182 Perspecfives in Microbiology 



excretion of either the toxic materials that had been trans- 

 located, or through other organic materials excreted in 

 response to effects of the fungicides on plant development. 

 Some of the substances decreased the numbers of bacteria 

 in the rhizosphere, whereas others increased them. 



More definite evidence of movement of organic ma- 

 terials through plants was recently reported by Preston, 

 Mitchell, and Reeve (32). When the tops of certain plants 

 were treated with a-methoxyphenylacetic acid and the 

 treated plants were grown in the same pot with untreated 

 plants, the chemical moved downward through the treated 

 plants, out of the roots, and was taken up by the others. 

 When grown in solutions, the treated plants liberated the 

 chemical into the water. The effect was noted only with 

 this one plant-growth-modifying compound and not with 

 others, yet it establishes the possibility of the excretion 

 through roots of organic materials applied to aerial plant 

 parts. Winter (47) has included the following among the 

 organic materials excreted by plant roots: phosphatides, 

 amino acids, thiamin, biotin, mesoinositol, para-amino- 

 benzoic acid, carbohydrates, tannins, and alkaloids. Harley 

 (15) listed sugars, amino acids, vitamins, and other organic 

 substances as materials excreted by roots. The total number 

 of excreted substances must be large. Obviously, more 

 information on the movement of organic materials from 

 plant roots is needed. 



Under conditions of high air humidity and low content 

 of soil moisture, the plant may affect microbial develop- 

 ment in still another way. Breazeale and McGeorge (1) 

 reported that, under these conditions, water is transported 

 from the air through the plant to the rhizosphere, and in 

 this way the soil moisture content can be increased. In 

 one of their experiments, tomato plants grown in soil in 

 which the water content had been reduced to the point 

 where the plants began to wilt were placed in an atmos- 

 phere of 80 to 90 per cent humidity. The plants recovered 



