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THE QUARTERLY REVIEW OF BIOLOGY 



Jeffs (36), working under che writer's 

 direction, was the first to study these, 

 using corn, radish, and mustard in satu- 

 rated atmosphere. He determined first 

 the grand period of growth of these hairs, 

 taking readings at fifteen minute intervals 

 for periods up to ten hours. He found 

 that the grand period extends, under these 

 conditions, for from ten to fifteen hours. 

 The curve is of the typical sigmoid form 

 with an acceleration during the first two 

 hours, and finally a retardation, during 

 two or three hours. 



In the case of root hair growth, how- 

 ever, there seems to be a definite explana- 

 tion of the acceleration during the first 

 part of the grand period of growth, which 

 is peculiar to the root hair situation. It is 

 to be observed that this period of accelera- 

 tion does not occur in root hairs growing 

 in aqueous media (11, 14, 15, 16, 17). In 

 water root hairs elongate at a constant 

 rate from the start. It has been noted 

 above that root hairs in air occur much 

 nearer the tip of the root than they do in 

 water; and Jeffs (fig. 4) found that this 

 involved an overlapping of the region of 

 root hairs with the region of root elonga- 

 tion. That is, the root hairs which are 

 just emerging in air, are on the upper 

 portion of the region of root elongation. 

 This is shown by the fact that these very 

 young root hairs are carried along down- 

 ward very slowly during the first two or 

 three hours of their development. The 

 rate at which they are thus carried along, 

 which is referred to as their lateral move- 

 ment, becomes less and less during this 

 period, so that at the close of the interval 

 they cease this movement. It is observed 

 that this retardative lateral movement is 

 coincident with the acceleration of their 

 elongation. It is thus apparent that as 

 the epidermal cell decreases its rate of 

 elongation vertically, it increases its rate 

 of elongation horizontally, that is the 



elongation of its root hair. Thus it ap- 

 pears that cell enlargement may proceed 

 during this process at a rather constant 

 rate. The absence of such a period of 

 acceleration of root hair elongation for 

 roots in water simply means that the 

 cells or the root have already ceased 

 vertical elongation in the region which is 

 producing root hairs. The graph for the 

 grand period of growth for root hairs in 

 water is then not the typical sigmoid 

 form, but consists of a rather straight line 

 with, in some cases, a retardation to zero 

 near the end of the period. 



An intensive study has been undertaken 

 by the writer (14) of just one variety, 

 Georgia collards, of just one species of 

 plant, Brass ka oleracea. To this same 

 species belong cabbage, cauliflower, kohl- 

 rabi, Brussels sprouts, etc. Seedlings are 

 chosen as nearly as possible of the same 

 age, and placed in a chamber on the stage 

 of the horizontal microscope, in such a 

 position that the root is immersed in a 

 solution, while the shoots develop in the 

 air. The solution in the chamber is kept 

 flowing at a rather constant rate, so that 

 during the period of the experiment, which 

 is 16 hours, there is practically no change 

 in the chemical composition of the solu- 

 tion. The solutions thus far used have 

 been of single calcium compounds, namely, 

 the hydroxide, chloride, and nitrate. The 

 experiments were performed in duplicate 

 for each molar and hydrogen ion concen- 

 tration. Previous to insertion of the 

 seedlings a solution of known molar 

 concentration and known degree of acidity 

 or alkalinity was aerated with carbon- 

 dioxide-free air, so that it contained an 

 abundant supply of oxygen, but little or 

 no carbon dioxide. The entire experiment 

 was performed in a dark room of reason- 

 ably constant temperature. The only 

 illumination consisted of a very weak red 

 light which was used during the period 



