Chapter XXIV: Studies in Cryptogams

The pupil who has acquired skill in the use of the compound microscope may desire to make more extended excursions into the cryptogamous orders. The following plants have been chosen as examples in various groups. Ferns are sufficiently discussed in the preceding chapter.

BACTERIA

If an infusion of ordinary hay is made in water and allowed to stand,
it becomes turbid or cloudy after a few days, and a drop under the
microscope will show the presence of minute oblong cells swimming in
the water perhaps by means of numerous hair-like appendages, that
project through the cell wall from the protoplasm within. At the
surface of the dish containing the infusion the cells are non-motile
and are united in long chains. Each of these cells or organisms is a
_bacterium_ (plural, _bacteria_). (Fig. 135.)

Bacteria are very minute organisms,--the smallest known,--consisting
either of separate oblong or spherical cells, or of chains, plates,
or groups of such cells, depending on the kind. They possess a
membrane-like wall which, unlike the cell walls of higher plants,
contains nitrogen. The presence of a nucleus has not been definitely
demonstrated. Multiplication is by the fission of the vegetative
cells; but under certain conditions of drought, cold, or exhaustion
of the nutrient medium, the protoplasm of the ordinary cells may
become invested with a thick wall, thus forming an _endospore_ which
is very resistant to extremes of environment. No sexual reproduction
is known.

Bacteria are very widely distributed as parasites and saprophytes in
almost all conceivable places. _Decay_ is largely caused by bacteria,
accompanied in animal tissue by the liberation of foul-smelling
gases. Certain species grow in the reservoirs and pipes of water
supplies, rendering the water brackish and often undrinkable. Some
kinds of _fermentation_ (the breaking down or decomposing of organic
compounds, usually accompanied by the formation of gas) are due to
these organisms. Other bacteria oxidize alcohol to _acetic acid_, and
produce _lactic acid_ in milk and _butyric acid_ in butter. Bacteria
live in the mouth, stomach, intestines, and on the surface of the
skin of animals. Some secrete gelatinous sheaths around themselves;
others secrete sulfur or iron, giving the substratum a vivid color.

Were it not for bacteria, man could not live on the earth, for not
only are they agents in the process of decay, but they are concerned
in certain healthful processes of plants and animals. We have learned
in Chap. VIII how bacteria are related to nitrogen-gathering.

Bacteria are of economic importance not alone because of their effect
on materials used by man, but also because of the _disease-producing
power_ of certain species. _Pus_ is caused by a spherical form,
_tetanus_ or _lock-jaw_ by a rod-shaped form, _diphtheria_ by short
oblong chains, _tuberculosis_ or “_consumption_” by more slender
oblong chains, and _typhoid fever_, _cholera_, and other diseases
by other forms. Many _diseases of animals and plants_ are caused by
bacteria. Disease-producing bacteria are said to be _pathogenic_.

The ability to grow in other nutrient substances than the natural
one has greatly facilitated the study of these minute forms of life.
By the use of suitable culture media and proper precautions, _pure
cultures_ of a particular disease-producing bacterium may be obtained
with which further experiments may be conducted.

Milk provides an excellent collecting place for bacteria coming
from the air, from the coat of the cow and from the milker. Disease
germs are sometimes carried in milk. If a drop of milk is spread on
a culture medium (as agar), and provided with proper temperature,
the bacteria will multiply, each one forming a colony visible to the
naked eye. In this way, the number of bacteria originally contained
in the milk may be counted.

Bacteria are disseminated in water, as the germ of typhoid fever and
cholera; in milk and other fluids; in the air; and on the bodies of
flies, feet of birds, and otherwise.

Bacteria are thought by many to have descended from algæ by the loss
of chlorophyll and decrease in size due to the more specialized
acquired saprophytic and parasitic habit.

ALGÆ

The algæ comprise most of the green floating “scum” which covers the
surfaces of ponds and other quiet waters. The masses of plants are
often called “frog spittle.” Others are attached to stones, pieces of
wood, and other objects submerged in streams and lakes, and many are
found on moist ground and on dripping rocks. Aside from these, all
the plants commonly known as seaweeds belong to this category; these
latter are inhabitants of salt water.

The simplest forms of algæ consist of a single spherical cell, which
multiplies by repeated division or fission. Many of the forms found
in fresh water are filamentous, _i.e._ the plant body consists of
long threads, either simple or branched. Such a plant body is termed
a _thallus_. This term applies to the vegetative body of all plants
that are _not differentiated into stem and leaves_. Such plants are
known as _thallophytes_ (p. 181). All algæ contain chlorophyll, and
are able to assimilate carbon dioxid from the air. This distinguishes
them from the fungi.

_Nostoc._--On wet rocks and damp soil dark, semitransparent irregular
or spherical gelatinous masses about the size of a pea are often
found. These consist of a colony of contorted filamentous algæ
embedded in the jelly-like mass. The chain of cells in the filament
is necklace-like. Each cell is homogeneous, without apparent
nucleus, and blue-green in color, except one cell which is larger
and clearer than the rest. The plant therefore belongs to the group
of _blue-green algæ_. The jelly probably serves to maintain a more
even moisture and to provide mechanical protection. Multiplication is
wholly by the breaking up of the threads. Occasionally certain cells
of the filament thicken to become _resting-spores_, but no other
spore formation occurs.

_Oscillatoria._--The blue-green coatings found on damp soil and
in water frequently show under the microscope the presence of
filamentous algæ composed of many short homogeneous cells (Fig. 264).
If watched closely, some filaments will be seen to wave back and
forth slowly, showing a peculiar power of movement characteristic
of this plant. Multiplication is by the breaking up of the threads.
There is no true spore formation.

SPIROGYRA.--One of the most common forms of the green algæ is
spirogyra (Fig. 265). This plant often forms the greater part of the
floating green mass (or “frog spittle”) on ponds. The threadlike
character of the thallus can be seen with the naked eye or with a
hand lens, but to study it carefully a microscope magnifying two
hundred diameters or more must be used. The thread is divided into
long cells by cross walls which, according to the species, are either
straight or curiously folded (Fig. 266). The chlorophyll is arranged
in _beautiful spiral bands_ near the wall of each cell. From the
character of these bands the plant takes its name. Each cell is
provided with a _nucleus_ and other _protoplasm_. The nucleus is
suspended near the center of the cell (_a_, Fig. 265) by delicate
strands of protoplasm radiating toward the wall and terminating
at certain points in the chlorophyll band. The remainder of the
protoplasm forms a thin layer lining the wall. The interior of the
cell is filled with cell-sap. The protoplasm and nucleus cannot be
easily seen, but if the plant is stained with a dilute alcoholic
solution of eosin they become clear.

Spirogyra is propagated vegetatively by the breaking off of parts of
the threads, which continue to grow as new plants. Resting-spores,
which may remain dormant for a time, are formed by a process known
as _conjugation_. Two threads lying side by side send out short
projections, usually from all the cells of a long series (Fig. 266).
The projections or processes from opposite cells grow toward each
other, meet, and fuse, forming a connecting tube between the cells.
The protoplasm, nucleus, and chlorophyll band of one cell now pass
through this tube, and unite with the contents of the other cell. The
entire mass then becomes surrounded by a thick cellulose wall, thus
completing the _resting-spore_, or _zygospore_ (_z_, Fig. 266).

_Zygnema_ is an alga closely related to spirogyra and found in
similar places. Its life history is practically the same, but it
differs from spirogyra in having _two star-shaped chlorophyll bodies_
(Fig. 267) in each cell, instead of a chlorophyll-bearing spiral band.

_Vaucheria_ is another alga common in shallow water and on damp
soil. The thallus is much branched, but the threads are not divided
by cross walls as in spirogyra. The plants are attached by means of
colorless root-like organs which are much like the root hairs of the
higher plants: these are _rhizoids_. The chlorophyll is in the form
of _grains scattered through the thread_.

Vaucheria has a special mode of asexual reproduction by means of
swimming spores or _swarm-spores_. These are formed singly in a
short enlarged lateral branch known as the _sporangium_. When the
sporangium bursts, the entire contents escape, forming a single large
swarm-spore, which swims about by means of numerous lashes or cilia
on its surface. The swarm spores are so large that they can be seen
with the naked eye. After swimming about for some time they come to
rest and germinate, producing a new plant.

The formation of resting-spores of vaucheria is accomplished by means
of special organs, _oögonia_ (_o_, Fig. 268) and _antheridia_ (_a_,
Fig. 268). Both of these are specially developed branches from the
thallus. The antheridia are nearly cylindrical, and curved toward
the oögonia. The upper part of an antheridium is cut off by a cross
wall, and within it numerous ciliated _sperm-cells_ are formed. These
escape by the ruptured apex of the antheridium. The oögonia are more
enlarged than the antheridia, and have a beak-like projection turned
a little to one side of the apex. They are separated from the thallus
thread by a cross wall, and contain a single large green cell, the
_egg-cell_. The apex of the oögonium is dissolved, and through the
opening the sperm-cells enter. Fertilization is thus accomplished.
After fertilization the egg-cell becomes invested with a thick wall
and is thus converted into a resting-spore, the _oöspore_.

_Fucus._--These are rather large specialized algæ belonging to the
group known as brown seaweeds and found attached by a disk to the
rocks of the seashore just below high tide (Fig. 269). They are firm
and strong to resist wave action and are so attached as to avoid
being washed ashore. They are very abundant algæ. In shape the plants
are long, branched, and multicellular, with either flat or terete
branches. They are olive-brown. Propagation is by the breaking off of
the branches. No zoöspores are produced, as in many other seaweeds;
and reproduction is wholly sexual. The _antheridia_, bearing
_sperm-cells_, and the _oögonia_, each bearing eight _egg-cells_,
are sunken in pits or _conceptacles_. These pits are aggregated in
the swollen lighter colored tips of some of the branches (_s_, _s_,
Fig. 269). The egg-cells and sperm-cells escape from the pits and
fertilization takes place in the water. The matured eggs, or spores,
reproduce the fucus plant directly.

_Nitella._--This is a large branched and specialized fresh-water alga
found in tufts attached to the bottom in shallow ponds (Fig. 270).
Between the whorls of branches are long _internodes consisting of a
single cylindrical cell_, which is one of the largest cells known in
vegetable tissue. Under the microscope the walls of this cell are
found to be lined with a layer of small stationary chloroplastids,
within which layer the protoplasm, under favorable circumstances,
will be found in motion, moving up one side and down the other (in
rotation). Note the clear streak up the side of the cell and its
relation to the moving current.

FUNGI

Some forms of fungi are familiar to every one. Mushrooms and
toadstools, with their varied forms and colors, are common in fields,
woods, and pastures. In every household the common molds are familiar
intruders, appearing on old bread, vegetables, and even within
tightly sealed fruit jars, where they form a felt-like layer dusted
over with blue, yellow, or black powder. The strange occurrence of
these plants long mystified people, who thought they were productions
of the dead matter upon which they grew, but now we know that a mold,
as any other plant, cannot originate spontaneously; it must start
from something which is analogous to a seed. The “seed” in this case
is a _spore_. A spore may be produced by a _vegetative process_
(growing out from the ordinary plant tissues), or it may be the
result of a _fertilization process_.

_Favorable conditions for the growth of fungi._--Place a piece of
bread under a moist bell jar and another in an uncovered place near
by. Sow mold on each. Note the result from day to day. Moisten a
third piece of bread with weak copper sulfate (blue vitriol) or
mercuric chlorid solution, sow mold, cover with bell jar, note
results, and explain. Expose pieces of different kinds of food in
a damp atmosphere and observe the variety of organisms appearing.
Fungi are saprophytes or parasites, and must be provided with organic
matter on which to grow. They are usually most abundant in moist
places and wet seasons.

_Mold._--One of these molds (_Mucor mucedo_), which is very common on
all decaying fruits and vegetables, is shown in Fig. 271, somewhat
magnified. When fruiting, this mold appears as a _dense mass of long
white hairs_, often over an inch high, standing erect from the fruit
or vegetable on which it is growing.

The life of this mucor begins with a minute rounded spore (_a_,
Fig. 272), which lodges on the decaying material. When the spore
germinates, it sends out a delicate thread that grows rapidly in
length and forms very many branches that soon permeate every part
of the substance on which the plant grows (_b_, Fig. 272). One of
these threads is termed a _hypha_. All the threads together form the
_mycelium_ of the fungus. The mycelium disorganizes the material in
which it grows, and thus the mucor plant (Fig. 271) is nourished. It
corresponds physiologically to the roots and stems of other plants.

When the mycelium is about two days old, it begins to form the
long fruiting stalks which we first noticed. To study them, use a
compound microscope magnifying about two hundred diameters. One of
the stalks, magnified, is shown in _a_, Fig. 274. It consists of a
rounded head, the _sporangium, sp_, supported on a long, delicate
stalk, the _sporangiophore_. The stalk is separated from the
sporangium by a wall which is formed at the base of the sporangium.
This wall, however, does not extend straight across the thread, but
it arches up into the sporangium like an inverted pear. It is known
as the _columella, c_. When the sporangium is placed in water, the
wall immediately dissolves and allows hundreds of spores, which
were formed in the cavity within the sporangium, to escape, _b_.
All that is left of the fruit is the stalk, with the pear-shaped
columella at its summit, _c_. The spores that have been set free by
the breaking of the sporangium wall are now scattered by the wind
and other agents. Those that lodge in favorable places begin to grow
immediately and reproduce the fungus. The others soon perish.

_a_, sporangium; _b_, sporangium bursting; _c_, columella.]

The mucor may continue to reproduce itself in this way indefinitely,
but these spores are very delicate and usually die if they do not
fall on favorable ground, so that the fungus is provided with another
means of carrying itself over unfavorable seasons, as winter. This
is accomplished by means of curious _thick-walled resting-spores_
or _zygospores_. The zygospores are formed on the mycelium buried
within the substance on which the plant grows. They originate in the
following way: Two threads that lie near together send out short
branches, which grow toward each other and finally meet (Fig. 273).
The walls at the ends, _a_, then disappear, allowing the contents to
flow together. At the same time, however, two other walls are formed
at points farther back, _b_, _b_, separating the short section, _c_,
from the remainder of the thread. This section now increases in size
and becomes covered with a thick, dark brown wall ornamented with
thickened tubercles. The zygospore is now mature and, after a period
of rest, it germinates, either producing a sporangium directly or
growing out as mycelium.

The zygospores of the mucors form one of the most interesting and
instructive objects among the lower plants. They are, however, very
difficult to obtain. One of the mucors (_Sporodinia grandis_) may be
frequently found in summer growing on toadstools. This plant usually
produces zygospores that are formed on the aërial mycelium. The
zygospores are large enough to be recognized with a hand lens. The
material may be dried and kept for winter study, or the zygospores
may be prepared for permanent microscopic mounts in the ordinary way.

_Yeast._--This is a very much reduced and simple fungus, consisting
normally of isolated spherical or elliptical cells (Fig. 275)
containing abundant protoplasm and probably a nucleus, although the
latter is not easily observed. It propagates rapidly by _budding_,
which consists of the gradual extrusion of a wart-like swelling that
is sooner or later cut off at the base by constriction, thus forming
a separate organism. Although simple in structure, the yeast is found
to be closely related to some of the higher groups of fungi as shown
by the method of spore formation. When grown on special substances
like potato or carrot, the contents of the cell may _form spores
inside of the sac-like mother cell_, thus resembling the sac-fungi to
which blue mold and mildews belong. The yeast plant is remarkable on
account of its power to induce alcoholic fermentation in the media in
which it grows.

There are many kinds of yeasts. One of them is found in the common
_yeast cakes_. In the process of manufacture of these cakes, the
yeast cells grow to a certain stage, and the material is then dried
and fashioned into small cakes, each cake containing great numbers of
the yeast cells. When the yeast cake is added to dough, and proper
conditions of warmth and moisture are provided, the yeast grows
rapidly and breaks up the sugar of the dough into carbon dioxid
and alcohol. This is _fermentation_. The gases escape and puff up
the dough, causing the _bread to rise_. In this loosened condition
the dough is baked; if it is not baked quickly enough, _the bread_
“_falls_.” Shake up a bit of yeast cake in slightly sweetened water:
the water soon becomes cloudy from the growing yeasts.

_Parasitic fungi._--Most of the molds are saprophytes. Many other
fungi are parasitic on living plants and animals (Fig. 285). Some
of them have complicated life histories, undergoing many changes
before the original spore is again produced. The _willow mildew_ and
the common _rust of wheat_ will serve to illustrate the habits of
parasitic fungi.

The _willow mildew (Uncinula salicis)_.--This is one of the sac
fungi. It forms white downy patches on the leaves of willows (Fig.
276). These patches consist of numerous interwoven threads that may
be recognized under the microscope as the mycelium of the fungus. The
mycelium in this case lives on the surface of the leaf and nourishes
itself by sending short branches into the cells of the leaf to absorb
food materials from them.

Numerous _summer-spores_ are formed of short, erect branches all over
the white surface. One of these branches is shown in Fig. 277. When
it has grown to a certain length, the upper part begins to segment or
divide into spores which fall and are scattered by the wind. Those
falling on other willows reproduce the fungus there. This process
continues all summer, but in the later part of the season provision
is made to maintain the mildew through the winter. If some of the
white patches are closely examined in July or August, a number of
little black bodies will be seen among the threads. These little
bodies are called _perithecia_, shown in Fig. 278. To the naked eye
they appear as minute specks, but when seen under a magnification of
200 diameters they present a very interesting appearance. They are
hollow spherical bodies decorated around the outside with a fringe
of crook-like hairs. The _resting-spores_ of the willow mildew are
produced in sacs or _asci_ inclosed within the leathery perithecia.
Figure 279 shows a cross-section of a perithecium with the asci
arising from the bottom. The spores remain securely packed in the
perithecia. They do not ripen in the autumn, but fall to the ground
with the leaf, and there remain securely protected among the dead
foliage. The following spring they mature and are liberated by the
decay of the perithecia. They are then ready to attack the unfolding
leaves of the willow and repeat the work of the summer before.

_The wheat rust._--The development of some of the rusts, as the
common _wheat rust (Puccinia graminis)_, is even more interesting
and complicated than that of the mildews. Wheat rust is also a true
parasite, affecting wheat and a few other grasses. The mycelium
here cannot be seen by the unaided eye, for it consists of threads
which are present within the host plant, mostly in the intercellular
spaces. These threads also send short branches, or _haustoria_ (Fig.
132), into the neighboring cells to absorb nutriment.

The _resting-spores_ of wheat rust are produced in late summer, when
they may be found in black lines breaking through the epidermis of
the wheat stalk (black-rust stage). They are formed in masses, called
_sori_ (Fig. 280), from the ends of numerous crowded mycelial strands
just beneath the epidermis of the host. The individual spores are
very small and can be well studied only with a microscope of high
power (× about 400). They are brown two-celled bodies with a thick
wall (Fig. 281). Since they are the resting or winter-spores, they
are termed _teleutospores_ (“completed spores”). Usually they do not
fall, but remain in the sori during winter. The following spring each
cell of the teleutospore puts forth a rather stout thread, which
does not grow more than several times the length of the spore and
terminates in a blunt extremity. This germ tube, _promycelium_, now
becomes divided into four cells by cross walls, which are formed from
the top downwards. Each cell gives rise to a short, pointed branch
which, in the course of a few hours, forms at its summit a single
spore called a _sporidium_. This in turn germinates and produces a
mycelium. In Fig. 282 a germinating teleutospore is drawn to show the
promycelium, _p_, divided into four cells, each producing a short
branch with a little _sporidium, s_.

A most remarkable circumstance in the life history of the wheat rust
is the fact that the mycelium produced by the sporidium _can live
only in barberry leaves_, and it follows that if no barberry bushes
are in the neighborhood the sporidia finally perish. Those which
happen to lodge on a barberry bush germinate immediately, producing a
mycelium that enters the barberry leaf and grows within its tissues.
Very soon the fungus produces a new kind of spores on the barberry
leaves. These are called _æcidiospores_. They are formed in long
chains in little fringed cups, or _æcidia_, which appear in groups on
the lower side of the leaf (Fig. 283). These orange or yellow æcidia
are termed _cluster-cups_. In Fig. 284 is shown a cross-section
of one of the cups, outlining the long chains of spores, and the
mycelium in the tissues.

The æcidiospores are formed in the spring, and after they have been
set free, some of them lodge on wheat or other grasses, where they
germinate immediately. The germ-tube enters the leaf through a
stomate, whence it spreads among the cells of the wheat plant. In
summer one-celled reddish uredospores (“blight spores,” red-rust
stage) are produced in a manner similar to the teleutospores. These
are capable of germinating immediately, and serve to disseminate the
fungus during the summer on other wheat plants or grasses. Late in
the season, teleutospores are again produced, completing the life
cycle of the plant.

Many rusts besides _Puccinia graminis_ produce different spore forms
on different plants. The phenomenon is called _heterœcism_, and
was first shown to exist in the wheat rust. Curiously enough, the
peasants of Europe had observed and asserted that barberry bushes
cause wheat to blight long before science explained the relation
between the cluster-cups on barberry and the rust on wheat. The true
relation was actually demonstrated, as has since been done for many
other rusts on their respective hosts, by sowing the æcidiospores on
healthy wheat plants and thus producing the rust. The _cedar apple_
is another rust, producing the curious swellings often found on the
branches of red cedar trees. In the spring the teleutospores ooze out
from the “apple” in brownish yellow masses. It has been found that
these attack various fruit trees, producing æcidia on their leaves.
Fig. 285 explains how a parasitic fungus works.

_Puffballs_, _mushrooms_, _toadstools_, _and shelf fungi_.--These
represent what are called the _higher fungi_, because of the size
and complexity of the plant body as well as from the fact that they
seem to stand at the end of one line of evolution. The mycelial
threads grow together in extensive strands in rotten wood or in the
soil, and send out large complex growths of mycelium in connection
with which the spores are borne. These aërial parts are the only
ones we ordinarily see, and which constitute the “mushroom” part
(Fig. 131). Only asexual spores (_basidiospores_) are produced, and
on short stalks (_basidia_) (Fig. 286). In the puffballs the spores
are inclosed and constitute a large part of the “smoke.” In the
mushrooms and toadstools they are borne on _gills_, and in the shelf
fungi (Fig. 134) on the walls of minute pores of the underside. The
mycelium of these shelf fungi frequently lives and grows for a long
time concealed in the substratum before the visible fruit bodies are
sent out. Practically all timber decay is caused by such growth, and
the damage is largely done before the fruiting bodies appear. For
other accounts of mushrooms, see Chap. XIV.

_tr_, trama tissue; _sh_, hymenium; _b_, basidium; _st_, sterigma; _sp_, spore. (Atkinson.)]

LICHENS

Lichens are so common everywhere that the attention of the student
is sure to be drawn to them. They grow on rocks, trunks of trees
(Fig. 287), old fences, and on the earth. They are thin, usually gray
ragged objects, apparently lifeless. Their study is too difficult for
beginners, but a few words of explanation may be useful.

Lichens were formerly supposed to be a distinct or separate division
of plants. They are now known to be organisms, each species of which
is a constant association of a fungus and an alga. The thallus is
ordinarily made up of fungous mycelium or tissue within which the
imprisoned alga is definitely distributed. The result is a growth
unlike either component. This association of alga and fungus is
usually spoken of as _symbiosis_, or mutually helpful growth, the
alga furnishing some things, the fungus others, and both together
being able to accomplish work that neither could do independently. By
others this union is considered to be a mild form of parasitism, in
which the fungus profits at the expense of the alga. As favorable to
this view, the facts are cited that each component is able to grow
independently, and that under such conditions the algal cells seem to
thrive better than when imprisoned by the fungus.

Lichens propagate by means of _soredia_, which are tiny parts
separated from the body of the thallus, and consisting of one or more
algal cells overgrown with fungus threads. These are readily observed
in many lichens. They also produce spores, usually ascospores, which
are always the product of the fungus element, and which reproduce the
lichen by germinating in the presence of algal cells, to which the
hyphæ immediately cling.

Lichens are found in the most inhospitable places, and, by means of
acids which they secrete, they attack and slowly disintegrate even
the hardest rocks. By making thin sections of the thallus with a
sharp razor and examining under the compound microscope, it is easy
to distinguish the two components in many lichens.

LIVERWORTS

The liverworts are peculiar flat green plants usually found on wet
cliffs and in other moist, shady places. They frequently occur
in greenhouses where the soil is kept constantly wet. One of the
commonest liverworts is _Marchantia polymorpha_, two plants of which
are shown in Figs. 288, 289. The plant consists of a ribbon-like
thallus that creeps along the ground, becoming repeatedly forked as
it grows. The end of each branch is always conspicuously notched.
There is a prominent midrib extending along the center of each branch
of the thallus. On the under side of the thallus, especially along
the midrib, there are numerous rhizoids which serve the purpose of
roots, absorbing nourishment from the earth and holding the plant in
its place. The upper surface of the thallus is divided into minute
rhombic areas that can be seen with the naked eye. Each of these
areas is perforated by a small breathing pore or stomate that leads
into a cavity just beneath the epidermis. This space is surrounded
by chlorophyll-bearing cells, some of which stand in rows from the
bottom of the cavity (Fig. 290). The delicate assimilating tissue is
thus brought in close communication with the outer air through the
pore in the thick, protecting epidermis.

FIG. 289.

PLANTS OF MARCHANTIA.]

At various points on the midrib are little cups containing small
green bodies. These bodies are buds or _gemmæ_ which are outgrowths
from the cells at the bottom of the cup. They become loosened and are
then dispersed by the rain to other places, where they take root and
grow into new plants.

The most striking organs on the thallus of marchantia are the
peculiar stalked bodies shown in Figs. 288, 289. These are termed
archegoniophores and _antheridiophores_ or _receptacles_. Their
structure and function are very interesting, but their parts are so
minute that they can be studied only with the aid of a microscope
magnifying from 100 to 400 times. Enlarged drawings will guide the
pupil.

The _antheridiophores_ are fleshy, lobed disks borne on short stalks
(Fig. 291). The upper surface of the disk shows openings scarcely
visible to the naked eye. However, a section of the disk, such as is
drawn in Fig. 291, shows that the pores lead into oblong cavities
in the receptacle. From the base of each cavity there arises a
thick, club-shaped body, the _antheridium_. Within the antheridium
are formed many sperm-cells which are capable of swimming about in
water by means of long lashes or cilia attached to them. When the
antheridium is mature, it bursts and allows the ciliated sperm cells
to escape.

The _archegoniophores_ are also elevated on stalks (Fig. 289).
Instead of a simple disk, the receptacle consists of nine or
more finger-like rays. Along the under side of the rays, between
delicately fringed curtains, peculiar flask-like bodies, or
_archegonia_, are situated. The archegonia are not visible to the
naked eye. They can be studied only with the microscope (× about
400). One of them much magnified is represented in Fig. 292. Its
principal parts are the long _neck, a_, and the rounded _venter, b_,
inclosing a large free cell--the egg-cell.

We have seen that the antheridium at maturity discharges its
sperm-cells. These swim about in the water provided by the dew and
rain. Some of them finally find their way to the archegonia and
egg-cells, the latter being fertilized, as pollen fertilizes the
ovules of higher plants.

After fertilization the egg-cell develops into the spore capsule
or _sporogonium_. The mature spore capsules may be seen in Fig.
293. They consist of an oval spore-case on a short stalk, the base
of which is imbedded in the tissue of the receptacle, from which
it derives the necessary nourishment for the development of the
sporogonium. At maturity the sporogonium is ruptured at the apex,
setting free the spherical spores together with numerous filaments
having spirally thickened walls (Fig. 294). These filaments are
called _elaters_. When drying, they exhibit rapid movements by means
of which the spores are scattered. The spores germinate and again
produce the thallus of marchantia.

MOSSES (Bryophyta)

If we have followed carefully the development of marchantia, the
study of one of the mosses will be comparatively easy. The mosses
are more familiar plants than the liverworts. They grow on trees,
stones, and on the soil both in wet and dry places. One of the
common larger mosses, known as _Polytrichum commune_, may serve as
an example, Fig. 295. This plant grows on rather dry knolls, mostly
in the borders of open woods, where it forms large beds. In dry
weather these beds have a reddish brown appearance, but when moist
they form beautiful green cushions. This color is due, in the first
instance, to the color of the old stems and leaves, and, in the
second instance, to the peculiar action of the green living leaves
under the influence of changing moisture-conditions. The inner or
upper surface of the leaf is covered with thin, longitudinal ridges
of delicate cells which contain chlorophyll. These cells are shown in
cross-section in Fig. 296, as dots or granules. All the other tissue
of the leaf consists of thick-walled, corky cells which do not allow
moisture to penetrate. When the air is moist the green leaves spread
out, exposing the chlorophyll cells to the air, but in dry weather
the margins of the leaves roll inward, and the leaves fold closely
against the stem, thus protecting the delicate assimilating tissue.

The _antheridia_ and _archegonia_ of polytrichum are borne in groups
at the ends of the branches on different plants (many mosses bear
both organs on the same branch). They are surrounded by involucres
of characteristic leaves termed _perichætia_ or _perichætal leaves_.
Multicellular hairs known as _paraphyses_ are scattered among the
archegonia and antheridia. The involucres with the organs borne
within them are called _receptacles_, or, less appropriately, “moss
flowers.” As in marchantia, the organs are very minute and must be
highly magnified to be studied.

The antheridia are borne in broad cup-like receptacles on the
antheridial plants (Fig. 297). They are much like the antheridia of
marchantia, but they stand free among the paraphyses and are not
sunk in cavities. At maturity they burst and allow the sperm cells
or _spermatozoids_ to escape. In polytrichum, when the receptacles
have fulfilled their function, the stem continues to grow from the
center of the cup (_m_, Fig. 295). The archegonia are borne in other
receptacles on different plants. They are like the archegonia of
marchantia except that they stand erect on the end of the branch.

The _sporogonium_ which develops from the fertilized egg is shown
in _a_, _b_, Fig. 295. It consists of a long, brown stalk bearing
the spore-case at its summit. The base of the stalk is imbedded in
the end of the moss stem by which it is nourished. The capsule is
entirely inclosed by a hairy cap, the _calyptra, b_. The calyptra is
really the remnant of the archegonium, which, for a time, increases
in size to accommodate and protect the young growing capsule. It is
finally torn loose and carried up on the spore-case. The mouth of
the capsule is closed by a circular lid, the _operculum_, having a
conical projection at the center.

The operculum soon drops, or it may be removed, displaying a fringe
of sixty-four teeth guarding the mouth of the capsule. This ring of
teeth is known as the _peristome_. In most mosses the teeth exhibit
peculiar hygroscopic movements; _i.e._ when moist they bend outwards,
and upon drying curve in toward the mouth of the capsule. This
motion, it will be seen, serves to disperse the spores gradually over
a long period of time.

Not the entire capsule is filled with spores. There are no elaters,
but the center of the capsule is occupied by a columnar strand of
tissue, the _columella_, which expands at the mouth into a thin,
membranous disk, closing the entire mouth of the capsule except the
narrow annular chink guarded by the teeth. In this moss the points of
the teeth are attached to the margin of the membrane, allowing the
spores to sift out through the spaces between them.

When the spores germinate they form a green, branched thread, the
_protonema_. This gives rise directly to moss plants, which appear
as little buds on the thread. When the moss plants have sent their
little rhizoids into the earth, the protonema dies, for it is no
longer necessary for the support of the little plants, and the moss
plants grow independently.

_Funaria_ is a moss very common on damp, open soil. It forms green
patches of small fine leaves from which arise long brown stalks
terminated by curved capsules (Fig. 298). The structure is similar to
that of polytrichum, except the absence of plates on the under side
of the leaves, the continuous growth of the stem, the curved capsule,
double peristome, monœcious rather than diœcious receptacles, and
nearly glabrous unsymmetrical calyptra.

EQUISETUMS, OR HORSETAILS (Pteridophyta)

There are about twenty-five species of equisetum, constituting the
only genus of the unique family _Equisetaceæ_. Among these _E.
arvense_ (Fig. 299) is common on clayey and sandy soils.

In this species the work of nutrition and that of spore production
are performed by separate shoots from an underground rhizome. The
fertile branches appear early in spring. The stem, which is 3
to 6 inches high, consists of a number of cylindrical, furrowed
internodes, each sheathed at the base by a circle of scale leaves.
The shoots are of a pale yellow color. They contain no chlorophyll,
and are nourished by the food stored in the rhizome (Fig. 299).

The spores are formed on specially developed fertile leaves or
_sporophylls_ which are collected into a spike or cone at the end of
the stalk (_a_, Fig. 299). A single sporophyll is shown at _b_. It
consists of a short stalk expanded into a broad, mushroom-like head.
Several large _sporangia_ are borne on its under side. The spores
formed in the sporangia are very interesting and beautiful objects
when examined under the microscope (× about 200). They are spherical,
green bodies, each surrounded by two spiral bands attached to the
spore at their intersection, s. These bands exhibit hygroscopic
movements by means of which the spores become entangled, and are held
together. This is of advantage to the plant, as we shall see. All
the spores are alike, but some of the _prothallia_ grow to a greater
size than the others. The large prothallia produce only archegonia
while the smaller ones produce _antheridia_. Both of these organs are
much like those of the ferns, and fertilization is accomplished in
the same way. Since the prothallia are usually diœcious, the special
advantage of the spiral bands, holding the spores together so that
both kinds of prothallia may be in close proximity, will be easily
understood. As in the fern, the fertilized egg-cell develops into an
equisetum plant.

_st_, sterile shoot; _f_, fertile shoot showing the spike at _a_; _b_, sporophyll, with sporangia; _s_, spore.]

The sterile shoots (_st_, Fig. 299) appear much later in the season.
They give rise to repeated whorls of angular or furrowed branches.
The leaves are very much reduced scales, situated at the internodes.
The stems are provided with chlorophyll and act as assimilating
tissue, nourishing the rhizome and the fertile shoots. Nutriment is
also stored in special tubers developed on the rhizome.

Other species of equisetum have only one kind of shoot--a tall, hard,
leafless, green shoot with the spike at its summit. Equisetum stems
are full of silex, and they are sometimes used for scouring floors
and utensils; hence the common name “scouring rush.”

ISOËTES (Pteridophyta)

_Isoëtes_ or quillwort is usually found in water or damp soil on the
edges of ponds and lakes. The general habit of the plant is seen
in Fig. 300, _a_. It consists of a short, perennial stem bearing
numerous erect, quill-like leaves with broad sheathing bases. The
plants are commonly mistaken for young grasses.

Isoëtes bears two kinds of spores, large roughened ones, the
_macrospores_, and small ones or _microspores_. Both kinds are formed
in _sporangia_ borne in an excavation in the expanded base of the
leaf. The macrospores are formed on the outer and the microspores on
the inner leaves. A sporangium in the base of a leaf is shown at _b_.
It is partially covered by a thin membrane, the _velum_. The minute
triangular appendage at the upper end of the sporangium is called the
_ligule_.

The spores are liberated by the decay of the sporangia. They form
rudimentary prothallia of two kinds. The microspores produce
prothallia with _antheridia_, while the macrospores produce
prothallia with _archegonia_. Fertilization takes place as in the
mosses or liverworts, and the fertilized egg-cell, by continued
growth, gives rise again to the isoëtes plant.

CLUB-MOSSES (Pteridophyta)

The club-mosses are low trailing plants of moss-like looks and
habit, although more closely allied to ferns than to true mosses.
Except one genus in Florida, all our club mosses belong to the
genus _Lycopodium_. They grow mostly in woods, having 1-nerved
evergreen leaves arranged in four or more ranks. Some of them make
long strands, as the ground pine, and are much used for Christmas
decorations. The spores are all _of one kind_ or form, borne in
_1-celled sporangia_ that open on the margin into two valves. The
sporangia are borne in some species (Fig. 301) as small yellow bodies
in the axils of the ordinary leaves near the tip of the shoot; in
other species (Fig. 302) they are borne in the axils of small scales
that form a catkin-like spike. The spores are very numerous, and
they contain an oil that makes them inflammable. About 100 species
of lycopodium are known. The plants grown by florists under the name
of lycopodium are of the genus _Selaginella_, more closely allied to
isoëtes, bearing two kinds of spores (microspores and macrospores).

ANIMAL BIOLOGY