56 



Irrigation and Drainage 



root -hairs and held in a body about the young root, while B is 

 intended to show the appearance of the plant with the soil grains 

 washed away. So, too, in Fig. 8 is shown the root of wheat soon 



after germination, and again four 

 weeks later, after the root has ad- 

 vanced into new soil, and the root- 

 hairs have died away behind and 

 new ones formed. 



The soil grains of a good soil 

 are very small, the majority of 

 them even much less than j^o of 

 an inch in diameter. Indeed, in a 

 heavy clay soil one -half of the dry 

 weight may be made up of soil 

 grains as small as 25000 of an inch 

 in diameter. Now, the fine root- 

 hairs make their way in between 

 these minute soil grains, and even 

 change their shape to fit them- 

 selves closely upon their surfaces 

 in many cases. 



The soil particles are them- 

 selves invested with a thin layer 

 of water, even in the condition 

 which we know as air- dry, and 

 as these minute root-hairs apply 

 themselves closely to the surfaces 

 of the soil grains, they are brought into immediate contact with 

 the soil moisture. Indeed, capillarity has the same tendency to 

 invest the root-hairs with a film of moisture that it has the soil 

 grains, and we may suppose, in the absence of direct observation, 

 that the root -hairs all the time carry a film of moisture equal in 

 thickness to that which invests the soil grains of like diameters, 

 except in so far as the film of water is thinned out by the flow 

 through the walls of the root-hairs and away through the root to 

 meet the demands in the green parts of the plants. Such a thin- 

 ning out of the film of water on the root -hairs does take place 



Fig. 8. Root-hairs of wheat, A when 

 very young, B four weeks later. 

 (After Sachs.) 



