LUTHER BURBANK 



joining 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 phenom- 

 enon of the "rise of sap." 



Regardless of the precise explanation, however, 

 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 ultimate 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 surface 

 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 ca- 

 nals that may be likened to the bronchial tubes of 

 an animal or of man, except that they are infini- 

 tesimal in size. 



Through these channels, air is brought in con- 

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

 riods of growth, there is a constant, even though 

 slow, interchange between the air in the inter- 



[20] 



