138 THE VEGETATIVE PUNCTIONS OF PLANTS 



in an abnormal manner. ^ Under conditions of normal illu- 

 mination (granted, of course, that all other conditions are 

 normal), the physiological processes of the plant are regu- 

 lated in a normal manner; but when the illumination is 

 abnormal, healthy regulation ceases, and the organ be- 

 haves abnormally. This abnormal behavior includes the 

 failure of chlorophyll to develop, and irregularities of 

 growth. With many plants the stems grow abnormally 

 long and slender, suggesting that absence of light favors 

 more rapid growth in length. 



Fig. g6. — Potted plants of an oxalis showing the position of the leaflets 

 during the day {A) and the night {B) — the so-called "sleep" of plants. 

 (Photo by the author.) 



134. Relation of Leaves to Light. — More than all other 

 organs of the plant, the foKage-leaf is developed and ad- 

 justed with reference to illumination. Its form, dimen- 

 sions, and internal structure, and its attitude and posi- 

 tion on the stem are chiefly expressions of the surround- 



» It is often stated in "popular" writings that stems grown in darkness' 

 "reach for" or "seek" the light. A careful consideration by the student 

 of all that these terms imply, when predicated of a plant, will lead at once 

 to a recognition of their incorrectness, and even of their absurdity. 



