424 PLANT MORPHOLOGY 



some numbers that exists between the gametophyte and sporophyte. 

 The phenomena of apogamy and apospory have been cited as evidence in 

 favor of this theory, but they are merely digressions from the normal life 

 cycle. 



The antithetic theory seems to be more in accord with actual conditions 

 in the plant kingdom. It contends that the gametophyte is the original 

 generation and the sporophyte a later one interpolated in the life history 

 between fertilization and meiosis. It holds that the sporophyte has 

 evolved from the zygote, an initial stage occurring in such algae as Oedo- 

 gonium, where the zygote gives rise to four meiospores. A second stage 

 might be represented by Coleochacte, where the zygote, after undergoing 

 meiosis, forms a small group of spore-producing cells (up to 32). In a 

 third stage, as seen in Riccia, the zygote develops into a very simple spo- 

 rophyte in which meiosis is delayed until spores are formed. In the evo- 

 lution of the plant kingdom, such a delay may have taken place in a single 

 step by mutation. 



Where the zygote becomes a thick-walled resting cell, its nucleus divides 

 reductionally upon germination. This is a feature only of fresh-water 

 green algae. Where the zygote germinates at once, it gives rise to a 

 diploid vegetative body. This occurs in the marine green algae, in the 

 brown algae, and in most of the red algae. ^ In most algae with a diploid 

 vegetative phase, there is a distinct alternation of generations, exceptions 

 being such forms as Acetabularia, Codium, Bryopsis, and the Fucales. 



It is apparent that the behavior of the zygote is related to environ- 

 mental conditions. Seasonal variations are more pronounced in bodies 

 of fresh water than in the ocean. A resting zygote, usually formed near 

 the close of the growing season, carries the plant over a period of unfavor- 

 able conditions. Algae with a resting zygote display no diploid vegeta- 

 tive phase. Thus the origin of alternation of generations may be sought 

 in a determination of the factors that induce prompt germination of the 

 zygote with an accompanying postponement of meiosis. Perhaps these 

 factors have been responsible for a mutation that has resulted in the 

 establishment of a diploid generation. 



Alternation of generations, once established in a group of plants, con- 

 ferred such advantages that it would tend to be retained as a permanent 

 feature. These advantages are: (1) An increase in the number of individ- 

 uals produced as a result of a single gametic union, thereby conferring on 

 them any beneficial results of such a union which, in many of the lower 

 plants, sometimes occurs with a great deal of rarity. Instead of produc- 



1 In the red algae the zygote always germinates promptly, but subsequent stages are 

 variable and complicated by the formation of carpospores. These are haploid in the 

 two lowest orders, where the zygote is the only diploid cell in the life cycle, and diploid 

 in the other orders, where they give rise to sporophytes. 



