In this instance all the generations are asexual, but the most common
case is for the sexual and the asexual generations to alternate. I will
describe as examples the reproduction of a moss, a fern, and a
dicotyledon.
In such a typical moss as Funaria, we have the following cycle of
developments: The sexual generation is a dioecious leafy structure,
having a central elongated axis, with leaves arranged regularly around
and along it. At the top of the axis in the male plant rise the
antheridia, surrounded by an envelope of modified leaves called the
perigonium. The antheridia are stalked sacs, with a single wall of
cells, and the spiral antherozoids arise by free-cell formation from the
cells of the interior. They are discharged by the bursting of the
antheridium, together with a mucilage formed of the degraded walls of
their mother cells.
In the female plant there arise at the apex of the stem, surrounded by
an envelope of ordinary leaves, several archegonia. These are of the
ordinary type of those organs, namely, a broad lower portion, containing
a naked oosphere and a long narrow neck with a central canal leading to
the oosphere. Down this canal pass one or more antherozoids, which
become absorbed into the oosphere, and this then secretes a wall, and
from it grows the second or asexual generation. The peculiarity of this
asexual or spore-bearing plant is that it is parasitic on the sexual
plant; the two generations, although not organically connected, yet
remain in close contact, and the spore-bearing generation is at all
events for a time nourished by the leafy sexual generation.
The spore-bearing generation consists of a long stalk, closely held
below by the cells of the base of the archegonium; this supports a
broadened portion which contains the spores, and the top is covered with
the remains of the neck of the archegonium forming the calyptra.
The spores arise from special or mother-cells by a process of division,
or it may be even termed free-cell formation, the protoplasm of each
mother-cell dividing into four parts, each of which contracts, secretes
a wall, and thus by rejuvenescence becomes a spore, and by the
absorption of the mother-cells the spores lie loose in the spore sac.
The spores are set free by the bursting of their chamber, and each
germinates, putting out a branched thread of cells called a protonema,
which may perhaps properly be termed a third generation in the cycle of
the plant; for it is only from buds developed on this protonema that the
leafy sexual plant arises.
The characteristics, then, of the mosses are, that the sexual generation
is leafy, the one or two asexual generations are thalloid, and that the
spore-bearing generation is in parasitic connection with the sexual
generation.
In the case of the fern, these conditions are very different.
The sexual generation is a small green thalloid structure called a
prothallium, which bears antheridia and archegonia, each archegonium
having a neck-canal and oosphere, which is fertilized just as in the
moss.
But the asexual generation derived from the oospore only for a short
while remains in connection with the prothallium, which, of course,
answers to the leafy portion of the moss. What is generally known as the
fern is this asexual generation, a great contrast to the small leafless
moss fruit or sporogonium as it is called, to which it is
morphologically equivalent. On the leaves of this generation arise the
sporangia which contain the spores. The spores are formed in a manner
very similar to those of the mosses, and are set free by rupture of the
sporangium.
The spore produces the small green prothallium by cell-division in the
usual way, and this completes the cycle of fern life.
The alternation of generations, which is perhaps most clear and typical
in the case of the fern, becomes less distinctly marked in the plants of
higher organization and type.
Thus in the Rhizocarpæ there are two kinds of spores, _microspores_ and
_macrospores_, producing prothallia which bear respectively antheridia
and archegonia; in the Lycopodiaceæ, the two kinds of spores produce
very rudimentary prothallia; in the cycads and conifers, the microspore
or pollen grain only divides once or twice, just indicating a
prothallium, and no antheridia or antherozoids are formed. The
macrospore or embryo-sac produces a prothallium called the endosperm, in
which archegonia or corpuscula are formed; and lastly, in typical
dicotyledons it is only lately that any trace of a prothallium from the
microspore or pollen cell has been discovered, while the macrospore or
embryo-sac produces only two or three prothallium cells, known as
antipodal cells, and two or three oospheres, known as germinal vesicles.
This description of the analogies of the pollen and embryo-sac of
dicotyledons assumes that the general vegetative structure of this class
of plants is equivalent to the asexual generation of the higher
cryptogams. In describing their cycle of reproduction I will endeavor to
show grounds for this assumption.
We start with the embryo as contained in the seed. This embryo is the
product of fertilization of a germinal vesicle by a pollen tube. Hence,
by analogy with the product of fertilization of rhizocarp's, ferns, and
mosses, it should develop into a spore bearing plant. It does develop
into a plant in which on certain modified leaves are produced masses of
tissue in which two kinds of special reproductive cells are formed. This
is precisely analogous to the case of gymnosperms, lycopods, etc., where
on leaf structures are formed macro and micro sporangia.
To deal first with the microsporangium or pollen-sac. The pollen cells
are formed from mother cells by a process of cell division and
subsequent setting free of the daughter cells or pollen cells by
rejuvenescence, which is distinctly comparable with that of the
formation of the microspores of Lycopodiaceæ, etc. The subsequent
behavior of the pollen cell, its division and its fertilization of the
germinal vesicle or oosphere, leave no doubt as to its analogy with the
microspore of vascular cryptogams.
Secondly, the nucleus of the ovule corresponds with the macrosporangium
of Selaginella, through the connecting link of the conifers, where the
ovule is of similar origin and position to the macrosporangium of the
Lycopodiaceæ. But the formation of the macrospore or embryo-sac is
simpler than the corresponding process in cryptogams. It arises by a
simple enlargement of one cell of the nucleus instead of by the division
of one cell into four, each thus becoming a macrospore. At the top of
this macrospore or embryo-sac two or three germinal vesicles are formed
by free cell formation, and also two or three cells called antipodal
cells, since they travel to the other end of the embryo-sac; these
latter represent a rudimentary prothallium. This formation of germinal
vesicles and prothallium seems very different from the formation of
archegonia and prothallium in Selaginella, for instance; but the link
which connects the two is in the gymnosperms, where distinct archegonia
in a prothallium are formed.
Thus we see that the flowering plant is essentially the equivalent of
the asexual fern, and of the sporogonium of the moss, and the pollen
cell and the embryo-sac represent the two spores of the higher
cryptogams, and the pollen tube and the germinal vesicles and antipodal
cells are all that remain of the sexual generation, seen in the moss as
a leafy plant, and in the fern as a prothallium. Indeed, when a plant
has monoecious or dioecious flowers, the distinction between the asexual
and the sexual generation has practically been lost, and the
spore-bearing generation has become identified with the sexual
generation.
Having now described the formation of the pollen and the germinal
vesicles, it only remains to show how they form the embryo. The pollen
cell forms two or three divisions, which are either permanent or soon
absorbed; this, as before stated, is the rudimentary male prothallium.
Then when it lies on the stigma it develops a long tube, which passes
down the style and through the micropyle of the ovule to the germinal
vesicles, one of which is fertilized by what is probably an osmotic
transference of nuclear matter. The germinal vesicle now secretes a
wall, divides into two parts, and while the rest of the embyro-sac fills
with endosperm cells, it produces by cell division from the upper half a
short row of cells termed a suspensor, and from the lower half a mass of
cells constituting the embryo. Thus while in the moss the asexual
generation or sporogonium is nourished by the sexual generation or leafy
plant, and while in the fern each generation is an independent
structure, here in the dicotyledon, on the other hand, the asexual
generation or embryo is again for a time nourished in the interior of
the embryo-sac representing the sexual generation, and this again
derives its nourishment from the previous asexual generation, so that as
in the moss, there is again a partial parasitism of one generation on
the other.
To sum up the methods of plant reproduction: They resolve themselves
into two classes.
1st. Purely vegetative.
2d. Truly reproductive by special cells.
In the second class, if we count conjugation as a simple form of
fertilization, there are only two types of reproductive methods.
1st. Reproduction from an asexual spore.
2d. Reproduction from an oospore formed by the combination of two sexual
cells.
In the vast majority of plant species these two types are used by the
individuals alternately.
The extraordinary similarity of the reproductive process, as shown in
the examples I have given, Achlya, Spirogyra, and Vaucheria among algæ,
the moss, the fern, and the flowering plant, a similarity which becomes
the more marked the more the details of each case and of the cases of
plants which form links between these great classes are studied, points
to a community of origin of all plants in some few or one primeval
ancestor. And to this inference the study of plant structure and
morphology, together with the evidence of palæobotany among other
circumstances, lends confirmatory evidence, and all modern discoveries,
as for instance that of the rudimentary prothallium formed by the pollen
of angiosperms, tend to the smoothing of the path by which the descent
of the higher plants from simpler types will, as I think, be eventually
shown.
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