292 



BOTANY: PRINCIPLES AND PROBLEMS 



plants. Their spores germinate on the surface of the leaf or 

 stem and, entering the tissues, branch through them in all direc- 

 tions, absorbing food and often causing the death of the host 

 plant. Definite fruiting bodies are produced at the surface of 

 various organs of the host, where specialized hyphae become 

 constricted to form spores, which separate and are carried away 



Fig. 167. — ^Zfewgo, one of the Peronosporales. A, section through a leaf-blade 

 showing portion of a "blister" produced by the fungus. The hyphae penetrate 

 between the cells of the leaf, sending into them small, sucker-like structures. 

 Under the epidermis are produced rows of conidia. B, sexual organs, the anther- 

 idial filament (at right) discharging a male nucleus into the oogonium, at left. 



by the wind. Non-sexual, aerial spores produced in this manner 

 are called conidia, and are of frequent occurrence among fungi. 

 Sexual organs appear in the deeper tissues. The egg nucleus in 

 the oogonium is fertilized by a male nucleus from an adjacent 

 antheridial filament, and a thick-walled oospore is formed. 

 Such destructive plant parasites as the potato blight and the 

 grapevine mildew belong to this order. 



Ascomycetes or Sac Fungi. — This enormous group of plants 

 includes over 20,000 species. They show but little resemblance 

 to the algae, and although all must have come originally from 

 some chlorophyll-bearing forms, their exact ancestry is not clear. 

 These fungi are typically land-inhabiting plants and include 

 both saprophytic and parasitic species. Both groups differ 

 from the Phycomycetes in the fact that their hyphae are divided 

 into cells by cross-walls, and that sexual processes are much 

 reduced or altogether lacking. The plant body commonly consists 



