THE ROLE OF STUDY OF ALGAE IN DEVELOPMENT OF BOTANY 479 



the two generations are identical in vegetative structure. For the Rhodo- 

 phyceae, Yamanouchi (1906) showed that sexual thalli of Polysiphonia are 

 haploid and that tetrasporic thalli are diploid. Other phycologists soon showed 

 that a similar condition obtains in other Rhodophyceae with thalli producing 

 tetraspores. The alternation thus discovered in Dictyota and in the Rhodo- 

 phyceae differs from that found in bryophytes and other land plants in the 

 following two respects: the two generations are vegetatively identical, and 

 each is independent of the other from the beginning. An alternation in which 

 the two generations are identical in size and vegetative structure has been 

 called isomorphic alternation by Fritsch (1935). The contrasting type where 

 the two differ in size and vegetative structure has been called heteromorphic 

 alternation. Among algae this was first found in the Phaeophyceae and is 

 now known to occur in many of them. Discovery of a heteromorphic life 

 cycle among Phaeophyceae was not made until gametophytes were grown in 

 cultures started from zoospores discharged from unilocular sporangia. The 

 gametophytes developing from these zoospores are always filamentous and 

 of microscopic size. Union of gametes produced by gametophytes results in a 

 diploid sporophyte that is often of macroscopic size and of complex structure. 



A life cycle in which a multicellular haploid generation alternates with a 

 one-celled diploid phase was first found by Allen (1905) in Coleochaete, a 

 member of the Chlorophyceae. With the finding of the same for certain 

 other Chlorophyceae, phycologists tended to make the generalization that 

 this is a characteristic of the entire class. This generalization became invalid 

 when an isomorphic alternation was found in Enteromorpha (Hartmann, 

 1929) and in Cladophora (F0yn, 1929). A similar life cycle is also known 

 for certain other Chlorophyceae. The only reported case of a heteromorphic 

 alternation of multicellular generations among Chlorophyceae is that of 

 Stigeoclonium (Juller, 1937). 



The alternation of generations among algae contributes to an understanding 

 of that among higher plants. Firstly, it shows that evolution of an alternation 

 of generations is not dependent upon migration from an aquatic to a terrestrial 

 environment. Secondly, it shows that an alternation of generations has ap- 

 peared many times instead of but once during the evolution of plants. 

 Thirdly, algae do not show whether the sporophyte arose by modification of 

 the gametophyte, or is to be interpreted as an entirely new generation inter- 

 calated between two successive gametophytic generations. One may have very 

 positive views concerning the relative merits of the modification (homologous) 

 and the intercalation (antithetic) theories of origin of an alternation of gen- 

 erations, but the algae do not furnish conclusive proof for either theory. 



Algae have been of service in the advancement of plant physiology because 

 they are the best experimental material in certain fields of this discipline. 

 Valonia and Halicystis have proven especially suitable for studies on per- 

 meability. These algae have cells that are up to half an inch in diameter and 



