THE VEGETATIVE FUNCTIONS OF PLANTS 
in an abnormal manner. 1 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. 96. Potted plants of an oxalis showing the position of the leaflets 
during the day (A) and the night (5) the so-called "sleep" of plants. 
134. Relation of Leaves to Light. More than all other 
organs of the plant, the foliage-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- 
1 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. 
