FACTORS OF ENVIRONMENT. 481 



the nitrate and nitrite bacteria, etc., which prepare food for the 

 higher plants. 



940. The responsive factor in plants. The power which the 

 plant possesses to relate itself to environment, and to undergo 

 slight changes in form, texture, etc., which adapt it better to 



Fig. 4870. 

 Willow trees pollarded by man 



different conditions, is a most important factor. This is shown in 

 the power of growth and response to environment in the assump- 

 tion of favorable positions by its different parts from the seedling 

 stage onward. This response of plants to environment is further 

 manifested in the diurnal and nocturnal movement of leaves, the 

 nutation of stems, etc. 



