THE LEAF AS A PHYSICAL STUDY. 137 



occasioned by the light and heat of tlie sun. Snrelv we onght to 

 derive satisfaction from the thouglit that, as we till our fields daring 

 the intense heat of the summer, the sunlight that exhausts us is 

 at the same time giving us more oxygen to breathe, and is storing 

 up food for our futiue sustenance in the plant we cultivate. The 

 thought -will not render the toil less fatiguing, but the consolation 

 comes from the knowledge that we get the upper hand of nature 

 when we harvest the crop. 



To understand the full office of the leaf we must know that the 

 sap that flows so freely in tlie spring is largel}' composed of water 

 absorbed by tlic roots of the plants. This water holds in solution 

 minute quantities of gas and mineral salts, and adds thereto on its 

 upward way dextrine and sugar, which it dissolves out of the cells 

 as it comes in contact with them, gaining in density as it nears the 

 leaf. Within the leaf it parts with much of its water, having no 

 longer a use therefor, receiving in its place carbon, and the digested 

 juices acted upon by the chlorophyl, as it passes througli the surface 

 of the leaf; from thence, b}' means of the leaf-stalk, it passes into 

 the cellular and woody tissues of the bark, and continues its down- 

 ward passage, making deposits of food, first in the cells of the pith 

 at the base of ever}' incipient bud ; then a copious store in the 

 cambium region ; giving also a good portion to the medullar}- ra3's, 

 some to be carried outward to the cortical la3-er, and some inward 

 for solidifying the wood ; and lastly the richest portion is sent to 

 the root, ever}' branch and fibre being filled. The return sap also 

 contains nitrogen, to a limited extent, and minute portions of mineral 

 matter. From this nitrogen are first organized the protein sub- 

 stances, analogous in composition to the living tissues of animals ; 

 and cellulose, the peculiar principle of vegetable tissue, having in 

 it the exact elements of water, C12 Hm Ojq. The action of chloro- 

 phyl upon this substance develops gum, sugar, and starch, which 

 are nutritive products common to all plants, and are stored away 

 for future use, as fat is stored away in our own and all animal 

 systems. As examples we have sugar stored up in the root of the 

 beet, and in the stalks of corn, sugar-cane, and sorghum ; and starch 

 in the tubers of the potato. Tliese substances, with cellulose, are 

 all composed of carbon and the elements of water, often in identi- 

 cal proportions. Thus, cane sugar, C12 H12 Oi2 ; grape sugar, C12 

 Hii On ; gum, C12 Hjo Oio ; starch, Cj, Hjo 0^ ; cellulose, C12 Hjo Ojo. 

 The ease with which these five general products of leaf elaboration 

 are converted into one another, both in the growing plant and in the 



