190 PLANT BIOLOGY 



grandis) may be frequently found in summer growing on toad- 

 stools. This plant usually produces zygospores that 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. 



Yeast. — This is a very much reduced and simple fungus, con- 

 sisting normally of isolated spherical or elliptical cells (Fig. 275) 

 containing abundant protoplasm and prob- 

 ably a nucleus, although the latter is not 

 easily observed. It propagates rapidly by 

 budding, which consists of the gradual extru- 

 sion of a wart-like swelling that is sooner or 

 later cut off at the base by constriction, thus 

 forming a separate organism. Although sim- 

 Fig 27=; —Yeast I^ e m structure > the yeast is found to be 

 ' Plants closely related to some of the higher groups of 



fungi as shown by the method of spore forma- 

 tion. When grown on special substances like potato or carrot, the 

 contents of the cell may form spores inside of the sac-like mother 

 cell, thus resembling the sac-fungi to which blue mold and mildews 

 belong. The yeast plant is remarkable on account of its power to 

 induce alcoholic fermentation in the media in which it grows. 



There are many kinds of yeasts. One of them is found in the 

 common yeast cakes. In the process of manufacture of these 

 cakes, the yeast cells grow to a certain stage, and the material is 

 then dried and fashioned into small cakes, each cake containing 

 great numbers of the yeast cells. When the yeast cake is added 

 to dough, and proper conditions of warmth and moisture are pro- 

 vided, the yeast grows rapidly and breaks up the sugar of the 

 dough into carbon dioxid and alcohol. This is fermentation. 

 The gases escape and puff up the dough, causing the bread to rise. 

 In this loosened condition the dough is baked ; if it is not baked 

 quickly enough, the bread "falls." Shake up a bit of yeast cake 

 in slightly sweetened water : the water soon becomes cloudy from 

 the growing yeasts. 



Parasitic fungi. — Most of the molds are saprophytes. Many 

 other fungi are parasitic on living plants and animals (Fig. 285). 

 Some of them have complicated life histories, 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). — This is one of the sac 

 fungi. It forms white downy patches on the leaves of willows 



