PHYLOGENY 



Figure 35. Ulothrix (A), Drapamaldia (B), and Ulva (C). (Redrawn from Fuller 

 and Tippo, "College Botany," 2nd Ed., Henry Holt & Co., 1954.) 



adult plants. Thus most of the life cycle of Ulothrix is passed with only 

 the haploid chromosome complement, while the zygote alone is diploid. 



Drapamaldia is advanced over Ulothrix principally by the more com- 

 plex development of the vegetative body. While the latter is composed 

 only of simple, unbranched filaments, the former has a major, basal fila- 

 ment from which many branches arise, and from these there are secondary 

 branches. Reproduction again is accomplished either by zoospores or by 

 isogametes. 



Ulva, the common sea lettuce, is advanced over the previously discussed 

 algae in several ways. The vegetative body forms large, leaf-like sheets 

 which, for the first time, are more than one cell thick. Reproduction is 

 again either by zoospores or by isogametes. But there is a significant de- 

 velopment in the life cycle. In all of the algae discussed above, the reduc- 

 tion divisions occur in the zygote, with the result that in the whole life 

 cycle only the zygote is diploid, in Ulva, however, the divisions of the 

 zygote are ordinarv mitotic divisions, with the result that a dij^loid plant 

 is formed. Some of the cc>lls of the adult plant tluMi undcMgo the meiotic 

 divisions, which result in the formation ol haploid z()()spor(\s. This diploid 

 plant which reproduces by the formation of haploid zoospores is called 

 a sporophijfc. The spores then develop into haploid plants much as do the 

 spores of other algae. These haploid plants then reproduce l)v isogamy. 

 This haploid, gamet(>-forming jilant is called a (i^amclophijlc. The sporo- 

 phytic and gametophytic generations of Ulva are morphologically indis- 



lOS 



