54 DISEASE IN PLANTS. 



the living root-hairs as instruments as pieces of 

 living machinery for the active absorption of 

 water, with substances dissolved in it, from the 

 soil ; and it will also be evident, I think, that no 

 one can form a proper conception of this matter 

 of absorption, so important in all agricultural 

 questions, unless he pays attention to these 

 biological phenomena. It was hopeless to expect 

 to understand these matters merely in the light of 

 chemical analyses of plants and soils, and one 

 expression of this hopelessness was the belief in 

 the power of roots to select only the substances 

 useful to it. We now know that the expression 

 " selective power of roots " has a totally different 

 meaning from that implied in the minds of the 

 last generation of agriculturalists, and it would 

 be easy to devise experiments, with solutions of 

 different strength, where the plant should be made 

 to take up relatively large quantities of harmless, 

 but useless minerals, etc., and to starve in the 

 midst of plenty of the elements proper to its 

 structure, siip.ply because the former are offered in 

 a form in which they easily traverse the proto- 

 plasm of the root-hairs, while the latter arc 

 presented in a form unsuitable for absorption. 

 That all these matters are of importance in regard 

 to manuring and choice of soils, etc., needs no 

 emphasising. 



These remarks, of course, do not detract from 

 the value of good comparative chemical analyses, 

 when viewed in the light of physiological know- 

 ledge, as I need hardl)' sa)- ; but they do, and 



