EVOLUTION OF THE PLANT KINGDOM 423 



phytes the life cycle includes only one kind of individual. In most green 

 algae the vegetative body is haploid and meiosis occurs in connection with 

 the germination of the zygote. Here a prolonged haploid growth phase 

 alternates with a single diploid cell, the zygote. Although the zygote 

 usually gives rise to four spores, it could hardly be regarded as a sporo- 

 phyte, and so there is no true alternation of generations. 



In many diatoms, Siphonocladiales, Siphonales, and in all the Fucales 

 the vegetative body is diploid, meiosis occurring when gametes are formed, 

 as in animals. Here a prolonged diploid growth phase alternates with a 

 few haploid cells, the gametes, and again there is no true alternation of 

 generations. Thus, where the life cycle includes only one vegetative 

 phase, this may be either haploid, meiosis directly following fertilization, 

 or diploid, meiosis immediately preceding fertilization. 



An alternation of generations occurs wherever a diploid growth phase 

 intervenes between fertilization and meiosis, and a haploid growth phase 

 between meiosis and fertilization. Such a condition is displayed by Ulva 

 and a few other green algae, by most brown and red algae, and by all 

 bryophytes and pteridophytes. It is also characteristic of the spermato- 

 phytes, although in angiosperms the haploid phase (gametophyte) con- 

 sists of only a very few cells or nuclei. Obviously this is a result of reduc- 

 tion, while in Fucus, for example, where a somewhat similar condition 

 prevails, reduction may have taken place but evidence is lacking. 



In Fucus, where spores are absent from the life cycle, a diploid plant 

 body produces gametes. In Ulothrix and Oedogonium, on the other hand, 

 a haploid body may produce either gametes or spores. Such spores are 

 vegetative spores and are not homologous with the four spores derived 

 from the zygote or with the spores borne by the sporophyte of the higher 

 plants, both of which are meiospores and belong to the sexual life cycle. 

 Either a haploid or a diploid plant body may be propagated vegetatively 

 by various means without fertilization and meiosis being involved. Veg- 

 etative reproduction is always asexual. 



Origin of Alternation of Generations. Regarding the origin of alterna- 

 tions, two different theories have been advanced, the homologous theory 

 and the antithetic theory. The homologous theory was based originally 

 on conditions in those algae in which a single plant can produce either 

 spores or gametes. It assumes that these functions later became sep- 

 arated into two distinct individuals, sporophyte and gametophyte, alter- 

 nating regularly in the life cycle. The theory furnishes a more adequate 

 explanation of alternation where the two generations are alike vegeta- 

 tively than w^here they are unlike. Against it may be urged the fact that 

 vegetative spores produced by a haploid plant body and meiospores pro- 

 duced by a zygote or a sporophyte are not morphologically equivalent. 

 Also, the homologous theory fails to account for the difference in chromo- 



