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single cells are visible and easily manipulated is preferable be- 

 cause of the ability to obtain more constant nutrient condi- 

 tions. If successive measurements are to be made over a period 

 of time, it is also desirable that the cells be stationary. It is 

 almost necessary that the enlargement take place in onl_, one 

 direction so that the increments may represent as nearly as 

 possible the absolute growth during the period of observation. 

 It is also a matter of decided advantage that large numbers of 

 cells may be located in a field of the microscope so that their 

 increases in size may be measured simultaneously. One must 

 certainly have an abundance of material of the same phys- 

 iological age. 



The root hairs of many terrestrial plants would seem to 

 possess all of these requirements. Seedlings may, in many 

 cases be produced within a moist chamber within a short 

 period of time, furnishing an almost unlimited amount of ma- 

 terial. The seedlings of many kinds of plants will continue to 

 form root hairs when they are transferred to a nutrient culture 

 solution. 



ORIGIN OF ROOT HAIRS 



Each root hair is an extension of a single epidermal cell 

 which is subject to neither division nor marked differentiation 

 throughout its normal development. The usual direction of 

 growth is in a line perpendicular to the main axis of the root. 



Even a brief resume of the various theories concerning the 

 development and function of root hairs would require more time 

 than is available. If I may summarize, with no sense of finality, 

 the following points may serve to present the ideas now con- 

 sidered to be most tenable. From the experiments of Reinhardt 

 in which he placed minute particles of red lead upon the tip of 

 the root hair and watched the change of position as the hair 

 grew, we may believe that some root hairs grow at the tip. 

 That this method of growth is not universal, however, has been 

 shown more recently by Ziegenspek. He has found that in 

 Hydrochairs the growth takes place intercalarly near the base 

 of the hair. 



COMPOSITION OF THE ROOT HAIR WALL 



Studies of the nature of the cell wall have produced a large 

 amount of conflicting evidence. From the behavior of the hairs 



