244 Vegetable Physiology. 



of other leaves. How singular that in the wonderful a< 

 tion of these different structures to the different circumstance 

 and positions ill which the plants are to grow, any one should 

 fail to perceive the foresight, wisdom, and skill of a Divi n 

 Being. Many other instances of this adaptation of structure 

 to circumstances might be given. It will be sufficient to refer 

 to the organization of the Cactus, as described on page SJ19 

 which will also explain how those plants are enabled to endure 

 for a long time the confined air of hot houses, in which ordinary 

 plants will wither and perish. 



THE FUNCTIONS OF LEAVES. 



In the leaves, as already stated, that process goes on, which 

 converts the crude moisture absorbed by the roots into nutri- 

 tious sap, which supplies the necessary materials for the 

 growth and production of new parts, and also for those pro- 

 ducts which plants offer for articles of diet, use, or medicine. 

 This is called the process of elaboration, and in it several dis- 

 tinct changes take place. The first is the concentration of 

 the fluid by the loss of a considerable portion of its moisture 

 by a perspiration like that of animals, the leaves giving off, 

 under certain circumstances, a quantity of watery vapor. 

 That such a vapor is really exhaled may easily be tested by 

 placing a large wide-mouthed glass vessel, with its mouth 

 downwards, over a growing plant, or the surface of a meadow, 

 in a warm summer day. Its interior will be rendered dim by 

 the accumulation of the vapor, which will soon collect into 

 drops, and run down the inner surface. It has been calculated 

 that an acre of grass land transpires in this way no less than 

 six thousand four hundred quarts of water in twenty-four 

 hours. Experiments have been made to show that this tran- 

 spiration takes place through the stomata. A very simple 

 method of showing that this is the case, may be made by hold- 

 ing a piece of glass near the under surface of the leaf of a 

 vigorous vine, when it will soon be found to be covered with 

 moisture, while no effect will be perceived if it is held near 

 the upper surface. When the stomata exist on both surfaces 

 in equal numbers, they seem to transpire alike, and if on nei- 

 ther surface, no effect is seen. Again, if a plant, actively 



