ernation 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 Rhizocarpae there are two kinds of spores, _microspores_ and
_macrospores_, producing prothallia which bear respectively antheridia
and archegonia; in the Lycopodiaceae, 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 Lycopodiaceae, 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 S