MOSSES (MUSCI).
=513.= We are now ready to take up the more careful study of the
moss plant. There are a great many kinds of mosses, and they differ
greatly from each other in the finer details of structure. Yet there
are certain general resemblances which make it convenient to take for
study almost any one of the common species in a neighborhood, which
forms abundant fruit. Some, however, are more suited to a first study
than others. (Polytrichum and funaria are good mosses to study.)
=514. Mnium.=—We will select here the plant shown in fig. 280.
This is known as a mnium (M. affine), and one or another of the species
of mnium can be obtained without much difficulty. The mosses, as we
have already learned, possess an axis (stem) and leaf-like expansions,
so that they are leafy-stemmed plants also. Certain of the branches of
the mnium stand upright, or nearly so, and the leaves are all of the
same size at any given point on the stem, as seen in the figure. There
are three rows of these leaves, and this is true of most of the mosses.
=515.= The mnium plants usually form quite extensive and pretty
mats of green in shady moist woods or ravines. Here and there among the
erect stems are prostrate ones, with two rows of prominent leaves so
arranged that it reminds one of some of the leafy-stemmed liverworts.
If we examine some of the leaves of the mnium we see that the greater
part of the leaf consists of a single layer of green cells, just as
is the case in the leafy-stemmed liverworts. But along the middle
line is a thicker layer, so that it forms a distinct midrib. This is
characteristic of the leaves of mosses, and is one way in which they
are separated from the leafy-stemmed liverworts, the latter never
having a midrib.
[Illustration: Fig. 280.
Portion of moss plant of Mnium affine, showing two sporogonia from one
branch. Capsule at left has just shed the cap or operculum; capsule at
right is shedding spores, and the teeth are bristling at the mouth.
Next to the right is a young capsule with calyptra still attached; next
are two spores enlarged.]
=516. The fruiting moss plant.=—In fig. 280 is a moss plant “in
fruit,” as we say. Above the leafy stem a slender stalk bears the
capsule, and in this capsule are borne the spores. The capsule then
belongs to the _sporophyte phase_ of the moss plant, and we should
inquire whether the entire plant as we see it here is the sporophyte,
or whether part of it is gametophyte. If a part of it is gametophyte
and a part sporophyte, then where does the one end and the other begin?
If we strip off the leaves at the end of the leafy stem, and make a
longisection in the middle line, we should find that the stalk which
bears the capsule is simply stuck into the end of the leafy stem, and
is not organically connected with it. This is the dividing line, then,
between the gametophyte and the sporophyte. We shall find that here the
archegonium containing the egg is borne, which is a surer way of
determining the limits of the two phases of the plant.
=517. The male and female moss plants.=—The two plants of mnium
shown in figs. 281, 282 are quite different, as one can easily see,
and yet they belong to the same species. One is a female plant, while
the other is a male plant. The sexual organs then in mnium, as in many
others of the mosses, are borne on separate plants. The archegonia are
borne at the end of the stem, and are protected by somewhat narrower
leaves which closely overlap and are wrapped together. They are similar
to the archegonia of the liverworts.
[Illustration: Fig. 281. Female plant (gametophyte) of a moss (mnium),
showing rhizoids below, and the tuft of leaves above which protect the
archegonia.]
[Illustration: Fig. 282. Male plant (gametophyte) of a moss (mnium)
showing rhizoids below and the antheridia at the center above
surrounded by the rosette of leaves.]
The male plants of mnium are easily selected, since the leaves at the
end of the stem form a broad rosette with the antheridia, and some
sterile threads packed closely together in the center. The ends of the
mass of antheridia can be seen with the naked eye, as shown in fig.
282. When the antheridia are ripe, if we make a section through a
cluster, or if we merely tease out some from the end with a needle in
a drop of water on the slide, then prepare for examination with the
microscope, we can see the form of the antheridia. They are somewhat
clavate or elliptical in outline, as seen in fig. 284. Between them
there stand short threads composed of several cells containing
chlorophyll grains. These are sterile threads (paraphyses).
=518. Sporogonium.=—In fig. 280 we see illustrated a sporogonium
of mnium, which is of course developed from the fertilized egg-cell of
the archegonium. There is a nearly cylindrical capsule, bent downward,
and supported on a long slender stalk. Upon the capsule is a peculiar
cap,[29] shaped like a ladle or spatula. This is the remnant of the
old archegonium, which, for a time surrounded and protected the young
embryo of the sporogonium, just as takes place in the liverworts. In
most of the mosses this old remnant of the archegonium is borne aloft
on the capsule as a cap, while in the liverworts it is thrown to one
side as the sporogonium elongates.
[Illustration: Fig. 283. Section through end of stem of female plant of
mnium, showing archegonia at the center. One archegonium shows the egg.
On the sides are sections of the protecting leaves.]
[Illustration: Fig. 284. Antheridium of mnium with jointed paraphysis
at the left; spermatozoids at the right.]
=519. Structure of the moss capsule.=—At the free end on the moss
capsule as shown in the case of mnium in fig. 280, after the remnant
of the archegonium falls away, there is seen a conical lid which fits
closely over the end. When the capsule is ripe this lid easily falls
away, and can be brushed off so that it is necessary to handle the
plants with care if it is desired to preserve this for study.
=520.= When the lid is brushed away as the capsule dries more we
see that the end of the capsule covered by the lid appears “frazzled.”
If we examine this end with the microscope we see that the tissue of
the capsule here is torn with great regularity, so that there are two
rows of narrow, sharp teeth which project outward in a ring around the
opening. If we blow our “breath” upon these teeth they will be seen to
move, and as the moisture disappears and reappears in the teeth, they
close and open the mouth of the capsule, so sensitive are they to the
changes in the humidity of the air. In this way all of the spores are
prevented to some extent from escaping from the capsule at one time.
=521.= Note. If we make a section longitudinal of the capsule of
mnium, or some other moss, we find that the tissue which develops the
spores is much more restricted than in the capsule of the liverworts
which we have studied. The spore-bearing tissue is confined to a single
layer which extends around the capsule some distance from the outside
of the wall, so that a central cylinder is left of sterile tissue. This
is the columella, and is present in nearly all the mosses. Each of the
cells of the fertile layer divides into four spores.
[Illustration: Fig. 285. Two different stages of young sporogonium of
a moss, still within the archegonium and wedging their way into the
tissue of the end of the stem. _h_, neck of archegonium; _f_, young
sporogonium. This shows well the connection of the sporophyte with the
gametophyte.]
=522. Development of the sporogonium.=—The egg-cell after
fertilization divides by a wall crosswise to the axis of the
archegonium. Each of these cells continues to divide for a time, so
that a cylinder pointed at both ends is formed. The lower end of
this cylinder of tissue wedges its way down through the base of the
archegonium into the tissue of the end of the moss stem as shown in
fig. 285. This forms the foot through which the nutrient materials
are passed from the gametophyte to the sporogonium. The upper part
continues to grow, and finally the upper end differentiates into the
mature capsule.
=523. Protonema of the moss.=—When the spores of a moss germinate
they form a thread-like body, with chlorophyll. This thread becomes
branched, and sometimes quite extended tangles of these threads are
formed. This is called the protonema, that is _first thread_. The older
threads become finally brown, while the later ones are green. From this
protonema at certain points buds appear which divide by close oblique
walls. From these buds the leafy stem of the moss plant grows. Threads
similar to these protonemal threads now grow out from the leafy stem,
to form the rhizoids. These supply the moss plant with nutriment, and
now the protonema usually dies, though in some few species it persists
for long periods.
Classification of the Mosses.
CLASS MUSCINEÆ (MUSCI).
=524. Order Sphagnales.=[30]—This order includes the peat mosses. There
is but one family (Sphagnaceæ) and but a single genus (Sphagnum). The
peat mosses are widely distributed over the globe, chiefly occurring
in moors, or “bogs,” usually low ground around the shores of lakes,
ponds, or along streams, but they often occur on wet dripping rocks in
cool shady places. Small ponds are sometimes filled in by their growth.
As the sphagnum growing in such an abundance of water only partially
decays, “ground” is built up rather rapidly, and the sphagnum remains
are known as “peat.” This “ground”-building peculiarity of sphagnum
sometimes enables the plant (often in conjunction with others) to fill
in ponds completely. (See Atoll Moor, Chapter LV.)
The gametophyte of sphagnum, like that of all the mosses, is dimorphic,
but the first part (or protonema) which develops from the spores is
thalloid, and therefore more like the thallose liverworts. The leafy
axis (or gametophore) which develops from the thalloid form is very
characteristic (see Chapter LV).
The archegonia are borne on the free end of the main axis, while the
antheridia are borne on short branches which are brightly colored, red,
yellow, etc. The sporophyte (sporogonium) is globose and possesses a
broad foot anchored in the end of a naked prolongation of the end of
the leafy gametophore. This naked prolongation of the gametophore looks
like the stalk of the sporogonium, but a study of its connection with
the sporogonium shows that it is part of the gametophyte, which is only
developed after the fertilization of the egg in the archegonium. In the
sporogonium there is a short columella, and the archesporium is in the
form of an inverted cup.
=525. Order Andreæales.=[31]—This order includes the single genus
Andreæa. The plants are small but form extensive mats, growing on rocks
in arctic or alpine regions usually. They are sometimes found in great
abundance on bare, rather dry rocks on mountains. The protonema is
somewhat thalloid. The sporogonium opens by splitting longitudinally
into four valves. An elongated columella is present so that the
archesporium is shaped like an inverted test tube.
=526. Order Archidiales.=[32]—This order contains the single genus
Archidium, and by some is placed as an aberrant genus in the Bryales.
There is no columella in the simple sporogonium. The archesporium
occupies all the internal part of the sporogonium, some cells being
fertile and others sterile.
=527. Order Bryales.=[33]—These include the higher mosses, and a very
large number of genera and species. The protonema is filamentous and
branched except in a few forms where it is partly thalloid as in
Tetraphis (= Georgia). (Tetraphis pellucida is a common moss on very
rotten logs. The capsule has four prominent teeth.) In a few of the
lower genera (Phascum, Pleuridium, etc.) the capsule opens irregularly,
but in the larger number the capsule opens by a lid (operculum). A
cylindrical columella is present, and the archesporium is in the form
of a tube open at both ends. (Examples: Polytrichum, Bryum, Mnium,
Hypnum, etc.)
=528.= TABLE SHOWING RELATION OF GAMETOPHYTE AND SPOROPHYTE IN THE
LIVERWORTS AND MOSSES.
----------------------------------------------------------------------
GAMETOPHYTE.
(Prominent part of the plant. Leads an independent existence.)
-------------+--------------+---------------+-------------------------
| Vegetative | Vegetative | Sexual Organs.
| Stage. |Multiplication.|
-------------+--------------+---------------+-------------------------
Riccia. |Thallus |Sometimes by |Immersed by surrounding,
|flattened, |branching and |upward growth of thallus.
|ribbon-like, |dying away of +--------------+----------
|forked, |older parts. |Antheridia, |Archegonia,
|or nearly | |with |with egg
|circular. | |spermatozoids.|in each.
-------------+--------------+---------------+-------------------------
Marchantia. |Thallus |By dying away | Borne on special
|flattened, |of older parts,| receptacles on
|ribbon-like, |and by gemmæ. | different plants.
|forked, | +--------------+----------
|male and | |Antheridia, |Archegonia,
|female | |with |borne on
|plants bear | |spermatozoids |female
|gametophores. | |borne on |gametophore
| | |antheridio- | (or
| | | phores, |archegonio-
| | |or male | phore),
| | |gametophores. |each with
| | | |an egg.
-------------+--------------+---------------+-------------------------
Jungermannia |A plant with |By dying away | On different plants.
(or |apparent |of older parts.+--------------+----------
Cephalozia,|leaves and | |Antheridia, |Archegonia,
Porella |stem; margins | |with |each with
etc.) |of thallus | |spermatozoids,|egg, on
|have become | |in axils of |female
|cut into | |leaves of |plant.
|lobes. Male | |male plant. |
|and female | | |
|plants. | | |
| | | |
| | | |
-------------+--------------+---------------+--------------+----------
Mosses. |Plant with |By branching, | On different plants.
{Mnium, |apparent |by growth of +--------------+----------
{Funaria, |leafy axis, |protonema from |Antheridia, |Archegonia,
{Polytrichum|3 rows of |axis, leaves, |with |each with
{etc. |leaves |or even |spermatozoids,|egg, on
|(similar to |sporogonium. |at end of |female
|jungermannia),|(In somegenera |stem of male |plant.
|borne on an |by gemmæ.) |plant. |(Calyptra
|earlier | | |found on
|protonemal | | |sporogonium
|stage Male | | |is remnant
|and female | | |of archeg-
|plants. | | | onium.)
| | | |
| | | |
-------------+--------------+---------------+--------------+----------
----------------------------------------------------------+---------
SPOROPHYTE. |
(Attached to gametophyte and dependent |
on it for nourishment.) |
---------------+--------------+------------+--------------+Beginning
| Beginning | Sterile | Fertile | of
| of | Part. | Part. |Gameto-
| Sporophyte. | | | phyte.
---------------+--------------+------------+--------------+
Riccia. |Fertilized |Wall of |Central mass |
|egg. (Develops|sporogonium,|(archesporium)|
|sporogonium.) |of one-layer|develops .....| Spores.
| |cells. | |
---------------+--------------+------------+--------------+---------
Marchantia. |Fertilized |Sterile part|Central part |
|egg. (Develops|of stalked |of capsule |
|sporogonium.) |capsule is |(archesporium)| Spores.
| |stalk, wall |develops .....|
| |of capsule |and elaters. |
| |of several | |
| |layers, | |
| |elaters. | |
---------------+--------------+------------+--------------+---------
Jungermannia |Fertilized |Sterile part|Central part |
(or |egg. (Develops|of stalked |of capsule |
Cephalozia, |sporogonium.) |capsule is |(archesporium)| Spores.
Porella, | |stalk, wall |develops .....|
etc.) | |of capsule |and elaters. |
| |of several | |
| |layers, | |
| |elaters. | |
----------------+--------------+------------+--------------+---------
Mosses |Fertilized |Sterile part|Cylindrical |
{ Mnium, |egg. (Develops|of stalked |layer of |
{ Funaria, |sporogonium.) |capsule is |cells around |
{ Polytrichum,| |stalk, wall |columella is |
{ etc. | |of capsule |the | Spores.
| |of several |archesporium; |
| |layers, |it |
| |columella, |develops .....|
| |lid, teeth | |
| |etc., of | |
| |the highly | |
| |specialized | |
| |capsule. | |
---------------+--------------+------------+--------------+---------
FOOTNOTES:
[29] Called the calyptra.
[30] As subclass in Engler and Prantl.
[31] As subclass in Engler and Prantl.
[32] As subclass in Engler and Prantl.
[33] As subclass in Engler and Prantl.