LIVERWORTS (HEPATICÆ).
=473.= We come now to the study of representatives of another
group of plants, a few of which we examined in studying the organs of
assimilation and nutrition. I refer to what are called the liverworts.
Two of these liverworts belonging to the genus riccia are illustrated
in figs. 30, 252.
Riccia.
=474. Form of the floating riccia (R. fluitans).=—The general
form of floating riccia is that of a narrow, irregular, flattened,
ribbon-like object, which forks repeatedly, in a dichotomous manner,
so that there are several lobes to a single plant. It receives its
name from the fact that at certain seasons of the year it may be found
floating on the water of pools or lakes. When the water lowers it comes
to rest on the damp soil, and rhizoids are developed from the under
side. Now the sexual organs, and later the fruit capsule, are developed.
=475. Form of the circular riccia (R. crystallina).=—The circular
riccia is shown in fig. 252. The form of this one is quite different
from the floating one, but the manner of growth is much the same.
The branching is more compact and even, so that a circular plant is
the result. This riccia inhabits muddy banks, lying flat on the wet
surface, and deriving its soluble food by means of the little rootlets
(rhizoids) which grow out from the under surface.
[Illustration: Fig. 252. Thallus of Riccia crystallina.]
Here and there on the margin are narrow slits, which extend nearly to
the central point. They are not real slits, however, for they were
formed there as the plant grew. Each one of these V-shaped portions of
the thallus is a lobe, and they were formed in the young condition of
the plant by a branching in a forked manner. Since growth took place
in all directions radially the plant became circular in form. These
large lobes we can see are forked once or twice again, as shown by the
seeming shorter slits in the margin.
=476. Sexual organs.=—In order to study the sexual organs we
must make thin sections through one of these lobes lengthwise and
perpendicular to the thallus surface. These sections are mounted for
examination with the microscope.
=477. Archegonia.=—We are apt to find the organs in various
stages of development, but we will select one of the flask-shaped
structures shown in fig. 253 for study. This flask-shaped body we see
is entirely sunk in the tissue of the thallus. This structure is the
female organ, and is what we term in these plants the _archegonium_. It
is more complicated in structure than the oogonium. The lower portion
is enlarged and bellied out, and is the venter of the archegonium,
while the narrow portion is the neck. We here see it in section. The
wall is one cell layer in thickness. In the neck is a canal, and in
the base of the venter we see a large rounded cell with a distinct and
large nucleus. This cell is the _egg_ cell.
=478. Antheridia.=—The antheridia are also borne in cavities sunk
in the tissue of the thallus. There is here no illustration of the
antheridium of this riccia, but fig. 259 represents an antheridium of
another liverwort, and there is not a great difference between the two
kinds. Each one of those little rectangular sperm mother cells in the
antheridium changes into a swiftly moving body like a little club with
two long lashes attached to the smaller end. By the violent lashing of
these organs the spermatozoid is moved through the water, or moisture
which is on the surface of the thallus. It moves through the canal of
the archegonium neck and into the egg, where it fuses with the nucleus
of the egg, and thus fertilization is effected.
=479. Embryo.=—In the plants which we have selected thus far for
study, the egg, immediately after fecundation, we recollect, passed
into a resting state, and was enclosed by a thick protecting wall.
But in riccia, and in the other plants of the group which we are now
studying, this is not the case. The egg, on the other hand, after
acquiring a thin wall, swells up and fills the cavity of the venter.
Then it divides by a cross wall into two cells. These two grow, and
divide again, and so on until there is formed a quite large mass
of cells rounded in form and still contained in the venter of the
archegonium, which itself increases in size by the growth of the cells
of the wall.
[Illustration: Fig. 253.
Archegonium of riccia, showing neck, venter, and the egg; archegonium
is partly surrounded by the tissue of the thallus. (Riccia
crystallina.)]
[Illustration: Fig. 254.
Young embryo (sporogonium) of riccia, within the venter of the
archegonium; the latter has now two layers of cells. (Riccia
crystallina.)]
=480. Sporogonium of riccia.=—The fruit of riccia, which is
developed from the fertilized egg in the archegonium, forms a rounded
capsule still enclosed in the venter of the archegonium, which grows
also to provide space for it. Therefore a section through the plant at
this time, as described for the study of the archegonium, should show
this capsule. The capsule then is a rounded mass of cells developed
from the egg. A single outer layer of cells forms the wall, and
therefore is sterile. All the inner cells, which are richer in
protoplasm, divide into four cells each. Each of these cells becomes
a spore with a thick wall, and is shaped like a triangular pyramid
whose sides are of the same extent as the base (tetrahedral). These
cells formed in fours are the _spores_. At this time the wall of the
spore-case dissolves, the spores separate from each other and fill the
now enlarged venter of the archegonium. When the thallus dies they are
liberated, or escape between the loosely arranged cells of the upper
surface.
[Illustration: Fig. 255. Nearly mature sporogonium of Riccia
crystallina; mature spore at the right.]
[Illustration: Fig. 256.
Riccia glauca; archegonium containing nearly mature sporogonium. _sg_,
spore-producing cells surrounded by single layer of sterile cells, the
wall of the sporogonium.]
=481. A new phase in plant life.=—Thus we have here in the
sporogonium of _riccia_ a very interesting phase of plant life, in
which the egg, after fertilization, instead of developing directly into
the same phase of the plant on which it was formed, grows into a quite
new phase, the sole function of which is the development of spores.
Since the form of the plant on which the sexual organs are developed
is called the _gametophyte_, this new phase in which the spores are
developed is termed the _sporophyte_.
Now the spores, when they germinate, develop the _gametophyte_, or
thallus, again. So we have this very interesting condition of things,
the thallus (gametophyte) bears the sexual organs and the unfertilized
egg. The fertilized egg, starting as it does from a single-celled
stage, develops the sporogonium (sporophyte). Here the single-cell
stage is again reached in the spore, which now develops the thallus.
=482. Riccia compared with coleochæte, œdogonium, etc.=—We have
said that in the sporogonium of riccia we have formed a new phase in
plant life. If we recur to our study of coleochæte we may see that
there is here possibly a state of things which presages, as we say,
this new phase which is so well formed in riccia. We recollect that
after the fertilized egg passed the period of rest it formed a small
rounded mass of cells, each of which now forms a zoospore. The zoospore
in turn develops the normal thallus (gametophyte) of the coleochæte
again. In coleochæte then we have two phases of the plant, each having
its origin in a one-celled stage. Then if we go back to œdogonium,
we remember that the fertilized egg, before it developed into the
œdogonium plant again (which is the gametophyte), at first divides into
_four_ cells which become zoospores. These then develop the œdogonium
plant.
Note. Too much importance should not be attached to
this seeming homology of the sporophyte of œdogonium,
coleochæte, and riccia, for the nuclear phenomena
in the formation of the zoospores of œdogonium and
coleochæte are not known. They form, however, a very
suggestive series.
Marchantia.
=483.= The marchantia (M. polymorpha) has been chosen for study
because it is such a common and easily obtained plant, and also for
the reason that with comparative ease all stages of development can
be obtained. It illustrates also very well certain features of the
structure of the liverworts.
[Illustration: Fig. 257. Male plant of marchantia bearing
antheridiophores.]
The plants are of two kinds, male and female. The two different organs,
then, are developed on different plants. In appearance, however, before
the beginning of the structures which bear the sexual organs they
are practically the same. The thallus is flattened like nearly all
of the thalloid forms, and branches in a forked manner. The color is
dark green, and through the middle line of the thallus the texture is
different from that of the margins, so that it possesses what we term a
midrib, as shown in figs. 257, 261. The growing point of the thallus is
situated in the little depression at the free end. If we examine the
upper surface with a hand lens we see diamond-shaped areas, and at the
center of each of these areas are the openings known as the stomates.
[Illustration: Fig. 258. Section of antheridial receptacle from male
plant of Marchantia polymorpha, showing cavities where the antheridia
are borne.]
=484. Antheridial plants.=—One of the male plants is figured at
257. It bears curious structures, each held aloft by a short stalk.
These are the antheridial receptacles (or male gametophores). Each
one is circular, thick, and shaped somewhat like a biconvex lens. The
upper surface is marked by radiating furrows, and the margin is
crenate. Then we note, on careful examination of the upper surface,
that there are numerous minute openings. If we make a thin section of
this structure perpendicular to its surface we shall be able to unravel
the mystery of its interior. Here we see, as shown in fig. 258, that
each one of these little openings on the surface is an entrance to quite
a large cavity. Within each cavity there is an oval or elliptical
body, supported from the base of the cavity on a short stalk. This is
an antheridium, and one of them is shown still more enlarged in fig.
259. This shows the structure of the antheridium, and that there are
within several angular areas, which are divided by numerous straight
cross-lines into countless tiny cuboidal cells, the _sperm mother
cells_. Each of these, as stated in the former chapter, changes into a
swiftly moving body resembling a serpent with two long lashes attached
to its tail.
[Illustration: Fig. 259. Section of antheridium of marchantia, showing
the groups of sperm mother cells.]
[Illustration: Fig. 260. Spermatozoids of marchantia, uncoiling and one
extended, showing the two cilia.]
=485.= The way in which one of these sperm mother cells changes
into this spermatozoid is very curious. We first note that a coiled
spiral body is appearing within the thin wall of the cell, one end of
the coil larger than the other. The other end terminates in a slender
hair-like outgrowth with a delicate vesicle attached to its free end.
This vesicle becomes more and more extended until it finally breaks and
forms two long lashes which are clubbed at their free ends as shown in
fig. 260.
[Illustration: Fig. 261. Marchantia polymorpha, female plants bearing
archegoniophores.]
[Illustration: Fig. 262. Marchantia polymorpha, showing origin of
gametophore.]
=486. Archegonial plants.=—In fig. 261 we see one of the female
plants of marchantia. Upon this there are also very curious structures,
which remind one of miniature umbrellas. The general plan of the
archegonial receptacle (or female gametophore), for this is what these
structures are, is similar to that of the antheridial receptacle,
but the rays are more pronounced, and the details of structure are
quite different, as we shall see. Underneath the arms there hang down
delicate fringed curtains. If we make sections of this in the same
direction as we did of the antheridial receptacle, we shall be able to
find what is secreted behind these curtains. Such a section is figured
at 266. Here we find the archegonia, but instead of being sunk in
cavities their bases are attached to the under surface, while the
delicate, pendulous fringes afford them protection from drying. An
archegonium we see is not essentially different in marchantia from
what it is in riccia, and it will be interesting to learn whether the
sporogonium is essentially different from what we find in riccia.
=487. Homology of the gametophore of marchantia.=—To see the
relation of the gametophore to the thallus of marchantia take portions
of the thallus bearing the female receptacle. On the under side note
that the prominent midrib continues beyond the thin lateral expansions
and arches upward in the sinus or notch at the end, or at the side
where the branch of the thallus has continued to grow beyond. The stalk
of the gametophore is then a continuation of the midrib of the thallus.
On the apex of this are organized several radial growing points which
develop the digitate or ray-like receptacle. The gametophore is thus a
specialized branch of the thallus. When young, or in many cases when
nearly or quite mature, the gametophore, as one looks at the upper
surface of the thallus, appears to arise from the upper surface, as in
fig. 261. This is because the thin lateral expansions of the thallus
project forward and overlap in advance of the stalk. It is sometimes
necessary to tear these overlapping edges apart to see the real origin
of the gametophore. But in quite old plants these expanded portions are
farther apart and show clearly that the stalk arises from the midrib
below and arches upward in the sinus, as in fig. 262.