106 PLANT PHYSIOLOGY 
pressure necessary to take in the requisite large quanti- 
ties of water that increase the bulk of the cell so greatly 
during the phase of enlargement. Then sufficient food 
substances must also be present to supply in the process 
of respiration the energy necessary for growth. Further- 
more the water supply must be ample, for growth ceases 
immediately if the cells of the plant are not kept strongly 
turgid, hence the reason that in a dry season a plant may 
remain alive for months on a minimum of water, but 
scarcely grow at all. The temperature also has a 
marked influence on growth. The cardinal points of 
temperature for growth are often quite different from 
those for photosynthesis or respiration in the same plant. | 
In some plants that come up through the snow the 
optimum temperature for growth may be but little 
above 0° C., while in Indian corn, for example, the opti- 
mum lies between 37° and 42° C. 
151. The effect of light upon growth is noteworthy. 
Careful records of the rate of growth with automatically 
recording instruments show that, given constant tem- 
perature, the growth is much more rapid in darkness 
than in light. If the rays from the blue end of the 
spectrum are excluded growth is scarcely if at all checked 
by light. The absence of light, however, although favor- 
ing the elongation of the plant, prevents the normal form- 
ation of leaves. This is possibly due in part to lack of 
food, but it seems probable that a definite stimulus on the 
part of light is needed before leaves will be produced in 
thenormalform and size. Plants kept in the dark become 
much elongated (remaining pale in color) with only rudi- 
ments of leaves. Such plants are said to be etiolated. 
To a certain degree this is useful to a plant in that a tuber 
or seed buried too deep produces an abnormally elongated 
shoot which may thus be able to reach the light. 
