A frequent consequence of homoplastic adaptation is conver gent evolution, with a high degree of similarity, or even of stan dardization as the final result. Examples of this are frequent in the vegetative system: for instance, in the phyllodes of Acacia and Oxalis, and the xerophytic leafage of Veronica and Cupressus: also in the prevalence of trimerous and pentamerous flowers. But it is in the propagative organs that it is most remarkable, leading to the high uniformity of the archegonia seen throughout the higher cryptogams, of the megaspores of the heterosporous types, and of the embryo-sacs in Angiosperms. Such standardiza tion of parts, which can hardly have been in any of these examples strictly homogenetic, greatly increases the difficulty of their phylogenetic treatment, so far as it is based upon features apparently so similar.
Alternation of Generations.—A few examples must suffice in illustration of the working out of the principles thus underlying the more recent developments of plant-morphology. The first is the phenomenon of Alternation of Generations. In mosses and ferns the life-history comprises two distinct phases, or genera tions, which appear in alternate succession. In the ferns the one is the leafy fern-plant, which as it bears sporangia containing spores, is called the sporophyte: the other is a small green scale, the prothallus, which bears the sexual organs, or gametangia, con taining the gametes, known as the spermatozoid, and ovum or egg : it is therefore called the gametophyte. These alternate gen erations together constitute a life-cycle of two somatic stages, each limited by a unicellular state, for each spore may germinate to form a prothallus, and each fertilized egg may grow into a new fern-plant (see PTERIDOPHYTA). Such a cycle exists in the life history of all mosses and ferns, while its correlative stages appear also in modified form in seed-plants : moreover it is now proved that a like alternation exists also in many Algae and Fungi. In fact an alternating cycle, with varying proportion of somatic development, underlies the morphology of all plants that possess sexuality.
Strasburger (1897) related this cycle with the cytological state of the respective stages (see CYTOLOGY). Since in fertilization (syngamy) the number of chromosomes is doubled, the generation that springs from it is (2x) diploid (e.g., fern-plant, moss sporo gonium) : in the tetrad-division that precedes spore-formation, a corresponding reduction (meiosis) is effected, and the generation that springs from the spore is (x) haploid (e.g., fern-prothallus, moss-plant). It is a morphological problem of the first rank to determine the evolutionary relation of these generations to one another, whether in the individual, the phylum, or in plants at large.
Whatever the historical origin of alternation may have been, certain features in its further development were probably as fol lows. The land-habit of the Archegoniatae appears to have en couraged or fixed a biological difference between the generations : the gametophyte is characteristically semi-aquatic, while sexuality which is its end depends upon external liquid water : the sporo phyte is characteristically a land-living body, often rooted in the soil, while the dissemination of the spores depends normally on dry conditions. Thus the alternate generations of the Arche goniatae accord well with an amphibial life, one being distinctly sub-aerial, the other not fully freed from dependence on external water. But the tie of aquatic fertilization (zoidiogamic) is incon venient for land-living organisms : hence it is no surprise to find that the higher flowering plants have broken loose from it, by adopting fertilization by a pollen-tube (siphonogamic). The gametophyte by successive steps of reduction loses its independ ence as a separate generation, while the sporophyte becomes dominant. This state may be held as the last essential step in the adaptation of plant-life to a land-habit.
A second illustration of the working out of the principles sum marised above may be taken from the leaf. Of all the vegetative organs the leaf raises questions most open to debate. In the older morphology an ideal leaf was envisaged, and other types of leaf were regarded as modifications of it. But the duty of an evolutionist is to look not to an idea, but to the facts supplied by the lower organisms. They show that, whether in the gametophyte or the sporophyte, a leafless state preceded foliar development. Moreover both fossils and living plants suggest that all leaf-like appendages did not spring from a common source, but polyphy letically.
The effect of such considerations is to relax the old rigid con ception of "the leaf" as a morphological constant. Biologically a leafy shoot appears as a favourable advance on a simpler organi zation, bringing with it an increasing proportion of absorptive surface to bulk. The existence of a demand for this is common to enlarging organisms, and the foliar development appears as a consequence in any or all such. The facts indicate an independent response in many distinct phyletic lines, with the result that the leaves so produced would not be all homogenetic, but probably in high degree homoplastic.