222 



STRUCTURAL BOTANY 



kind in this manner, under such conditions as prevail 



in nature when the weather is at all dry. 



The great majority of the 

 Fungi, however, are adapted 

 to the same conditions of 

 life as the ordinary land 

 plants, on which so many of 

 them are parasitic, and this 

 implies that their reproduc- 

 tive bodies are fitted for 

 dissemination through com- 

 paratively dry air. In 

 Pythium and among its near 

 allies we can trace the steps 

 by which this adaptation to 

 an aerial environment has 

 been attained. In some 

 species of Pytliium, as, for 

 example, in the species P. 

 P>aryanum, which is so 

 common on Cress-seedlings, 

 it sometimes happens that 



FIG. w.pytuum. A, branch the sporangium does not 



of the mycelium, bearing three 



zoosporangia (s). Magnified torni ZOOSpores at all, but 



145 B, zoosporangium (s) grows out directly into a 



discharging its contents (6), J 



which are still enclosed in the hypha, thus Starting a new 



saffflSas-t. to. 1 *: p lant at . nce > without the 



zoospores. Magnified 145. c, intervention of the active 



germinating oospore (osp) form- rp l] q 



Lb> 



ing an asexual sporangium (s). 



Magnified 300. (After De this allows of propagation 



Bary.) , . , , 



taking place, even though 



there should not be water enough to float the zoospores. 

 The same thing happens in the closely-allied genus 



