70 JBotan^ 



out through the veins to every part of the leaf sur- 

 face. Thus it is exposed to light, air, sun-heat, until 

 it is chemicall}^ changed, cooked, as we may say, 

 into good plant food. The cells in the parenchyma 

 hold the coloring matter of the leaf, which is 

 called chlorophjdl, or green stuff. This is the chiet 

 agent in digesting plant food. There is also other 

 coloring matter known under one general name of 

 chromule. The j^resence of saj) in the leaf adds es- 

 pecial lustre to these coloring materials. In sj)ring, 

 when sap is abundant, the leaves are much richer 

 and brighter in color. 



When leaves have fallen in the cold season the 

 roots are also at rest in the soil, the stem ceases to 

 send up sap ; the whole plant seems asleep. You 

 may cut a sugar-maple stem in winter and no sap 

 flows ; if you cut it in spring the sap rushes out. 

 Grapevines and trees should be pruned in the late 

 autumn, so that the scar may harden over before sa]) 

 rises in the spring. This care will prevent leakage, 

 weakening to the plant. 



In the potato and other plants having tubers or 

 corms, a large portion of the ascended sap, laden with 

 the food-stuff gathered from the air by the leaves, is 

 sent back through the stem vessels to the under- 

 ground storehouses. When, as in the agave and other 

 like plants, the leaves are themselves the storehouses, 



