40 DISEASE IN PLANTS. 



plants developed no root-hairs, though they were 

 abundant in a i per cent, solution. 



Wheat plants with their roots in a i 5 per cent, 

 solution of potassium nitrate bore no root-hairs, 

 but they were numerous in a 2 per cent, solution 

 of the same salt. 



These are extreme cases, for, although the 

 roots were not killed, they were strongly inhibited 

 in their growth by the more concentrated solu- 

 tions. However, experiments of this kind at 

 least bring vividly before us what variations are 

 possible, and suggest that similar events on a 

 smaller scale may occur in a soil which yields 

 large quantities of soluble substances, e.g. when 

 freshly manured. Obviously these facts have 

 a practical significance as regards kind of soil, 

 drainage, season {e.g. drought or wet), etc. 



But there are other factors which rule the 

 development of root-hairs, and some experiments 

 by Lesage show that the correlations between 

 the development of root-hairs and roots are 

 probably much more complex than had been 

 suspected ; for he finds that if the lateral rootlets 

 of a Bean, in a water culture, are suppressed, 

 the main rootlet develops numerous and very 

 long hairs to compensate the loss in surface, a 

 matter of obvious importance in the discussion 

 of cases where roots have been injured in the 

 soil. 



Before proceeding further it is necessary to 

 look a little more closely into the structure of 

 a single hair. 



