54 AGRICULTURAL BACTERIOLOGY 



Fig. 17. Effect of Acid on Bacteria and Yeasts 



The lighter portion of the plate contains an acid agar, the darker portion a 



neutral agar. Threads carrying bacteria and yeast respectively were placed 



on the plate. The bacteria on the lower thread grew only on the neutral agar 



while the yeast grew on both but more profusely on the acid medium 



and also in those that are quite acid, as in fruit juices. 

 The molds will tolerate a still greater acidity. 



Light. One ordinarily thinks of light as favorable to 

 the development of animals. It is, of course, essential for 

 the growth of the green plants, since from it they secure 

 their energy. Any portion of the animal body not pro- 

 tected by a covering or by a pigment in the cells will be 

 quickly injured by the direct rays of the sun. Sunburn is 

 evidence of this action. By the development of pigment 

 or the darkening of the skin, the injurious action of light 

 is prevented. The cells of microorganisms are likewise 

 easily destroyed by the direct rays of the sun. Artificial 



