THE SOIL. 69 



a coating on every particle, but the larger interspaces of 

 the soil are filled with air. When these also contain water 

 the soil is said to be saturated or " wet." 



The thicker roots of the plant fix it securely in the soil 

 but play practically no part in the absorption of the food. 

 This is done by the root- hairs on the rootlets which branch 

 out from the larger roots in all directions in search of 

 food. The root-hairs, found only on the newer portions 

 of the rootlets, are very minute tube -like growths from 

 the surface cells and into them the protoplasmic layer and 

 cell-sap of these cells extend. They have a mucilaginous 

 coating in which many soil particles are imbedded, and 

 thus they are brought intimately into contact with the 

 water of these particles, and through them with that of all 

 the other soil particles in their immediate neighbourhood. 

 At the root-hair there are therefore two watery solutions, 

 the cell- sap and the soil water separated from one another 

 by a membrane consisting of the cell wall and the proto- 

 plasmic layer. This membrane is very tender and is easily 

 permeable to water and to certain dissolved substances. 



All the water and mineral matters of plants enter by 

 the root- hairs, and to explain the passage of these from the 

 soil into the plant some reference must here be made to 

 the two processes of Diffusion and Osmosis. 



When a soluble substance is placed in water it dissolves 

 and in time will distribute itself uniformly through the 

 water. This it does by diffusion or the gradual wandering 

 away of its molecules in among the molecules of the water. 

 Osmosis cannot be so simply defined but it may be re- 

 garded as a case of diffusion complicated by the presence 



