272 



BOTANY: PRINCIPLES AND PROBLEMS 



dia and oogonia are formed much as in Oedogonium, and a 

 thick-walled oospore is produced. Following this, however, the 

 adjacent cells give rise to branches which grow up and surround 

 the oospore, forming a distinct spore case or fruiting body. The 

 oospore germinates into a group of cells each of which ultimately 

 forms a zoospore. This reproductive cycle foreshadows the 

 "alternation of generations" of the higher plants. In its struc- 



r^ m L 



Fig. 150. — Coleochaete. A, portion of fertile thallus. og, oogonium, contain- 

 ing a single large egg. an, antheridium. s, sperm, o, oospore or fertilized egg, 

 which is beginning to be surrounded by filaments growing up from below. B, 

 mature fructification, the oospore surrounded by an envelope of cells. C, the 

 contents of the fructification dividing up into zoospores. D, zoospores. (From 

 Goebel, after Pringsheim) . 



ture and life history, Coleochaete is the most specialized of the 

 green algae and is believed by many botanists to approach the 

 lowest bryophytes. 



3. Siphonales or Tubular Algae. — These are distinguished 

 from all other algae by the fact that the whole plant body, 

 whether it be a simple filament or a well-differentiated thallus, is 

 essentially a single cell. The cross walls which divide other 

 algae and all ordinary plants into small cells are absent, and 

 the mass of cytoplasm with its thousands of nuclei is therefore 

 able to circulate freely throughout the whole plant. Such a 

 multinucleate cell, of which these plants are extreme examples, is 

 known as a coenocyte. 



The Siphonales are chiefly marine forms, especially abundant 

 in the warmer seas. They usually produce zoospores and in 



