312 THE ALGAE 



partially coenocytic forms on the otiier are shown in scheme D 

 (Chapman, 1952, 1954). In tiie Siphonocladalean Hne the position 

 of Urospora with macroscopic gametophyte and dwarf sporophytic 

 Codiolum stage is of interest because from that one can see how 

 forms with heteromorphic generations could have arisen, e.g. 

 Spongomorpha. The other feature is the appearance of heterotrichy 

 in the parenchymatous group, and also in the most advanced forms 

 the disappearance of the haploid generation {Enter omorpha nana 

 and E. procera). There is also the disappearance of the haploid 

 generation associated with development of the vesicular forms of 

 the Siphonocladales. In the development of the Chlorophyceae as 

 represented by Scheme A it is important to note the development 

 of the heterotrichous habit in the more advanced members, and 

 also, commencing with plants that are universally haploid, there is 

 the intercalation of a sporophytic generation and in some cases the 

 disappearance of the gametophytic generation. The concept of the 

 heterotrichous habit was first advanced by Fritsch in 1929, and it is 

 clear that an appreciation of this habit is important in imderstand- 

 ing evolutionary problems among the algae. The major fact is that 

 the most advanced Chlorophyceae exhibit the heterotrichous con- 

 dition in a primitive state, whilst this state is found fully developed 

 only among the simpler Rhodophyceae and Phaeophyceae. 



A study of the life cycles within the Chlorophyceae (Fig. 186) 

 shows that there are three principal types with intermediates : 



(i) A unicellular or multicellular haploid generation with a 

 short-Uved unicellular diploid phase {ChlamydomonaSy Vol- 

 vox), 



(2) An alternation between morphologically identical diploid 

 and haploid generations (Ulva, Enteromorpha, Cladophora). 



(3) A multicellular or coenocytic diploid generation in which the 

 gametophyte has been eliminated, e.g. Siphonales. 



In the primitive condition if the haploid plants were monoecious 

 the first stage in evolution would presumably have been the develop- 

 ment of the dioecious condition; this may then well have been 

 followed by the intercalation of a sporophyte generation through 

 delay in occurrence of the reduction division (e.g. Monostroma) 

 leading to alternation of two generations. Recentiy Feldmann 

 (1952) has put forward the view that the primitive life cycle in all 

 the Euphycophyta is one in which there is alternation of two like 

 generations. Whilst this may be true for the Phaeophyceae and 



