PHYCOMYCETES 23 



peculiar type of sporangial germination present in Pythium, 

 emphasizes the fact that the sexual organs of the Oomycetes 

 in general function quite differently from those of the siphon- 

 aceous green algae, argues that the Ascomycetes and Basidio- 

 mycetes are connected with the higher Phycomycetes through 

 such forms as Endogone and Dipodascus, and combats the point 

 of view that the higher fungi arose from the red algae. The 

 student interested in the detailed argument is referred to the 

 papers of Atkinson and to others cited in his bibliographies. 

 An adequate elaboration of the subject here does not seem desir- 

 able. The writer, who was a student under Atkinson, is incHned 

 to accept in the main his point of view on the phylogeny of the 

 fungi, and feels, moreover, that, as more critical and comprehen- 

 sive work is done on the various groups of lower organisms, a 

 greater body of evidence will be assembled justifying it. 



Thallus. — GeneraHzations concerning the morphology of the 

 Phycomycetes are difficult on account of the great diversity of 

 the group. The vegetative portion of the plant in the primitive 

 members of the class is inconspicuous and simple. In the 

 higher forms it develops into a profusely branching and extensive 

 mycelium. Though the mycelium in the group as a whole is 

 characteristically coenocytic (i.e., non-septate and plurinucleate), 

 in some of the higher families septa are commonly present. In 

 the Mucorales they are found usually only in old hyphae. In 

 the Entomophthorales their formation occurs in most forms 

 rather early. Even in typically coenocytic thalh septa are 

 almost invariably formed in the delimitation of the reproductive 

 cells (sporangia, antheridia, and oogonia). In certain lower 

 groups {e.g., Cladochytriaceae) the mycehum is often evanescent. 

 In the Entomophthorales it commonly falls apart at the septa 

 into the component cells. 



When the mycelium is coenocytic it is considered by many 

 writers to be a single cell regardless of its extent or the profusion 

 of its branching. Others regard each nucleus with the immedi- 

 ately adjacent enveloping cytoplasm as constituting a cell. An 

 interesting application of the latter point of view is seen in the 

 use of the term gametes for the nuclei of the plurinucleate 

 sexual organs (coenogametes) of this group. Regardless of 

 the interpretation accepted, it is known that the cytoplasm and 

 nuclei commonly flow throughout the thallus, and there is 

 apparently no basis for an assumption that definite portions of 



