The Kinds of Work That Are Done by Plants 97 



This chapter has already attained to a length so great that I 

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

 matters are connected with 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. , 



FIG. 32. Yeast plants, each a single cell 



A Yeast plant is a Single OVOid which buds out from a parent cell; very 

 I.,, i . . , highly magnified. 



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 when 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 experiment. 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 



