LYCOPODIACEÆ, OR CLUB MOSSES.
THE Club Mosses are mostly perennial plants, with slender creeping
stems, often several feet or yards in length, occasionally erect, and
clothed by small, sessile, closely set, often imbricated leaves without
veins. They have in some instances a habit resembling that of Conifers.
The stems consist of a mass of thick-walled, often dotted cells,
enclosing one or many bunches of scalariform tissue, which sends off
branches to every leaf and bud. The scalariform tissue is accompanied by
fine, elongated, and sometimes rather coarser cells, which are
occasionally reticulated. The stem approaches to that of ferns, but the
bundles of vascular tissue are confined to the centre. The branches of
the stems are often bifurcate, and terminate in one or a pair of
cone-like spikes, which are either sessile or stalked. The sporangia are
sessile in the axils of the imbricated leaves or bracts that cover the
cones. Two kinds grow on the same plant, one of them bivalved,
containing a powdery substance, whose particles a high magnifying power
shows to be globular spermatozoids: these do not germinate; and the
other three-valved, enclosing comparatively large, nearly spherical
granules, marked with three prominent ridges, radiating from one
extremity. These certainly germinate, forming by cell-division a
prothallus of hexagonal cellular tissue, adherent to and confluent with
the spores, as in Marsileaceæ, or penetrating their cavity, but without
the protrusion of threads, as in ferns or mosses. On this prothallus,
archegonia are soon produced, the embryo being formed from a cell at its
base; this sends down roots on one side, and a minute stem with the two
primary leaves of the young plant at the other. The plant thus bears a
close resemblance to a young plant of the dicotyledons, and exhibits,
with regard to fructification, the highest organization of which the
Cryptogams are capable, although in their tissues the Lycopods are
inferior to the Equisetaceæ and Marsileaceæ.
There are four genera of this order, very widely dispersed, most
abundant and larger in the hot moist parts of India and the Indian
islands, but large tracts are covered with them within the arctic
circles and in temperate latitudes. There are 200 species of the genus
Lycopodium, or Wolfs-claw, and of these, six only are indigenous in
Britain; they grow in very exposed situations, as in the case of L.
clavatum, which lives on upland heaths and pastures, and which has a
procumbent stem, creeping for many feet or yards, sending out branches
in all directions, with a pair of cones at their extremities, and strong
roots at intervals to fix it to the ground. Lycopodium inundatum forms
large patches on the marshes in the south of England. New Zealand has
many more species of this genus than Britain, and some of the noblest
specimens. The dried spores of the L. clavatum are so inflammable, that
they have been used on the stage to produce the effect of lightning.
Some Club Mosses yield a blue dye, a colour which is now obtained of a
better quality from coal tar. Others possess cathartic properties, but
although they have been used as medicine, they are very dangerous from
the violence of their effects.
The genus Selaginella has a moss-like habit. The stem is generally
creeping, and flat-looking from being clothed with distichous leaves;
but sometimes it is tall and erect, having feathery branches clad with
leaves. The sporangia are sacs, with two or three valves containing
large spores and sub-globose antheridia, containing orange-coloured or
minute scarlet bodies, ultimately developed into spermatozoids. The
species are very numerous and greatly varied, and mostly tropical.
From a comparison of the structure of the stems of the fossil plants,
Sigillaria and Lepidodendron, Dr. Hooker concludes that they are highly
developed Lycopods, approaching closely in structure to the highest
class of plants. The stems of the former differ chiefly in size from
those of the Lycopodium, and the cones of the Lepidostrobus differ in
their greater development and in the thickness of their scales from
those of the Conifers. The spores of the Lepidostrobus ornatus too are
sphericotetrahedral, like those of the Lycopodium. But inferior as the
existing Lycopods are to their fossil ancestors, their analogy to the
Conifers gives them a more exalted position in the vegetable world than
their tall and graceful allies, ‘the tree ferns.’
The gradual change of structure from the lowest to the highest
cryptogamic form is accompanied by a singular variety in the mode of
reproduction, and a degree of vital energy scarcely to be expected, at
least in beings of such low organization as the yeast plant, which
produces gemmæ in vast and rapid profusion, each gem or bud being only a
facsimile of the parent cell. In like manner the mother plant is
reproduced by the germination of the green zoospores, the green globular
cells in the fronds of the lichens, the motile gonidia, and the discs in
the baskets of the Marchantia, especially those of the Marchantia
polymorpha, which have such vigorous vegetation that they form stomates
on whichever side is turned to the light, and roots on the other. The
result of all these is an individual perfectly similar to its parent,
like that produced by a bud and cutting of a tree, or the axillary fruit
buds of the Begonia. The leaves of that flowering plant, as well as
those of the Achimenes and Gloxinia, possess the property of reproducing
the parent plant, for, when laid on moist earth and slit in different
places, a young plant rises from the upper side of the fracture, and
roots shoot down from the under. Although this manner of growth
resembles the germination of the embryo of an archegonium, it is widely
different, for the embryonic cell is fertilized by the spermatozoids, so
there is a certain analogy but not the smallest affinity. The highest
vegetable classes can reproduce the mother plant in many ways, but they
have nothing akin to the alternation of generations exhibited by many of
the lower tribes, nor yet to conjugation like the Desmidiaceæ and
Diatoms. Possibly the spores resulting from these two modes of
reproduction, as well as the resting spores, may produce new species;
certainly those resulting from fructification do occasionally yield new
varieties.
Many spores produce the plant directly, others indirectly, as most of
the fungi; but if the definition of a perfect plant be that which bears
the fructification, the mycelium of a mushroom constitutes the plant,
for the mushroom itself is only a kind of sporangium or spore-case.
Nevertheless, the spores of the Puccinia and other microscopic fungi,
which are the cause of the rust and mildew in wheat, give rise to a kind
of prothallus, ‘a slight fore-shadowing’ of the prothallus of the
Marchantia and Mosses, which only produce sporangia, and those of the
Ferns, Horsetails, and Club Mosses, whose archegonia contain the embryo
of the plant itself. The fructification of the Lycopods is the highest
of which the Cryptogamia are capable, and brings them into a singular
analogy with the flowering class. For in the ovule, or seed-vessel of
the flower-bearing race, a large cell is formed, containing mucilaginous
matter, which, soon after fructification, is converted into a mass of
cellular tissue, which gives rise to the cell containing the embryo,
just as the prothallus of a Lycopod gives rise to the archegonium
containing the embryo of the plant. The linear leaves of existing
Lycopods and the cones of their fossil allies, are similar to the leaves
and cones of the Coniferæ: there is no affinity, but an analogy existing
between these two groups sufficient to make them form links between the
two great divisions of the vegetable kingdom. The Lycopodiaceæ probably
may be regarded as the highest of the Cryptogamia, and Coniferæ as the
lowest of the flower-bearing class.