ROOT HAIRS AND GROWTH 



349 



according to the species studied. It 

 would therefore seem that by increasing 

 the surface of the root they increase 

 enormously its ability to absorb. How- 

 ever, this deduction of Schwarz should 

 not be taken without more careful con- 

 sideration, as it is very easily miscon- 

 strued. It is likely, in fact, that they 

 do not increase the capacity of the root to 

 absorb water at all. All of the water 

 absorbed must enter the root proper 

 through the base of the root hair, and 

 since the walls of the surface of the root 

 are apparently perfectly permeable to 

 water, no more can pass into the root 

 proper through the base of one of these 

 hairs than could enter through the same 

 area if no root hair were present there at 

 all. That is, a root immersed in water 

 can probably absorb no more water by 

 virtue of its root hairs than it could ab- 

 sorb if it had no root hairs at all. The 

 advantage of the root hairs in the absorp- 

 tion of water is, however, that in the 

 usual situation in the soil, they extend out 

 and come into contact with supplies of 

 water which would not otherwise come 

 into contact with the root at all. They 

 therefore increase its ability to obtain 

 water from a more or less dry soil, though 

 they do not, apparently, increase its total 

 capacity to absorb water. 



With regard to salts, the root hairs 

 doubtless have the same advantage of 

 extending to new supplies of the salt not 

 otherwise accessible; but they also prob- 

 ably actually increase the amount of salt 

 absorbed due to the increased surface. 

 This is due in part to the fact that the 

 cells are not perfectly permeable to the 

 salt, and hence more can enter the root 

 through the base of the hair than could 

 enter through that area devoid of a hair. 

 There is also the effect of protoplasmic 

 streaming to be considered in connection 

 with salt absorption. As will be shown 



below the protoplasm in most, if not all, 

 root hairs is in a state of circulation to 

 and from the cell proper. This means 

 that once a molecule of salt enters the 

 cell it is likely to be quickly carried into 

 the cell proper, thus giving opportunity 

 perhaps for additional absorption of the 

 same salt by the root hair. 



However there have been a few studies 

 from time to time which seem to raise a 

 question as to the absorptive function of 

 root hairs. Mer (53) raised the question 

 in 1879, b ut presented no definite evidence 

 bearing on the point. Frank (2.6) in 1887 

 determined the location of absorbed KNO3 

 by staining roots in diphenylamin-H 2 S0 4 

 after they had been growing in a solution 

 of the nitrate. He found it in the region 

 of elongation upon which no root hairs 

 had been developed. Kny (40) repeated 

 the experiments in 1898 and obtained the 

 same results for aqueous media, but he 

 points out that in the soil the reaction 

 occurred in the region of root hairs since 

 they extend nearer to the tip in the form 

 used. Overton in 190Z found that root 

 hairs of Hydrocharis are plasmolyzed in 

 most, if not all, inorganic salt solutions 

 and hence he concluded that they do not 

 absorb. Coupin (6) in 1919 found that 

 roots grew about twice as fast when only 

 the tip was immersed as when the entire 

 root was under water. But he does not 

 preclude the absorption of water con- 

 densed on the root hair in the saturated 

 atmosphere above the water in the first 

 case, nor does he demonstrate that there 

 was an adequate oxygen supply for the 

 entirely immersed roots. In a later paper 

 he contends definitely that not only does 

 the root tip absorb, but also that the root 

 hairs do not absorb. However, he does 

 not present evidence which is convincing 

 in this regard. Turina (80) worked with 

 salts which he found were absorbed from 

 the solution and deposited in the root cap 



