98 PLANT MORPHOLOGY 



in sporangia. The sporangia remain unicellular but the gametangiaof 

 some algae have become multicellular by the formation of cross walls, 

 as in Ectocarpus and Cidleria. In Diciyota the antheridia are multicel- 

 lular and the oogonia are unicellular. 



Alternation of Generations. In nearly all the green algae the vegeta- 

 tive plant, of which there is but one kind, gives rise to gametes and is 

 haploid. Here the diploid condition, which always results from fertiliza- 

 tion, is restricted to the zygote itself, since the reduction of chromosomes 

 takes place when it germinates. This reduction always involves the 

 formation of four haploid nuclei. In Oedogonium each of the four zoo- 

 spores (meiospores) coming from the zygote contains one of these nuclei. 

 In Spirogyra three of the nuclei degenerate and the zygote gives rise 

 directly to a haploid vegetative body. In Coleochaete four haploid cells 

 are formed by the zygote, but each divides one, two, or three more times 

 before zoospores are organized. In the two lower orders of red algae 

 (Bangiales and Nemalionales) an analogous condition exists in the forma- 

 tion of carpospores. 



In some of the algae, notably in the diatoms, Acetahularia, C odium, 

 Bryopsis, and the Fucales, there is only one kind of vegetative body and 

 it is diploid, the reduction of chromosomes occurring in connection with 

 the formation of gametes, or in several nuclear divisions immediately 

 preceding their formation. This is also the condition in animals. 



Some botanists recognize an alternation of generations wherever there 

 is a diploid and a haploid phase in the life history, even though one or 

 the other is represented by only one cell — in other words, wherever there 

 is sexual reproduction. Such a broad use of the term makes it almost 

 meaningless. In algae displaying a true alternation of generations, a 

 more or less prolonged growth phase intervenes between fertilization 

 and meiosis, as well as between meiosis and fertilization. Here the life 

 history involves two distinct and independent vegetative bodies, a 

 haploid body (gametophyte) producing gametes and a diploid body 

 (sporophyte) producing spores. The gametophyte arises from a spore, 

 the sporophyte from a zygote. The reduction of chromosomes occurs 

 when the spores are formed. A true alternation of generations is found 

 in only a very few green algae, such as Ulva and Cladophora, in all brown 

 algae except the Fucales, and in all red algae except the Bangiales and 

 Nemalionales. The alternation may be isomorphic, with both genera- 

 tions alike vegetatively, as in Ulva, Cladophora, Ectocarpus, Dictyota, 

 and Polysiphonia, or it may be heteromorphic, with both generations 

 unlike vegetatively, as in Cutleria and Laminaria. In all algae possessing 

 a true alternation of generations both gametophyte and sporophyte are 

 free-living; one is never dependent upon the other. 



Interrelationships. It is not possible to arrange the classes of algae 

 in such a way as to indicate their true relationship. The secjuence in 



