248 



GENERAL SCIENCE 



the roots of plants. Such soils are said to "leach," and fer- 

 tilizers applied to them do not permanently enrich them. 

 Their open porous state permits the rapid evaporation of 

 water from them, too, while the access of over much air to the 

 roots of plants is destructive. On the other hand, clay soils 

 filled with water may exclude air from the roots so completely 

 that plants do not thrive. Illustrations of this may often 



FIG. 80. Irrigation of trees. 



be seen in the stunted growths on low lands -that have been 

 overflowed, and where water has stood for some time during 

 the growing season for crops. 



Any consideration of the roots of plants, and of the manner 

 of supply of food material to a plant by osmotic flow into 

 the roots, emphasizes the necessity of a finely divided state 

 of the soil. It should be loose and friable enough for the 

 tender root structures to penetrate it readily in all directions, 

 but at the same time sufficiently compact so that the rootlets 



