62 PLANT MORPHOLOGY 



occurs by fission (in some unicellular forms), fragmentation, or by zoo- 

 spores, aplanospores, and akinetes. Sexual reproduction is either isoga- 

 mous or heterogamous. In the heterogamous forms the sperms are 

 ciliated, the eggs nearly always noncihated. Motile reproductive cells 

 generally have two or four cilia, equal in length and apically attached. 

 The zygote nearly always becomes a resting cell. 



Within the Chlorophyceae, three main evolutionary trends can be 

 recognized. The occurrence of ciliated reproductive cells in practically 

 all members, except the Conjugales, indicates that the common ancestor 

 of the group must have been a form like Chlamydomonas. The Volvo- 

 cales, retaining motility in vegetative cells, represent one hne of evolution. 

 It emphasizes the ciliated colonial type of organization that culminates in 



Volvox. 



A second line of development, represented by the Chlorococcales, also 

 emphasizes the colony but shows a loss of motility by the vegetative cells. 

 A tendency toward the formation of multinucleate cells appears in this 

 order. This leads to the development of coenocytic bodies, which 

 reaches a climax in the Siphonales. Protosiphon is a connecting link 

 between the Chlorococcales and Siphonales. Some regard the Siphono- 

 cladiales as a transitional stage leading to the evolution of the Siphonales; 

 others consider them an offshoot of that order, the incomplete formation 

 of walls being a recent development. Still others think that at least some 

 of the Siphonocladiales have arisen from the Ulotrichales. 



A third line of development within the Chlorophyceae is represented 

 by the Ulotrichales, an order in which several different types of multi- 

 cellular bodies have appeared. All of these grow by division of uni- 

 nucleate vegetative cells. The Oedogoniales may represent an offshoot 

 from this order, but a connection between the Conjugales and the 

 Ulotrichales seems rather remote, the lack of ciliated cells and pecuHar 

 type of sexual reproduction in the Conjugales being the chief obstacles. 

 The Ulotrichales are of great interest in being the order of green algae 

 most closely resembling the probable ancestors of the higher green plants. 

 The occurrence of plate-hke forms is particularly significant, inasmuch 

 as the vegetative body of the simpler bryophytes is a plate-like thallus. 



