THE THALLOPHYTA 



29] 



zygospores arc usually produced only when a plus plant and 

 a viinus plant come into contact. These fungi furnish an 

 interesting example of a physiological differentiation of sex 

 which is not accompanied by morphological differences. 



2. Saprolegtiiales or Water Molds (Fig. 16G).— In contrast 

 to the Mucorales, this group is entirely aquatic. Its members 

 live on the bodies of dead insects and other animals, and a few 

 are parasitic, attacking fish and amphibians. A cell cut off from 



Fig. 166. — Saprolegnia, one of the Saprolegtiiales. A, mycelium on an insect 

 in water. B, terminal sporangium producing zoospores. C, zoospores. D, 

 sexual organs; the two oogonia each containing several eggs. The one at the 

 left penetrated by antheridial filaments. 



the tip of a hypha develops into a sporangium and liberates a 

 large number of motile zoospores. Sexual reproduction is heter- 

 ogamous. A single-celled spherical oogonium is developed 

 and produces several eggs which are fertilized by the contents 

 of an antheridial filament, a slender branch which grows out from 

 the main hypha near the oogonium and fuses with it. In many 

 cases the egg develops into mature spores without having been 

 fertilized at all. 



Peronosporales or Blighls and Downy Mildcia-i (Fig. 167). — 

 The species composing this group arc all parasites on the higher 



