182 



STUDIES IN CRYPTOGAMS 



means of carrying itself over unfavorable seasons, as winter. This is 

 accomplished by means of curious thick-walled resting-spores or zygo- 

 spores. The zygospores are formed on the mycelium buried within 

 the substance on which the plant grows. They originate in 

 the following manner : Two threads which lie near to- 

 gether send out short branches, which grow toward each 

 other and finally meet (Fig. 320). The walls at 

 the ends, a, then disappear, allowing the contents 

 to flow together. At the same time, however, two 

 other walls are formed at points farther back, b, b, 

 separating the short section, c, from the remainder 

 of the thread. This section now increases in size 

 and becomes covered with a thick, dark brown wall 

 ornamented with thickened tubercles. The zygo- 

 spore is now mature and, after a period of rest, 

 it germinates, either producing a sporangium di- 

 rectly or growing out as mycelium. 



The zygospores of the mucors form one of the 

 most interesting and instructive objects among the 

 lower plants. They are, however, very difficult to 

 obtain. One of the mucors, Sporodinia grandis, 

 may be frequently found in summer growing on 

 toadstools. This plant usually produces zygospores, 

 which are formed on the aerial mycelium. The zygospores are large 

 enough to be recognized with a hand-lens. The material may be 

 dried and kept for winter study, or the zygospores may be prepared 

 for permanent microscopic mounts in the ordinary way. 



Willow mildew. Most of the moulds are saprophytes (181). 

 There are many other fungi which are parasitic on living plants and 

 animals. Some of them have interesting and complicated life-his- 

 tories, undergoing many changes before the original spore is again 

 produced. The willow 

 mildew and the common 

 rust of wheat will serve 

 to illustrate the habits of 

 parasitic fungi. 



The willow mildew, 

 Uncinula salicis, forms 

 white downy patches on 



the leaves of willows (Fig. 321). These patches consist of numer- 

 ous interwoven threads which may be recognized as the mycelium 

 of the fungus. The mycelium in this case lives on the surface of the 



320. Mucor showing 

 formation of zygo- 

 spore on the right ; 

 germinating zygo- 

 spore on the left. 



321. Colonies of willow mildew. 



