72 EXPERIMENTAL PLANT PHYSIOLOGY. 



and about the time the previous one has begun to lessen its rapidity 

 of growth, it has reached its maximum. In this manner the zone ol' 

 maximum growth progresses. (Fig. 66.) 



EXPERIMENT 90. 



ZONE OF MAXIMUM GROWTH OF STEMS. 



Growth of stems may be observed in the same manner as in the 

 last experiment. The elongation of the natural divisions, the inter- 

 nodes, can be measured and compared with one another. The 

 elongating part is greater than in roots — 35 millimeters in the Bean. 

 The internodes vary in length ; the middle ones are the longest. 

 Satisfactory results may be attained by the measurement of centi- 

 meter intervals on the stem of Bean or Corn. Compare movement 

 of zone of maximum growth with that in roots. 



EXPERIMENT 91. 



ZONE OF MAXIMUM GROWTH OF LEAVES. 



Cultivate Gourd or Tobacco plants in large pots, and after some 

 leaves have been formed place them under large bell-jars, and set 

 in light, but not in direct sun light, in a temperature as nearly 

 constant as possible. Before doing this mark off on the petiole or 

 midrib of a young leaf a scale as above. Compare observations 

 with results of above experiments. 



54. Influence of Light on Growth. — While light is necessary 

 for the formation of food by photosynthesis, and for the per- 

 formance of certain other functions, it at the same time 

 generally retards growth. Only the blue-violet end of the 

 spectrum exercises this retarding influence. By reason of 

 this influence the maximum growth of a great number of plants 

 occurs after they have been deprived of light for the longest 

 period, which is in the morning, or just before daylight. 

 Temperature is generally more favorable to growth during the 

 afternoon, and as a consequence the plant grows rapidly at 

 this time also. In fact the maximum growth often occurs then. 



