56 THE UNIVERSITY OF OKLAHOMA 



(1.) Root hairs spring from the upper region of root elonga- 

 tion and the root continues to elongate for an average of one and 

 a half to three hours, in the region in which new root hairs are 

 starting. 



(2.) The rate of root elongation in the region of newh' form- 

 ing root hairs decreases rapidly. The average of five corn roots 

 showed an initial rate of 112 microns per fifteen minute interval. 

 This decreased to 58.4 microns the last fifteen minute interval of 

 the first hour, and to 1.5 microns the third fifteen minute interval 

 of the third hour. This retardation in root elongation, was gradual 

 and uniform in rate. By the end of the third hour no furtlier 

 growth was noted. 



(3.) Root hairs grow slowly at first, gradually increasing 

 their rate of growth up to the time root elongation ceases and 

 there is a definite relation between root and root hair elongation. 

 At the time, or shortly after root elongation ceases, the rate of 

 growth of the root hairs in that region becomes stabilized and they 

 continue to grow with only slight variations up to about two hours 

 before root hair growth ceases. 



An average, taken from the above five corn roots showed an 

 initial rate for five root hairs (one from each root) of 6.5 microns 

 per fifteen minute interval. This increased to 16.1 microns per 

 interval at the end of the first hour and to 33.6 microns at the 

 end of the third hour. After this there was no further root 

 elongation in this region and the root hairs grew at an average 

 rate of 35 microns per fifteen minute interval for the next three 

 hours, which was as long as they were studied. However, several 

 root hairs were studied for a period of eight to ten hours after 

 root elongation ceased, and their variation in rate of growth in 

 all cases was slight. 



(4.) As a root hair matures it slows up in its growth rate 

 very rapidly. The average taken from five root hairs of radish 

 (from five different roots) gave the following results: 



Approximate length two hours before growth ceased 1824 mi- 

 crons. They were then elongating at an average rate of 44.5 mi- 

 crons per fifteen minute interval. One hour before growth ceased 

 they were growing 36 microns per interval, and fifteen minutes be- 

 fore growth ceased, they were growing 15 microns per fifteen inin- 

 ute period. Growth ceased on an average of two hours after 

 retardation in rate was noticeable. 



