190 TEXTBOOK OF BOTANY 



when the potato plant is exposed to a very low concentration of carbon 

 dioxide. 



As a rule there is a much greater growth of tops in proportion to the 

 growth of roots when plants in a moist climate are exposed to a high 

 temperature, dense shade, high nitrate supply in the soil, or to a low con- 

 centration of carbon dioxide in the air. On the contrary, there is a rela- 

 tively greater growth of roots in proportion to tops when the supply of 

 water or of salts of nitrogen in the soil is moderately low ( Fig. 67 ) . 



Fig. 67. Relative growth of tops and roots of squash and wheat seedlings that 

 grew in culture solutions without and with added nitrogen (N). Photos from 

 Mary E. Reid, Boyce Thompson Institute. 



The amount of food that accumulates in roots and underground stems 

 is influenced by the environmental conditions to which the tops of the 

 plants are exposed. 



Bluegrass in the shade of trees in a lawn that is frequently watered, 

 treated with ammonium sulfate, and mowed during the warmest months 

 of summer may not survive as well under this treatment as bluegrass 

 that is neither watered nor fertilized during this part of the season. 



The practice of pruning and pollarding trees checks the growth of 

 both tops and roots, but the growth of the roots is checked more than 

 that of the tops. 



In dry climates alfalfa is mowed 3 to 5 times in a season, but if it is 

 mowed as frequently as this in moist climates most of the plants die 

 within a year or two. 



