginning with the bryophytes.
The subject is a complicated one and somewhat difficult to grasp,
but it is important to form a correct idea of it and to fix clearly
in mind the different modes of reproduction as we proceed from the
lower to the higher forms of vegetation, since in this way alone can
their biological relationships and their order of succession in the
evolutionary scale be made intelligible.
VI. MOSSES
MATERIAL.—One of the most widely distributed of mosses is the
Sphagnum, or peat moss, so generally used by florists in packing
plants for shipment, and it can be obtained from them at almost all
times. It is rather difficult, however, to find specimens with the
fruiting organs, since they are rarely to be met with except in
late autumn or early spring. Other common forms are _Polytrichum_,
_Funaria_, and _Mnium_, any of which will meet all essential
conditions of the study outlined in the text.
[Illustration: FIGS. 482, 483.—Protonema of a moss: 482, germinating
spore; 483, protonema; _kn_, buds; _r_, rhizoids; _s_, spore.]
=396. The protonema or thallus stage.=—In mosses the sexual, or
gametophyte generation differs from that of liverworts in undergoing
two phases. The germinating cells of the sporophyte do not develop
immediately into the leafy stem, which is the typical gametophyte
of true mosses, but produce first a filamentous, creeping structure
called the _protonema_ (Fig. 483), that spreads over the ground
and forms the tangled green felt usually observed where mosses are
growing. Place a few of these filaments on a slide in water, and
examine under the microscope. Do they remind you of any of the
forms of algæ? Look near the base of the branches for knots or
enlargements, like those seen at _kn_, Fig. 483. These are buds
from which the leafy moss stems will develop. Do they correspond
to anything observed among the thallophytes? Notice the rootlike
filaments that extend under ground; how do they differ from the ones
above ground? Why are they colorless? How do you know that they are
not true roots? [67 (_a_), 379.] Sketch one of each kind of filament
sufficiently enlarged to show the cells composing it.
A protonema that arises directly from the spore is said to be
_primary_, while those which sometimes spring from rhizoids above
ground, or from stems or leaves, are _secondary_. The fact that a
protonema can bud from parts of the fruiting stems shows that the two
do not belong to different generations, but are merely successive
stages of a single generation, and both together compose the
gametophyte.
=397. The leafy stage.=—In their fully developed state the true
mosses show a marked advance in organization over the liverworts.
There is a distinct differentiation of the growing axis into stem
and leaves, though no true roots are formed. The leaves are arranged
spirally, on upright stems, while in the liverworts the vegetative
body is either a flat, spreading thallus, or the leaves are arranged
horizontally on opposite sides of a prostrate, or more or less
inclined, axis. Sometimes a second set occurs, on the upper side of
the axis, but in this case the leaves are usually much smaller and
inclined to the horizontal arrangement, as shown in Fig. 484.
[Illustration: FIG. 484.—Scapania, a liverwort with leafy thallus,
approaching the form of mosses and lycopodiums. (_From_ COULTER’S
“Plant Structures.”)]
[Illustration: FIG. 485.—Fruiting receptacle of a moss (_Phascum
cuspidatum_), bearing both antheridia, _an_, and archegonia, _ar_, at
the bifurcated apex; _b_, leaves; _p_, paraphyses.]
[Illustration: FIG. 486.—Fruiting stem of a moss (_Polytrichum
commune_) with ripe capsules: _s_, seta, or footstalk; _c_, capsule
with calyptra; _f_, capsule after the calyptra has fallen away; _d_,
operculum, or lid.]
=398. The reproductive organs.=—The antheridia and archegonia are
borne in groups at the end either of the main axes, or of lateral
branches (Figs. 485, 486), but as a rule only one archegonium is
fertilized, so the mature sporogonia are solitary. The plants may be
either diœcious or monœcious, as in Fig. 485; and in the latter case,
the reproductive organs may be borne on the same, or on different,
receptacles. The antheridia and the archegonia are both mixed with
club-shaped hairs called paraphyses (Fig. 485).
=399. The sporophyte.=—An examination of the fruiting capsule of any
of the true mosses will show that it consists of a long footstalk,
the _seta_, _s_, Fig. 486, bearing a capsule, or ripened sporogonium,
_f_, which is at first surmounted by a cap or hood, known as the
_calyptra_, _c_. The hood represents the excessively developed and
often highly specialized wall of the archegonium. It falls away at
maturity, and the spores are discharged through an opening made by
the removal of the _operculum_, or lid, _d_. The spores and the
capsule are both developed from the fertilized egg (oöspore), within
the archegonium, in much the same manner as in the liverworts, and
together constitute the sporophyte, or asexual generation. It never
leads a completely independent existence, but remains a partial
parasite on the mother plant, though the lower part of the young
sporogonium is usually provided with stomata and chlorophyll so
that it is capable of manufacturing food. In this respect it shows
a distinct advance on the corresponding phase of the liverworts—if
we except the single genus _Anthoceros_, which alone among the
liverworts has the cells of the sporogonium provided with chlorophyll.
=400. Alternation of generations.=—The process of reproduction
in mosses is so closely similar to that of liverworts that it is
unnecessary to repeat the details. There are some minor variations,
but in all essentials the processes are the same and may be
represented to the eye by the same formula.
=401. Relative position of mosses and liverworts in the line of
evolution.=—Though mosses, as a rule, show a higher degree of
organization than liverworts, in both generations, their development
has been _away_ from the general course of evolution followed by
the higher plants. This, as will be seen later, tends towards a
decreasing complexity of the gametophyte with increasing complexity
of the sporophyte, while the mosses show increasing complexity of
_both_. Like the order of birds in the animal kingdom, they form a
highly specialized and somewhat isolated group. While they may be
regarded as descendants from a common ancestral stock with the ferns
and club mosses, they have been switched off, so to speak, on a
side track of the great evolutionary trunk line, and their advance
on this side track has carried them to a point more remote from the
course along which the higher forms of plant life have traveled than
the distant junction at which they branched off from their less
progressive kindred, the humble liverworts.
VII. FERN PLANTS
MATERIAL.—Any kind of fern in the fruiting stage. Several different
varieties should be cultivated in the schoolroom for observation.
While gathering specimens, look along the ground under the fronds,
or in greenhouses where ferns are cultivated, among the pots and
on the floor, for a small, heart-shaped body like that represented
in Figs. 501, 502, called a _prothallium_. It is found only in
moist and shady places, and care should be taken in collecting
specimens, as in their early stages the prothallia bear a strong
resemblance to certain liverworts found in the same situations. The
best way is for each class to raise its own specimens by scattering
the spores of a fern in a glass jar, on the bottom of which is a
bed of moist sand or blotting paper. Cover the jar loosely with a
sheet of glass and keep it moist and warm, and not in too bright a
light. Spores of the sensitive ferns (_Onoclea_) will germinate in
from two to ten days, according to the temperature. Those of the
royal fern (_Osmunda_) germinate promptly if sown as soon as ripe,
but if kept even for a few weeks are apt to lose their vitality.
The spores of sensitive fern can be kept for six months or longer,
while those of the bracken (_Pteris_) and various other species
require a rest before germinating, so that in these cases it is
better to use spores of the previous season.
[Illustration: FIGS. 487-491.—A fern plant: 487, fronds and
rootstock; 488, fertile pinna: _s_, _s_, sori; 489, cross section of
a stipe, showing ends of the fibrovascular bundles; 490, a cluster of
sporangia, magnified; 491, a single sporangium still more magnified,
shedding its spores.]
=402. Study of a typical fern.=—Observe the size and general outline
of the fronds, and note whether those of the same plant are all
alike, or if they differ in any way, and how. Observe the shape
and texture of the divisions or pinnæ composing the frond, their
mode of attachment to the rachis, and whether they are simple, or
notched, or branched in any way. Hold a pinna up to the light and
notice the veining. Is it like any of the kinds described in 171,
172? In what respect is it different? This forked venation is a very
general characteristic of ferns. When the forks do not reticulate
or intercross, the veins are said to be free; are they free in your
specimen, or reticulated? Make a sketch, labeling the primary
branches of the frond, _pinnæ_ (sing., _pinna_), the secondary ones,
if any, _pinnules_, and the common stalk that supports them, _stipe_.
Note the color, texture, and surface of the stipe. If any appendages
are present, such as hairs, chaff, or scales (in Pteris, nectar
glands), notice whether they are equally distributed. If not, where
are they most abundant?
Examine the mode of attachment of the stipes to their underground
axis. Break one away and examine the scar. Compare with your drawings
of leaf scars and with Fig. 105. Do the stipes grow from a root or a
rhizome? How do you know? Do you find any remains of leafstalks of
previous years? How does the rootstock increase in length? Measure
some of the internodes; how much did it increase each year? Cut a
cross section and look for the ends of the fibrovascular bundles.
Trace their course through several internodes. Do they run straight,
or do they turn or bend in any way at the nodes? If so, where do they
go? Do you see anything like roots? Where do they originate? Put one
of them under the microscope and find out whether they are roots or
hairs.
True roots are first developed in the pteridophytes. Since those of
the fern spring from an underground stem, to what class of roots do
they belong? (83.)
=403. Minute study of a fern stem.=—Place a very thin section of
a fern rhizoma, or of the stipe of a frond, under the microscope.
Except in very young stems the vascular bundles are arranged in a
ring, or sometimes in two or more rings (Fig. 492), with plates of
strengthening tissue, _l_, _l_, between the inner and outer rings.
Notice the inner epidermal layer of hard brown tissue, and within
that, the soft parenchyma, which fills the rest of the interior. Test
it with iodine and observe how rich in starch it is. If the section
of a petiole is under observation, the details will be somewhat
different; would you expect to find as much starch in the stipe as in
the rootstock? Why, or why not?
Make a longitudinal section of a rhizome through the point where
a leafstalk is attached and trace the course of the bundles. This
will be facilitated if the specimen has stood in eosin solution a
few hours. Make enlarged drawings of both sections, labeling all the
parts.
[Illustration: FIG. 492.—Diagram of a cross section through the
stem of a fern (_Pteris_): _s_, _s_, _s_, rings of fibrovascular
bundles; _l_, _l_, plates of strengthening tissue, with a ring of
fibrovascular bundles between them; _lp_, zone of strengthening
fibers; _r_, cortex; _e_, epidermis.]
[Illustration: FIGS. 493-494.—Parts of fertile pinnæ: 493, of
_polypodium_, enlarged, showing the sori without indusium; 494, of
_pellea_, showing indusium formed by the revolute margin.]
Clearly differentiated conducting bundles occur in the mosses,
but they are of much simpler structure than in the pteridophytes,
consisting usually of a single central strand, and are found
more frequently in the leaves than in the stems. A true vascular
structure appears first in the pteridophytes, whence these plants are
distinguished as _vascular cryptogams_.
=404. Fructification.=—Examine the back of a fruiting frond; what
do you find there? These dots are the _sori_ (sing., _sorus_), or
spore clusters, and the fronds or pinnæ bearing them are said to be
_fertile_. Are there any differences of size, shape, etc., between
the fertile and the sterile fronds of your specimen? between the
fertile and the sterile pinnæ? On what part of the frond are the
fertile pinnæ borne? Notice the shape and position of the sori,
and their relation to the veins, whether borne at the tips, in the
forks, on the upper side (toward the margin), or the lower (toward
the midrib). Look for a delicate membrane (_indusium_) covering the
sori, and observe its shape and mode of attachment. If the specimen
under examination is a polypodium, there will be no indusium; if a
maidenhair, or a bracken, it will be formed of the revolute margin
of the pinna. In lady fern and Christmas fern (_Aspidium_), the sori
frequently become confluent, that is, so close together as to appear
like a solid mass. Sketch a fertile pinna as it appears under the
lens, bringing out all the points noted.
[Illustration: FIGS. 495-496.—Christmas fern (_Aspidium_): 495, part
of a fertile frond, natural size; 496, a pinna enlarged, showing the
sori confluent under the peltate indusia.]
[Illustration: FIGS. 497-500.—Spores of pteridophytes, magnified:
497, a fern spore; 498, 499, two views of a spore of a club moss;
500, spore of a common horsetail (_Equisetum arveuse_).]
=405. The spore cases.=—Look under the indusium at the cluster
of little stalked circular appendages (Fig. 490). These are the
_sporangia_, or spore cases, in which the reproductive bodies are
borne. Place one of them under the microscope, and it will be found
to consist of a little stalked circular body like a tennis racket
(Fig. 491), surrounded by a jointed ring called the _annulus_. Watch
a few moments and see if you can find out the use of the annulus. If
not, warm the slide and you will probably see the ring straighten
itself with a sudden jerk, rupturing the wall of the sporangium and
discharging the spores with considerable force. If this does not
happen, add a drop of strong glycerine to a specimen mounted in
water; the rupture will be apt to follow quickly. What causes it, in
either case? [56, (1); Exp. 19.]
=406. The sporophyte.=—The spores found in such abundance on the
fertile pinnæ; are all alike, and each one is capable of germinating
and continuing the work of reproduction as effectually as the sexual
spores of the bryophytes. The fertile frond, or part of a frond, on
which they are borne is called a _sporophyll_ (spore-bearing leaf),
and the entire plant is the _sporophyte_, which, with its crop of
spores, makes up one generation.
It is important to observe that in the ferns and in all pteridophytes
the sporophyte is the conspicuous and highly organized body that
is commonly recognized as the normal growing plant; while with the
bryophytes just the reverse holds true,—the sexual generation,
or gametophyte, represents the normal plant structure, while the
sporophyte is an insignificant appendage which never attains an
independent existence. Broadly speaking, in bryophytes, it is a spore
fruit; in the pteridophytes and spermatophytes a highly developed
plant.
[Illustration: FIGS. 501, 502.—Prothallium of a common fern
(_Aspidium_): 501, under surface, showing rhizoids, _rh_, antheridia,
_an_, and archegonia, _ar_; 502, under surface of an older
gametophyte, showing rhizoids, _rh_, young sporophyte, with root,
_w_, and leaf, _b_.]
=407. The gametophyte.=—When one of these asexual spores germinates,
it produces, not a fern plant like the one that bore it, but a small,
heart-shaped body like that shown in Fig. 501. Examine one of these
bodies carefully with a lens. Observe that there are no veins nor
fibrovascular bundles, and the whole body of the plant seems to
consist of one uniform tissue. Compare it with the forked apex of
a branching thallus of a liverwort. Do you perceive any points of
similarity? The two are, in fact, morphologically the same. This
heart-shaped body is called a _prothallium_, and is the gametophyte
of the fern. It may be of different shapes, and in some species is
branching and filamentous, like the protonema of a moss. Generally,
however, it is flat and more or less two-lobed, as shown in Fig.
501. It is small and inconspicuous and very short-lived, being of
importance only in connection with the work of reproduction.
Look with your lens for a cluster of small, bottle-shaped bodies
just below the deep cleft in the heart. If you cannot make out what
they are, put a thin section through a part of the prothallium
containing one under the microscope, and you will see that they are
the archegonia. Lower down among the rhizoids, near the pointed base,
will be found the antheridia. In some species the prothalli are
diœcious, one kind bearing antheridia, the other archegonia, but this
is rare among the true ferns.
[Illustration: FIG. 503.—Young archegonium of a fern, magnified: _K_,
neck canal cell; _K′_, ventral canal cell: _O_, egg cell.]
=408. Fertilization.=—This process is the same in all essentials as
in the bryophytes. As in other cryptogams, it can take place only
under water,—a circumstance which points to an aquatic origin for
this sub-kingdom, and through them to the entire flora of the globe.
The archegonia differ somewhat in shape from those of the liverworts
and mosses, but a section under the microscope will show that they
consist of essentially the same parts. On account of the similarity
of these organs, the pteridophytes and bryophytes are often classed
together as _Archegoniates_.
=409. Alternation of generations.=—Among the section of ferns that
we have been considering, the order of alternation corresponds in
all essentials to that prevailing among the bryophytes, and may
be represented by the same formula. The chief difference is in the
relatively much greater importance of the sporophyte, which may be
expressed by putting it first:—
_fg_ _fg_
╱ ╲ ╱ ╲
_S_→_o_→_G_ _oös_→_S_→_o_→_G_ _oös_→_S_→_o_→_G_ etc.
╲ ╱ ╲ ╱
_mg_ _mg_
But some of the pteridophytes—of which the Selaginella offers a
conspicuous example—have differentiated their asexual spores (_o_
of the formula) into two kinds, large and small, known respectively
as _megaspores_ and _microspores_. The prothallia developed by the
former bear archegonia containing female gametes only; those by the
latter, antheridia containing male gametes—while in the diœcious
bryophytes, the archegonial and antheridial thalli are produced by
spores of the same kind.
[Illustration: FIGS. 504.-508.—A kind of pteridophyte (_Selaginella
martensii_) with its organs of fructification: 504, a fruiting
branch; 505, a microsporophyll with a microsporangium, showing
microspores through a rupture in the wall; 506, a megasporophyll
with a megasporangium; 507, megaspores; 508, microspores. (_From_
COULTER’S “Plant Structures.”)]
The differentiation of the asexual spores in the higher pteridophytes
gives rise to corresponding changes in the sporangia that bear
them, and even in the sporophylls themselves, one kind bearing
microsporangia only, the other megasporangia. In this way the
differentiation of sex is pushed back, step by step, until it
virtually begins with the sporophyte, or asexual generation.
Using the same terms as before, and representing the microspores
by the abbreviation _mo_, the megaspores by _Mo_, the archegonial
gametophyte by _arG_, the antheridial by _anG_, the formula may be
modified to express this more complicated process of alternation, as
follows:—
_Mo_→_arG_→_fg_ _Mo_→_arG_→_fg_
╱ ╲ ╱ ╲
_s_ _oös_→_S_ _oös_→_S_ etc.
╲ ╱ ╲ ╱
_mo_→_anG_→_mg_ _mo_→_anG_→_mg_
Comparing this formula with the preceding, it will be seen that the
increased complexity affects the sporophyte at the expense of the
gametophyte, which has now become a mere dependent on the former.