2/O GENERAL MORPHOLOGY OF PLANTS 



place. The protoplasm in a yeast cell condenses into three or 

 four small globose shining bodies which are retained in the yeast 

 cell, the nucleus having previously been divided into four nuclei. 

 The yeast cell with the enclosed spores is regarded as an ascus. 

 The yeast plant, however, is supposed to be a degenerate form, 

 for some of the other sac .fungi as well as some of the spore-case 

 fungi (or alga-like fungi) under certain conditions degenerate into 

 yeast forms. No one, however, has ever succeeded in changing 

 the true yeasts into any of these better organized forms, though 

 some yeasts form a temporary mycelium which later breaks up 

 into yeast cells. The yeast used in bread " rising " and in brewing 

 beer is now a domestic or industrial form. There are, however, 

 many wild yeasts, which are abundant on all sorts of vegetables 

 and multiply in decaying parts. Some of these wild yeasts are 

 associated with certain bacteria, forming little nodules which have 

 been used by natives of certain countries in making fermented 

 drinks, as " ginger beer " from the " ginger- beer " plant, or 

 " kumiss " from mare's milk fermented by throwing in some of 

 these grains, which is used by some of the tribes of the Caucasus. 



The Lichens. 



432. Nature of lichens. The lichens are curious structures 

 composed of the elements of two different kinds of plants, a fungus 

 and an alga. The plant body of the lichen is made up of fungus 

 threads in the meshes of which the algae are enclosed. Many of 

 the lichens are greenish in color, this color being imparted to the 

 lichen thallus by the algal cells underneath the outer layer of 

 fungus threads. Others are brown, reddish, etc. Their method 

 of nutrition is interesting and illustrates one of the forms of 

 symbiosis (paragraph 206). The algal cells perform the function 

 of photosynthesis, so that the fungus element as well as the algal 

 is provided with the necessary carbohydrates. The fungus is 

 thus entirely dependent on the alga for its organic or carbohydrate 

 food. On the other hand the fungus, being external in most cases, 

 and forming the " rhizoids " and holdfasts, supplies the solution 



