VSibcn tbe Moo^s are %cnUQxccn 71 



a large amount of food is retained in the fleshy 

 tissues of the leaf. 



From the air the plant absorbs an abundance of 

 carbon, with less ox3^gen and hydrogen. From 

 these, and the material brought up by the sap from 

 the soil, the plant, chiefly by the agency of the 

 leaves, makes starch, sugar, resin, gum, oils, jelly, 

 and a great number of . substances useful to men, 

 and eventually becoming the food-supply of the 

 animal kingdom. 



Leaves receive constantly more or less moisture 

 from the air. This is chiefly absorbed by the under 

 surface of the leaf. Leaves that hang sidewise, 

 having the edges to earth and sky, are alike on both 

 sides ; leaves that are set horizontally, with the one 

 surface to the earth, have that surface softer and 

 more porous. 



The leaf surface being full of cells, exposed to air 

 and heat, their sap contents become rarified or given 

 ofif, and this causes an upward pressure of sap 

 through the stem-vessels to fill them ; these, being 

 crowded full, the sap, by natural gravity, begins to 

 seek the lower parts of the plant, and thus two con- 

 stant currents of circulation are kept up — a bringing 

 up of material, a carrying of this material changed 

 to food-stuff downward, and a distribution of it for 

 building purposes throughout the whole plant. 



