Relation of Root -hairs to Soil Grains 



57 



so long as they are in action, and it is this very process of thin- 

 ning which furnishes the conditions needed in order to keep them 

 supplied with water from the surfaces of the soil grains. 



The effect of surface tension, as it acts upon the water of a 

 well -drained soil, is to bring about a certain regularity of dis- 

 tribution of soil moisture over the surfaces of the soil grains, 

 which is determined by the sizes of the grains and by the dimen- 

 sions of the open spaces between them. This condition of things 

 may be represented by what is shown in Fig. 9 for a particular 

 soil, in which two root-hairs have found their way in among the 

 soil grains. 



To explain the action of the root, let us suppose that for 

 some reason there has been no movement of soil moisture and 

 no root action, so that everything has come to a condition o/ 

 rest, and we have what answers to the condition of wnter 

 standing in a tank where everything is still and the surface has 

 become level. We may now suppose that morning has come, 

 with the sun shining 

 brightly, so that the 

 breathing pores in 

 the green parts of 

 the plant have opened 

 wide, making it pos- 

 sible for both assim- 

 ilation and evapora- 

 tion to go on rapidly. 

 Under these condi- 

 tions the sap in the 

 tissues of the leaves, 

 stem and root will 

 gradually become 

 more dense than that 

 which is contained 

 in the root-hairs, which are encased in the film of soil mois- 

 ture. But no sooner is this condition of things established than 

 water in the root-hairs .will begin to move toward the root, 

 stem and leaves more rapidly than the denser sap enters them. 



Fig. 9. Distribution of water on the surfaces of soil 

 grains and of root-hairs, e, main root: 1, air-space; 

 2, soil grain ; 3, film of water ; hh, root-hairs. 

 (After Sachs.) 



