38 DISEASE IN PLANTS. 



developed along the course of these subterranean 

 boring organs. 



One of the most striking results of modern 

 enquir)' into these matters, is the discovery that 

 the number and superficial area of these root-hairs, 

 on one and the same plant, may vary to a large 

 extent according to the structure, as it were, of 

 the soil, and the degree of moisture it is capable 

 of retaining ; or, with the same soil, according to 

 the amount of water which it receives and holds. 

 Correlations have also been observed between the 

 development in length and surface of the rootlets 

 themselves. 



The following illustrations will suffice to show 

 this: 



Six young wheat-plants in soil kept constantly 

 wet, developed roots the total length of which 

 measured 365 mm. each, on the average, and 

 almost devoid of root-hairs. 



Six similar plants in soil onl}' moderately 

 moist, averaged 668 mm., and were well furnished 

 (though not densely covered) with root-hairs. 



Six similar plants in soil which would be 

 termed dry, averaged 371 mm., but were densely 

 covered with rich crops of root-hairs. 



Further researches have shown that the con- 

 ditions which rule the development of the root- 

 system and root-hairs in the soil are very complex, 

 and not always easy to trace. The most general 

 statements we can make are the following : 



There is an optimum degree of moisture in 

 the soil which promotes the maximum develop- 



