PLANT PHYSIOLOGY. 



97 



to total darkness, while the leaves are small and lacking in normal 

 color. 



(^■) Cover in like manner a portion of a cucumber-plant bearing 

 very young fruit. Notice that the fruit develops in darkness as well 

 — in size, at least — as in the light. 



(k) Grow some seedlings in full light and others in darkness, and 

 note that the latter are the longer. 



(I) Use an auxanometer (Fig. 50) for measuring the growth of 

 plants, and compare the day growth with the night growth. 



171. Gravitation. — Many cells always grow in a partic- 

 ular direction with respect to the earth's mass. Thus the 

 principal roots usually grow toward the earth, while most 

 stems grow away from it. When a seed germinates, its 

 roots invariably take a downward and its stems an upward 

 direction, and it does this regardless of its immediate sur- 

 roundings. This is well illustrated in the experiment 

 shown in Fig. 51, in which the stems invariably grow up- 

 ward, deeper and deeper into the ground and darkness, 

 while the roots grow down, out of the ground, and into 



Fig. 51. Fig. 52. 



Fig. 51.— Inverted flower-pot under a bell-jar. One tenth natural size. 

 Fig. 52.— Rotating apparatus, s, steel rod bearmg a pulley by means of 

 which it is rotated. One fifth natural size. 



the light. Experiments show that centrifugal force acts 

 precisely like gravitation. If we rotate a growing seed 



