The Kinds of Work That Are Done by Plants 97 



This chapter has ah-eady attained to a length so great that I 

 wish it were possible to end it right here. But certain additional 

 matters are connected wdth respiration so closely, and are be- 

 sides in themselves so important, that we must really keep on to 

 include them, though perhaps the reader will find it best to defer 

 a reading thereof for another occasion. These matters are fer- 

 mentation, decay, and disease. 



Fermentation is a phenomenon familiar to all, and best known, 

 perhaps, in the ''working" of preserves, which become ''strong" 

 i. e. alcoholic, while giving off tiny 

 bubbles of gas. The most typical 



kind of fermentation is that caused ""^^^ ' -^ 



by Yeast. Yeast, I venture to 

 remind the reader, is a very tiny >^ ^^ 



non-green plant which lives as a 

 saprophyte in sweet liquids. Mag- ( \ 



nified to a high degree by the mi- 

 croscope it looks much like our 

 picture (figure 32), though whiter, x- 00 v ^ i . u • , ,, 



■^ ^ ^ ' ' ° Fig. 32. — i east plants, each a single cell 



A Yeast plant is a single ovoid ^v■hich buds out from a parent cell; very 



cell which buds out into others, 



and these into others, in loose chains which fall easily apart, — 

 and so on, as long as the food supply lasts. And that is all, 

 except that when the liquid dries up, the cells produce very 

 thick-walled spores which float around in the air with the dust, 

 to start once more wiien they happen to fall into another sweet 

 liquid. It is by the growth of these cells that a sweet liquid is 

 "fermented" with a formation of alcohol and carbon dioxide. 

 This can be demonstrated very easily and clearly to the eye by 

 an interesting expermient. If one puts together in a glass flask 

 a solution of sugar and a cake of compressed (not dried) yeast, 

 and stands it in a warmish place, then within a very few 

 minutes tiny bubbles of gas begin to rise through the liquid, 

 producing a froth on its surface. If, now, the stopper of the flask 



