174 



THALLOPHYTES 



Among the most interesting of these parasitic fungi are the 

 rusts and smuts, some of which are very destructive to our grain 

 crops. Some of the rusts have the peculiar habit of requiring 

 two distinct host plants in order to complete their life cycle. 

 In such cases they have two or more stages, one occurring on one 

 host and the other on the other host; the spores in each case 

 will, as a rule, grow only on the opposite host plant. 



FIG. 112. Saprolegnia. a, infested fly; b, immature sporangium; c, mature sporangium; 

 d, same after the escape of the spores; e, free swimming spores. 



The fleshy fungi (Fig. 113), many of which are spoken of 

 as mushrooms, and toad-stools, usually grow either parasitically 

 or saprophytically on woody plants. Some are umbrella-shaped, 

 while others are shelving in character and still others are the 

 well-known puff-balls. 



The stipe or stem of the mushroom usually rests in a volva 

 or cup. The stipe supports the pileus, or cap, on the under side 



