GROWTH PHENOMENA CONTROLLED BY INTERNAL CONDITIONS 



219 



deliciosa are about 30 to 70 mm. in length, while those of Vitis velutina may 

 exceed 100 mm. The individual parts of the region of elongation in the root 

 show unequal rates of growth. The most rapidly elongating portion lies in the 

 center of the region, while the parts above and below grow more slowly. An 

 experiment in which young roots of Viciafaha seedlings were divided (by India 

 ink lines) into ten zones each a millimeter long, the zones being measured after 

 twenty-four hours, gave the following values for the increments of elongation of 

 the respective zones. The temperature was 20.5°C. and the zones were num- 

 bered from the tip upward. 



Number of 

 Zone 



X. 



IX. 



VIII. 



VII. 



VI. 



V. 



IV. 



III. 



II. 



I. 



Increment of 

 Elongation 

 mm. 

 0.1 

 0.2 

 0.3 

 0.5 

 1-3 

 1.6 



3-S 

 8.2 



S-8 



'-■S 



Fig. ioi. — Three stages in the elongation of a root of Vicia faba. 



In Fig. IOI are shown three stages in the elongation of the primary root of 

 a Vicia faba seedling. A shows the root divided into millimeter zones, at the 

 beginning of the experiment, and B and C show the same seedling after six hours 

 and after one day, respectively. 



