636 ETCHING OF MARBLE BY ROOTS 



It seems probable that the greater etching power of the roots of 

 pea plants in the presence of soil bacteria can be attributed to the 

 normal carbon dioxide excretion of the living root cells, together with 

 carbonic and other acids set free from dead or dying root cells whose 

 decomposition is accelerated by the presence of the bacteria. While 



A. 1^ 



Fig. 2. A, plant rools without bacteria; B, plant roots with bacteria. 



the results indicate that the dissolving power of roots is in part due 

 to the action of the bacteria, it must be admitted that there are many 

 factors involved. For example, the food supply of a plant must be 

 different in the presence of bacteria from that of a plant in soil free 

 of bacteria. 



