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 Selaginella, through the connecting link of the conifers, where the ovule is of similar origin and position to the macrosporangium of the Lycopodiaceae. 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 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.