CLUB MOSSES.
[Illustration: Fig. 326. Lycopodium clavatum, branch bearing two
fruiting spikes; at right sporophyll with open sporangium; single spore
near it.]
=578.= What are called the “club mosses” make up another group
of interesting plants which rank as allies of the ferns. They are not
of course true mosses, but the general habit of some of the smaller
species, and especially the form and size of the leaves, suggest a
resemblance to the larger of the moss plants.
=579. The clavate lycopodium.=—Here is one of the club mosses
(fig. 326) which has a wide distribution and which is well entitled to
hold the name of club because of the form of the upright club-shaped
branches. As will be seen from the illustration, it has a prostrate
stem. This stem runs for considerable distances on the surface of the
ground, often partly buried in the leaves, and sometimes even buried
beneath the soil. The leaves are quite small, are flattened-awl-shaped,
and stand thickly over the stem, arranged in a spiral manner, which
is the usual arrangement of the leaves of the club mosses. Here and
there are upright branches which are forked several times. The end of
one or more of these branches becomes produced into a slender upright
stem which is nearly leafless, the leaves being reduced to mere scales.
The end of this leafless branch then terminates in one or several
cylindrical heads which form the club.
=580. Fruiting spike of Lycopodium clavatum.=—This club is the
fruiting spike or head (sometimes termed a _strobilus_). Here the
leaves are larger again and broader, but still not so large as the
leaves on the creeping shoots, and they are paler. If we bend down some
of the leaves, or tear off a few, we see that in the axil of the leaf,
where it joins the stem, there is a somewhat rounded, kidney-shaped
body. This is the spore-case or sporangium, as we can see by an
examination of its contents. There is but a single spore-case for each
of the fertile leaves (sporophyll). When it is mature, it opens by a
crosswise slit as seen in fig. 326. When we consider the number of
spore-cases in one of these club-shaped fruit bodies we see that the
number of spores developed in a large plant is immense. In mass the
spores make a very fine, soft powder, which is used for some kinds of
pyrotechnic material, and for various toilet purposes.
[Illustration: Fig. 327. Lycopodium lucidulum, bulbils in axils of
leaves near the top, sporangia in axils of leaves below them. At right
is a bulbil enlarged.]
=581. Lycopodium lucidulum.=—Another common species is figured
at 327. This is Lycopodium lucidulum. The habit of the plant is quite
different. It grows in damp ravines, woods, and moors. The older
parts of the stem are prostrate, while the branches are more or less
ascending. It branches in a forked manner. The leaves are larger than
in the former species, and they are all of the same size, there being
no appreciable difference between the sterile and fertile ones. The
characteristic club is not present here, but the spore-cases occupy
certain regions of the stem, as shown at 327. In a single season one
region of the stem may bear spore-cases, and then a sterile portion
of the same stem is developed, which later bears another series of
spore-cases higher up.
=582. Bulbils on Lycopodium lucidulum.=—There is one curious way
in which this club moss multiplies. One may see frequently among the
upper leaves small wedge-shaped or heart-shaped green bodies but little
larger than the ordinary leaves. These are little buds which contain
rudimentary shoot and root and several thick green leaves. When they
fall to the ground they grow into new lycopodium plants, just as the
bulbils of cystopteris do which were described in the chapter on ferns.
=583.= Note.—The prothallia of the species of lycopodium which
have been studied are singular objects. In L. cernuum a cylindrical
body sunk in the earth is formed, and from the upper surface there
are green lobes. In L. phlegmaria and some others slender branched,
colorless bodies are formed which according to Treub grow as a
saphrophyte in decayed bark of trees. Many of the prothallia examined
have a fungus growing in their tissue which is supposed to play some
part in the nutrition of the prothallium.
The little club mosses (selaginella).
=584.= Closely related to the club mosses are the selaginellas.
These plants resemble closely the general habit of the club mosses, but
are generally smaller and the leaves more delicate. Some species are
grown in conservatories for ornament, the leaves of such usually having
a beautiful metallic lustre. The leaves of some are arranged as in
lycopodium, but many species have the leaves in four to six rows. Fig.
328 represents a part of a selaginella plant (S. apus). The fruiting
spike possesses similar leaves, but they are shorter, and their
arrangement gives to the spike a four-sided appearance.
[Illustration: Fig. 328. Selaginella with three fruiting spikes.
(Selaginella apus.)]
[Illustration: Fig. 329. Fruiting spike showing large and small
sporangia.]
[Illustration: Fig. 330. Large sporangium.]
[Illustration: Fig. 331. Small sporangium.]
=585. Sporangia.=—On examining the fruiting spike, we find as
in lycopodium that there is but a single sporangium in the axil of a
fertile leaf. But we see that they are of two different kinds, small
ones in the axils of the upper leaves, and large ones in the axils of
a few of the lower leaves of the spike. The _microspores_ are borne
in the smaller spore-cases and the _macrospores_ in the larger ones.
Figures 329-331 give the details. There are many microspores in a
single small spore-case, but 3-4 macrospores in a large spore-case.
=586. Male prothallia.=—The prothallia of selaginella are much
reduced structures. The microspores when mature are already divided
into two cells. When they grow into the mature prothallium a few more
cells are formed, and some of the inner ones form the spermatozoids,
as seen in fig. 332. Here we see that the antheridium itself is larger
than the prothallia. Only antheridia are developed on the prothallia
formed from the microspores, and for this reason the prothallia are
called _male prothallia_. In fact a male prothallium of selaginella is
nearly all antheridium, so reduced has the gametophyte become here.
[Illustration: Fig. 332. Details of microspore and male prothallium
of selaginella; 1st, microspore; 2d, wall removed to show small
prothallial cell below; 3d, mature male prothallium still within the
wall; 4th, small cell below is the prothallial cell, the remainder is
antheridium with wall and four sperm cells within; 5th spermatozoid.
After Beliaieff and Pfeffer.]
=587. Female prothallia.=—The female prothallia are developed
from the macrospores. The macrospores when mature have a rough, thick,
hard wall. The female prothallium begins to develop inside of the
macrospore before it leaves the sporangium. The protoplasm is richer
near the wall of the spore and at the upper end. Here the nucleus
divides a great many times, and finally cell walls are formed, so
that a tissue of considerable extent is formed inside the wall of the
spore, which is very different from what takes place in the ferns we
have studied. As the prothallium matures the spore is cracked at the
point where the three angles meet, as shown in fig. 334. The archegonia
are developed in this exposed surface, and several can be seen in the
illustration.
[Illustration: Fig. 333. Section of mature macrospore of selaginella,
showing female prothallium and archegonia. After Pfeffer.]
[Illustration: Fig. 334. Mature female prothallium of selaginella,
just bursting open the wall of macrospore, exposing archegonia. After
Pfeffer.]
[Illustration: Fig. 335. Seedling of selaginella still attached to the
macrospore. After Campbell.]
=588. Embryo.=—After fertilization the egg divides in such a way
that a long cell called a suspensor is cut off from the upper side,
which elongates and pushes the developing embryo down into the center
of the spore, or what is now the female prothallium. Here it derives
nourishment from the tissues of the prothallium, and eventually the
root and stem emerge, while a process called the “foot” is still
attached to the prothallium. When the root takes hold on the soil the
embryo becomes free.