ROOT HAIRS AND GROWTH 



347 



different degrees of alkalinity. We can 

 next use a neutral solution of calcium 

 nitrate of different concentrations. This 

 contains calcium ions (Ca) and nitrate 

 ions (N0 3 ). By adding calcium hy- 

 droxide to calcium nitrate we can study 

 the effects of different degrees of alkalinity 

 for different concentrations of calcium 

 nitrate. In this way we can obtain a 

 rather exact idea of the specific effect of 

 the nitrate ion (NO3) and of the hydroxyl 

 ion (OH). By adding nitric acid, HNO3, 

 to calcium nitrate, we can study the 

 effects of different degrees of acidity for 

 different concentrations of the calcium 

 nitrate, and in this way obtain information 

 regarding the effect of the hydrogen ion 

 (H). This same method can be repeated 

 with calcium sulphate, CaS0 4 , and cal- 

 cium phosphate, Ca 3 P0 4 , thus obtaining 

 a knowledge of the effect of the three 

 nutrient anions, nitrogen, sulphur, and 

 phosphorus, and also of one cation, cal- 

 cium. The other three most important 

 nutrient elements, potassium, magnesium, 

 and iron, are cations. It is therefore 

 necessary that they be studied by mixing 

 their salts with those of calcium, inasmuch 

 as calcium is necessary in order for root 

 hair elongation to occur. We shall there- 

 fore have to study a solution of calcium 

 nitrate, CaN0 3 , and potassium nitrate, 

 KNO3, in different proportions, different 

 concentrations, and different degrees of 

 acidity and alkalinity. In the same way 

 we can study magnesium nitrate, MgN0 3 , 

 and ferric nitrate, Fe(N0 3 ) 3 . By study- 

 ing the sulphates and the phosphates of 

 these compounds in the same fashion, and 

 then combining them in different ways, 

 we can gradually build up the balanced 

 culture solution with a thorough knowl- 

 edge of the specific and mutual effects of 

 each of its constituent parts. 



It is therefore important that a study 

 of root hairs be conducted intensively 



and extensively, in order to increase our 

 knowledge of the nature of cell enlarge- 

 ment as bearing on the general problem of 

 development, and from the standpoint of 

 our knowledge of the biological effects of 

 the respective nutrient, toxic, and stimu- 

 lating features of the environment of the 

 plant, especially of its roots. 



THE FUNCTION OF ROOT HAIRS 



Root hairs do not ordinarily occur 

 over the entire surface of the root. The 

 terminal portion of the root, consisting 

 of the root cap and the regions of cell 

 division and cell enlargement, does not 

 bear hairs. Also the older portion of the 

 root, that is, the part nearest the stem, 

 is usually devoid of hairs. In fact in most 

 roots the hairs are confined to a region 

 between one and four centimeters in 

 length near the tip of the main root and of 

 the secondary roots and their branches. 

 The hairless tip is usually of somewhat 

 less extent; and, aside from the root cap, 

 a protective structure which covers the 

 tip, it is a region of growth, giving rise by 

 cell division and cell enlargement to the 

 region of root hairs above. By virtue of 

 this elongation of the cells in the upper 

 part of this region of growth, the tip of 

 the root bearing the cap is pushed farther 

 and farther into the soil. The region of 

 root hairs is for the most part a region 

 of cell differentiation. In it the surface 

 cells produce protuberances which become 

 the root hairs. The interior cells, on the 

 other hand, become differentiated into the 

 various cells of the older portion of the 

 root, such as conductive cells, storage 

 cells, etc. It is thus in this region of 

 root hairs that the conductive tubes of 

 the root, which are continuous with those 

 in the stem, terminate as we follow them 

 down the root, or originate as we trace 

 them toward the stem. By virtue of the 

 fact that the region of root hairs is being 



