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 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.