on 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 algae,
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 palaeobotany 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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