PRODUCTIVE GARDENING 81 



osmotic action between these superficial cells and 

 the adjoining cells, which in turn pass the water, 

 with its modicum of nutrient salts, to yet deeper 

 layers of cells, and ultimately up along the stem 

 of the plant or tree — constituting the familiar 

 phenomenon of the "rise of sap." 



Regardless of the precise explanation, how- 

 ever, the fact is obvious and long familiar that 

 water bearing a certain quantity of minerals in 

 dilute solution is absorbed by the roots of the 

 plant and is carried up in due course to the ulti- 

 mate buds and growing tips and leaves. 



It has been known for a good while also that 

 the leaves of the plant have on their under sur- 

 face vast numbers of little mouths or stomata, 

 through which a certain amount of the water that 

 has come to them from the roots is transpired or 

 exhaled, and through which also air is inhaled. 



But it has only somewhat recently been learned 

 that the air which thus enters the structure of the 

 leaves is transmitted everywhere throughout the 

 tissues of the plant, through little crevices or 

 canals that may be likened to the bronchial tubes 

 of an animal or of man, except that they are 

 infinitesimal in size. 



Through these channels, air is brought in con- 

 tact with all the cells of the plant, and, during 

 periods of growth, there is a constant, even 



