ROOT HAIRS AND GROWTH 



35* 



plement of root hairs, it seems reasonable 

 to conclude that under normal conditions 

 the root hairs are of immense importance 

 in supplying the plant with water. 



Schwarz (73) also pointed out that root 

 hairs have another function in addition 

 to absorption, namely, anchorage. They 

 secrete material which dissolves the sur- 

 face of solid bodies in the soil, forming a 

 cement which fastens them firmly to these 

 soil particles. In fact, if a plant is pulled 

 from the soil, the root hairs will either 

 bring the soil particles to which they are 

 attached along with them, or they will 

 separate from the root, but not from the 

 soil particles. It therefore appears ob- 

 vious that they have a function in anchor- 

 ing the young seedlings . They could have 

 no such function in anchoring the older 

 plant, inasmuch as branching of the root 

 system and root contraction performs 

 this so much more effectively. In the 

 young seedling this function of anchorage 

 of the root hairs is very important. It is 

 not so much that it prevents the plant 

 from being pulled out of the ground. 

 Such a circumstance is a relatively uncom- 

 mon experience. It does however prevent 

 the soil from being completely washed 

 away from the seedling in case of watering 

 or of hard rains. But it has a still more 

 important relation than this. It prevents 

 the seedling from pushing itself out of the 

 ground. The region of cell elongation is 

 just below the region of root hairs. It is 

 that region in which the cells elongate 

 parallel to the axis of the root, and push 

 the root tip into the soil. In pushing the 

 tip into the soil, the latter obviously 

 encounters resistance. The exertion of a 

 considerable pressure is therefore neces- 

 sary. Were it not for the root hairs which 

 bind the region above to the soil, the ex- 

 ertion of this pressure of elongation would 

 result in the upper part of the root being 

 pushed up out of the soil, instead of the 



lower portion being pushed down into it. 

 This is then the most important, because 

 it is the most frequent anchoring function 

 of the root hairs. 



Hill (34) has pointed out an additional 

 function of the root hair, namely that it is 

 able to adjust the absorbing surface of the 

 root to the concentration of the soil solu- 

 tion. The latter is likely to become 

 greater as a result of evaporation of water 

 from the soil, or to become weaker owing 

 to rains or leaching. It is therefore desir- 

 able that the absorptive cells be newly 

 formed from day to day to cope properly 

 with these changing conditions. The 

 progressive elongation of the individual 

 root hairs, the progressive formation of 

 new root hairs, and the migration of the 

 root hair zone, accomplish this adjust- 

 ment in an admirable manner. 



STRUCTURE AND COMPOSITION OF ROOT HAIRS 



Meyen (56) in 1838 was the first to give 

 an accurate description of the structure and 

 development of root hairs. According to 

 Schwarz (73) they vary in maximum 

 length in land plants from 0.14 mm. in 

 Vicia to 3.15 mm. in Tradescantia. In 

 aquatic plants they may attain a length 

 of 5 mm., as in Potomageton, and of 8 mm., 

 as in Triune a. 



It is thus seen that a teleological expla- 

 nation of their existence will hardly do; 

 that is, we cannot say that a plant has root 

 hairs because it needs them in order to 

 absorb or to anchor the plant, inasmuch 

 as the plants having the longest root hairs 

 are those which need them neither for 

 anchorage nor for absorption. On the 

 other hand it is to be noted that the duck- 

 weed, Lemna, a free floating aquatic, does 

 not have root hairs at all; while Elodea, a 

 submersed aquatic, has them only when 

 the roots are in contact with mud. 



The question arises as to what cells of 

 the outer layer of the root produce root 



