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 

 the normal form 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. 



