LIVERWORTS CONTINUED.
[Illustration: Fig. 263.
Archegonial receptacles of marchantia bearing ripe sporogonia. The
capsule of the sporogonium projects outside, while the stalk is
attached to the receptacle underneath the curtain. In the left figure
two of the capsules have burst and the elaters and spores are escaping.]
=488. Sporogonium of marchantia.=—If we examine the plant shown
in fig. 181 we shall see oval bodies which stand out between the
rays of the female receptacle, supported on short stalks. These are
the sporogonia, or spore-cases. We judge at once that they are quite
different from those which we have studied in riccia, since those were
not stalked. We can see that some of the spore-cases have opened, the
wall splitting down from the apex in several lines. This is caused by
the drying of the wall. These tooth-like divisions of the wall now
curl backward, and we can see the yellowish mass of the spores in slow
motion, falling here and there. It appears also as if there were
twisting threads which aided the spores in becoming freed from the
capsule.
[Illustration: Fig. 264.
Section of archegonial receptacle of Marchantia polymorpha; ripe
sporogonia. One is open, scattering spores and elaters; two are still
enclosed in the wall of the archegonium. The junction of the stalk of
the sporogonium with the receptacle is the point of attachment of the
sporophyte of marchantia with the gametophyte.]
[Illustration: Fig. 265. Elater and spore of marchantia. _sp_, spore;
_mc_, mother cell of spores, showing partly formed spores.]
=489. Spores and elaters.=—If we take a bit of this mass of
spores and mount it in water for examination with the microscope, we
shall see that, besides the spores, there are very peculiar thread-like
bodies, the markings of which remind one of a twisted rope. These are
very long cells from the inner part of the spore-case, and their walls
are marked by spiral thickenings. This causes them in drying, and also
when they absorb moisture, to twist and curl in all sorts of ways. They
thus aid in pushing the spores out of the capsule as it is drying.
=490. Sporophyte of marchantia compared with riccia.=—We must
recollect that the sporogonium in marchantia is larger than in riccia,
and that it is also not lying in the tissue of the thallus, but is only
attached to it at one side by a slender stalk. This shows us an
increase in the size and complex structure of this new phase of the
plant, the _sporophyte_. This is one of the very interesting things
which we have to note as we go on in the study of the higher plants.
[Illustration: Fig. 266.
Marchantia polymorpha, archegonium at the left with egg; archegonium at
the right with young sporogonium; _p_, curtain which hangs down around
the archegonia; _e_, egg; _v_, venter of archegonium; _n_, neck of
archegonium; _sp_, young sporogonium.]
=491. Sporophyte dependent on the gametophyte for its
nutriment.=—We thus see that at no time during the development of
the sporogonium is it independent from the gametophyte. This new phase
of plants then, the sporophyte, has not yet become an independent
plant, but must rely on the earlier phase for sustenance.
=492. Development of the sporogonium.=—It will be interesting
to note briefly how the development of the marchantia sporogonium
differs from that of riccia. The first division of the fertilized egg
is the same as in riccia, that is a wall which runs crosswise of the
axis of the archegonium divides it into two cells. In marchantia the
cell at the base develops the stalk, so that here there is a radical
difference. The outer cell forms the capsule. But here after the wall
is formed the inner tissue does not all go to make spores, as is the
case with riccia. But some of it forms the elaters. While in riccia
only the outside layer of cells of the sporogonium remained sterile, in
marchantia the basal half of the egg remains completely sterile and
develops the stalk, and in the outer half the part which is formed from
some of the inner tissue is also sterile.
[Illustration: Fig. 267.
Section of developing sporogonia of marchantia; _nt_, nutritive tissue
of gametophyte; _st_, sterile tissue of sporophyte; _sp_, fertile part
of sporophyte; _va_, enlarged venter of archegonium.]
=493. Embryo.=—In the development of the embryo we can see all
the way through this division line between the basal half, which is
completely sterile, and the outer half, which is the fertile part. In
fig. 267 we see a young embryo, and it is nearly circular in section
although it is composed of numerous cells. The basal half is attached
to the base of the inner surface of the archegonium, and at this time
the archegonium still surrounds it. The archegonium continues to grow
then as the embryo grows, and we can see the remains of the shrivelled
neck. The portion of the embryo attached to the base of the archegonium
is the sterile part and is called the “foot,” and later develops the
stalk. The sporogonium during all the stages of its development derives
its nourishment from the gametophyte at this point of attachment at
the base of the archegonium. Soon, as shown in fig. 267 at the right,
the outer portion of the sporogonium begins to differentiate into
the cells which form the elaters and those which form spores. These
lie in radiating lines side by side, and form what is termed the
_archesporium_. Each fertile cell forms four spores just as in riccia.
They are thus called the mother cells of the spores, or spore mother
cells.
=494. How marchantia multiplies.=—New plants of marchantia are
formed by the germination of the spores, and growth of the same to the
thallus. The plants may also be multiplied by parts of the old ones
breaking away by the action of strong currents of water, and when they
lodge in suitable places grow into well-formed plants. As the thallus
lives from year to year and continues to grow and branch the older
portions die off, and thus separate plants may be formed from a former
single one.
[Illustration: Fig. 268. Marchantia plant with cupules and gemmæ;
rhizoids below.]
=495. Buds, or gemmæ, of marchantia.=—But there is another
way in which marchantia multiplies itself. If we examine the upper
surface of such a plant as that shown in fig. 268, we shall see that
there are minute cup-shaped or saucer-shaped vessels, and within them
minute green bodies. If we examine a few of these minute bodies with
the microscope we see that they are flattened, biconvex, and at two
opposite points on the margin there is an indentation similar to that
which appears at the growing end of the old marchantia thallus. These
are the growing points of these little buds. When they free themselves
from the cups they come to lie on one side. It does not matter on what
side they lie, for whichever side it is, that will develop into the
lower side of the thallus, and forms rhizoids, while the upper surface
will develop the stomates.
Leafy-stemmed liverworts.
=496.= We should now examine more carefully than we have done
formerly a few of the leafy-stemmed liverworts (called foliose
liverworts).
[Illustration: Fig. 269.
Section of thallus of marchantia. _A_, through the middle portion; _B_,
through the marginal portion; _p_, colorless layer; _chl_, chlorophyll
layer; _sp_, stomate; _h_, rhizoids; _b_, leaf-like outgrowths on
underside (Goebel).]
=497. Frullania= (Fig. 32).—This plant grows on the bark of
logs, as well as on the bark of standing trees. It lives in quite dry
situations. If we examine the leaves we will see how it is able to do
this. We note that there are two rows of lateral leaves, which are very
close together, so close in fact that they overlap like the shingles
on a roof. Then, as the creeping stems lie very close to the bark of
the tree, these overlapping leaves, which also hug close to the stem
and bark, serve to retain moisture which trickles down the bark during
rains. If we examine these leaves from the under side as shown in fig.
34, we see that the lower or basal part of each one is produced into a
peculiar lobe which is more or less cup-shaped. This catches water and
holds it during dry weather, and it also holds moisture which the plant
absorbs during the night and in damp days. There is so much moisture in
these little pockets of the under side of the leaf that minute animals
have found them good places to live in, and one frequently discovers
them in this retreat. There is here also a third row of poorly
developed leaves on the under side of the stem.
=498. Porella.=—Growing in similar situations is the plant known
as porella. Sometimes there are a few plants in a group, and at other
times large mats occur on the bark of a trunk. This plant, porella,
also has closely overlapping leaves in rows on opposite sides of the
stem, and the lower margin of each leaf is curved under somewhat as in
frullania, though the pocket is not so well formed.
The larger plants are female, that is they bear archegonia, while
the male plants, those which bear antheridia, are smaller and the
antheridia are borne on small lateral branches. The antheridia
are borne in the axils of the leaves. Others of the leafy-stemmed
liverworts live in damp situations. Some of these, as Cephalozia, grow
on damp rotten logs. Cephalozia is much more delicate, and the leaves
are farther apart. It could not live in such dry situations where the
frullania is sometimes found. If possible the two plants should be
compared in order to see the adaptation in the structure and form to
their environment.
[Illustration: Fig. 270. Thallus of a thalloid liverwort (blasia)
showing lobed margin of the frond, intermediate between thalloid and
foliose plant.]
=499. Sporogonium of a foliose liverwort.=—The sporogonium of the
leafy-stemmed liverworts is well represented by that of several genera.
We may take for this study the one illustrated in fig. 274, but
another will serve the purpose just as well. We note here that it
consists of a rounded capsule borne aloft on a long stalk, the stalk
being much longer proportionately than in marchantia. At maturity the
capsule splits down into four quadrants, the wall forming four valves,
which spread apart from the unequal drying of the cells, so that the
spores are set free, as shown in fig. 276. Some of the cells inside
of the capsule develop elaters here also as well as spores. These are
illustrated in fig. 278.
[Illustration: Fig. 271. Foliose liverwort, male plant showing
antheridia in axils of the leaves (a jungermannia).]
[Illustration: Fig. 272. Antheridium of a foliose liverwort
(jungermannia).]
[Illustration: Fig. 273. Foliose liverwort, female plant with rhizoids.]
=500.= In this plant we see that the sporophyte remains attached to
the gametophyte, and thus is dependent on it for sustenance. This is
true of all the plants of this group. The sporophyte never becomes
capable of an independent existence, and yet we see that it is becoming
larger and more highly differentiated than in the simple riccia.
[Illustration: Fig. 274. Fruiting plant of a foliose liverwort
(jungermannia). Leafy part is the gametophyte; stalk and capsule is the
sporophyte (sporogonium in the bryophytes).
[Illustration: Fig. 275. Opening capsule showing escape of spores and
elaters.
[Illustration: Fig. 276. Capsule parted down to the stalk.
[Illustration: Fig. 277. Four spores from mother cell held in a group.
[Illustration: Fig. 278. Elaters, at left showing the two spiral marks,
at right a branched elater.
Figs. 275-278.—Sporogonium of liverwort (jungermannia) opening by
splitting into four parts, showing details of elaters and spores.]
The Horned Liverworts.[25]
=501. The horned liverworts= take their name from the shape of
the sporogonium. This is long, slender, cylindrical, pointed, and very
slightly curved, suggesting the shape of a minute horn. Anthoceros is
one of the most common and widely distributed species. The plant grows
on damp soil or on mud.
Anthoceros.
=502. The gametophyte.=—The gametophyte is thalloid. It is thin,
flattened, green, irregularly ribbon-shaped and branched. It lies on
the soil and is more or less crisped or wavy, or curled, the edges
nearly plane, or somewhat irregular, and with minute lobes, or notches,
especially near the growing end. The general form and branching can
be seen in fig. 279. Where the plants are much crowded the thallus is
more irregular, and often possesses numerous small lateral branches
in addition to the main lobes. Upon the under side are the slender
rhizoids, which attach to the soil. With a hand lens there can be seen
also upon the under side small dark, rounded and thickened spots, where
an alga (nostoc) is located.
Sexual Organs of Anthoceros.
=502. The sexual organs of anthoceros= differ considerably from
those of the other liverworts studied. In the first place they are
immersed in the true tissue of the thallus, i.e., they do not project
above the surface.
[Illustration: Fig. 279.
Anthoceros gracilis. _A_, several gametophytes, on which sporangia
have developed; _B_, an enlarged sporogonium, showing its elongated
character and dehiscence by two valves, leaving exposed the slender
columella on the surface of which are the spores, _C_, _D_, _E_, _F_,
elaters of various forms, _G_, spores. (After Schiffner.)]
=503. Antheridia.=—The antheridium arises from an internal cell
of the thallus, a cell just below the upper surface. This cell develops
usually a group of antheridia which lie in a cavity formed around this
cell as the thallus continues to grow. They are situated along the
middle line of the thallus, and can be seen by making a section in this
direction. The antheridia are oval or rounded, have a wall of one layer
of cells which contains the sperm cells, and each antheridium has a
slender stalk. The sperms are like those of the true liverworts.
=504. Archegonia.=—The archegonia are also borne along the
middle line of the thallus. Each one arises at an early stage in the
development of the tissue of the thallus from a superficial cell,
but the archegonium does not project above the surface. The venter
therefore which contains the egg is deep down in the thallus, the wall
of the neck is formed from cells indistinguishable from the adjoining
cells of the thallus and opens at the surface.
Sporophyte of Anthoceros.
=505. The Sporogonium.=—The sporogonium is developed from the
fertilized egg, fertilization resulting of course from the fusion of
one of the sperms with the nucleus of the egg. From the lower part
of the embryo certain cells elongate and push out like rhizoids into
the thallus (gametophyte), but never reach the outside so that the
sporogonium derives its nutriment from the gametophyte in a parasitic
manner like the true liverworts. It is surrounded at the base by a
sheath, an outgrowth of the gametophyte.
=506. Growing point of the sporogonium.=—A remarkable thing
about the sporogonium of anthoceros, and its relatives, is that the
growing point instead of being situated at the free end is located near
the base, just above the nourishing foot. Thus the upper part of the
sporogonium is older. In the old sporogonia there may be ripe spores
near the free end, young ones near the middle, and undifferentiated
growing tissue near the base. A longitudinal section of a sporogonium
just as the spores are ripening will show this.
=507. Structure of the sporogonium.=—A longitudinal section
of the sporogonium shows that the spore-bearing tissue occupies a
comparatively small portion of the sporogonium. In the section there
is a narrow layer (two cells thick) on either side and joined at the
top. In the entire sporogonium this fertile tissue is in the shape of
an inverted test tube situated inside of the sporogonium. The wall of
the sporogonium is about four cells thick. The sterile tissue inside
of the spore-bearing tube is the columella. The cells of the wall
contain chlorophyll, and there are true stomata with guard cells in the
epidermal layer.
=508. Spores and elaters.=—In the spore-bearing tissue there
are two layers of cells (the archesporium). Each cell is a potential
mother cell. The cells, however, of alternate tiers do not form spores.
They elongate some what and are somewhat irregular and sometimes divide
or branch. They are supposed to represent rudimentary _elaters_. The
cells in the other tiers are actual mother cells, and each one forms
four spores.
=509. The sporophyte of anthoceros= represents the highest type
found in the liverworts. The spongy green parenchyma forming the
wall, with the stomata in the epidermal layer, fits this tissue for
the process of photosynthesis, so that this part of the sporophyte
functions as the green leaf of the seed plants. It has been suggested
by some that if the rhizoids on the nourishing foot could only extend
outside and anchor in the soil, the sporophyte of anthoceros could live
an independent existence. But we see that it stops short of that.
Classification of the Liverworts.
CLASS HEPATICÆ.
=510. Order Marchantiales.=[26]—There are two families represented
in the United States.
Family Ricciaceæ, including Riccia and Ricciocarpus.
Family Marchantiaceæ, including Marchantia, Fegatella (= Conocephalus),
Fimbriaria, Targionia, etc.
=511. Order Jungermanniales.=[27]—There are two subdivisions
of this order. _The Anacrogynæ_ include chiefly thalloid forms with
continued apical growth, the archegonia back of the apical cell.
Examples: Blasia, Aneura, Pellia, etc.
_The Acrogynæ_ include chiefly foliose forms, the archegonia arising
from the apical cell and in such cases interrupting apical growth.
Examples: Cephalozia, Frullania, Bazzania, Jungermannia, Ptilidium,
Porella, etc.
CLASS ANTHOCEROTES.
=512. The Anthocerotes= have formerly been placed with the
Hepaticæ as an order. But because of their wide divergence from
the other liverworts in the development of the sexual organs, and
especially in the structure of the sporophyte, they are now by some
separated as a distinct class. There is one order.
=Order Anthocerotales.=[28]—This includes one family
(Anthocerotaceæ) with Anthoceros and Notothylas in Europe and North
America, and Dendroceros in the tropics. The latter is epiphytic.
FOOTNOTES:
[25] May be used as an alternate study for marchantia.
[26] As subclass in Engler and Prantl.
[27] As subclass in Engler and Prantl.
[28] As subclass in Engler and Prantl.