[Chap. VI THE INTERPRETATION OF PLANT BEHAVIOR 53 



as if the processes were occurring to meet the needs and desires of the 

 plants and animals. 



During the last quarter-century great advance has been made in 

 analyzing the chemical and physical phenomena that underlie plant 

 behavior and plant processes. Much remains to be discovered in this 

 field, but a broad foundation has certainly been built, mostly by those 

 who have applied the principles of chemistry and physics to biological 

 processes and have attacked problems from the point of view of causal 

 sequences. 



A little better understanding of this point of view may be obtained 

 if we briefly sketch one of the examples of the effect of light on growth. 



Fig. 27. Potato sprouts that have grown from tubers in diflFerent light intensities: 

 1, in a greenhouse; 2, in a darkroom. Photo by F. H. Norris. 



