PYRUS. 



109 



The Food of Plants. — Whence and what are the ma- 

 terials for sustaining this growth ? Learn from the treat- 

 ment which your house plants receive. Their roots are im- 

 mersed in a pot of soil. You shower them at night with 

 water containing a little added ammonia. You open the 



8, cross-section of an exogenous stem of 2 years" growth: 1, pith ; 2, 3, annual lay- 

 ers of wood ; 4, the bark and white new layer {cambium) under it. 9, an endogenous 

 stem (Indian Corn), with no layers nor bark. 



windows in the morning to bathe them in fresh air. Then, 

 with warmth and sunshine, they ask no more. So the tree, 

 by its myriad of roots and rootlets, imbibes water containing 

 ammonia and various mineral matters in solution. Thence 

 this sap, as we call it, creeping from cell to cell of the root, 

 stem and branch, and dissolving the sugar, gum, &c., it finds 

 on the way, finally reaches the leaves. Here is the chemical 

 laboratory of the plant.* Much of the water having per- 

 formed its work of carrying up the raw material from the 

 earth, evaporates through the pores of the leaf.f Through 



* It is curious to notice how the trunk and branches of the tree arc all the work o^ 

 the frail and transient leaf. Slowly, year after year, generation after generation, it is 

 steadily elaborating, from air and rain and sunshine, these solid structures which are 

 to remain its enduring monument, when it has faded and crumbled to dust. 



t It has been found by experiment that the loaves of plants exhale moisture to an 

 enormous amount. An acre of beets, during a single day of sunshine, evaporates 

 from 17 to 19 thousand pounds of water. A Chi'slnut tree 35 years old, in 24 hours, 

 lost over G3 quarts of water. Tlie ui)ward pressure of the ascending saji is very great. 

 Experiments were made, in 1720, by Dr. Hales of England, proving that this force in 

 a Grapevine was equal to the weight of a column of water 4^^ feet high. Similar 

 experiments were made in 1873, by President Clark, of the Massachusetts Agricultural 

 College, on a native vine (Vitis ivstivalis). On May (Jay, a mercurial gauge wa^ 



