FUNGI 



189 



t° 



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 as follows 

 The threads of two sexually different plants that 

 lie near together send out short branches, which 

 grow toward each other and finally meet. (Fig. 347.) 

 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 tu- 

 bercles. The zygospore is now mature and, after 

 a period of rest, it germinates, either producing a 

 sporangium directly 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 molds are saprophytes (192). There 

 are many other fungi which are parasitic on living plants and animals. 

 Some of them have interesting and complicated life-histories, under- 

 going 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. 348.) 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 

 leaf and nourishes itself by sending short branches into the cells of the 

 leaf to absorb food-materials from them. 



347. Muoor showing 

 formation of zygo- 

 spore on the right; 

 germinating zygo- 

 spore on the left. 



348. Colonies of willow mildew. 



