LEAVES AND ROOTS— THEIR FUNCTIONS AND STRUCTURE. 



315 



sent, consider the structure and functions 

 of those organs which more immediately 

 call for observation, i. e., the leaves and 

 roots ; as it is in these that the most im- 

 portant changes take place, by means of 

 which the produce of the plant is perfected, 

 and upon which the agriculturist must be- 

 stow his attention, that he may reap his 

 due reward. 



The functions of the leaves are intricate 

 and not easil}^ understood. They are com- 

 posed of numerous vessels, reticulated and 

 interwoven, one with another — of cellular 

 tissue, which fills up the intervening spa- 

 ces — of a membranous covering — which, 

 on the upper surface, allows of transpira- 

 tion, which in plants may be compared to 

 the insensible perspiration of the surface, 

 and the humidity exhaled from the lungs 

 of animals ; and on the under surface, sio- 

 mata or reservoirs, for the reception of cer- 

 tain gases which are contained in the at- 

 mosphere. Here, then, we shall find a 

 most important part of the vegetable econo- 

 my taking place ; here. the sap of the plant 

 is elaborated into the proper juices, the ab- 

 solute article which that plant is to yield. 

 Hence we can understand why the vigor of 

 a plant depends upon the healthy green 

 state and luxuriance of its leaves ; and 

 hence we can understand why a plant, hav- 

 ing its leaves destroyed or eaten off, as soon 

 as they are unfolded, cannot be expected 

 to yield fruit in the following season. 



Let us enter farther into the structure of 

 the leaves. It must be plain, that if such 

 important ends are to be gained in the 

 leaves, we must expect an organization of 

 a complex order. Yet, the subject admits 

 of elucidation ; and a few observations may 

 divest it of much of its complexity. 



It is not unlikely that the circulation of 

 the sap in the leaf, may be compared to 

 the capillary system in animals ; and that 



while some of the ascending vessels termi- 

 nate in the returning vessels, yet that much 

 of the juices are transferred from one to 

 the other by inhibition. I have said that 

 the leaf is composed of numerous vessels, 

 interwoven througli each other. Hence 

 you will bear in mind that there are two 

 sets of vessels here intermixed ; the one, 

 containing the sap which has ascended 

 from the roots ; the other, those which are 

 to convey away that sap after it has been 

 elaborated into the proper juice of the plant. 

 It is in the leaves that the product of the 

 plant is formed, that the ascending sap i* 

 elaborated or assimilated into the proper 

 juice of the plant ; and this is effected b\' 

 the combined action of light, the solar ray, 

 and the atmosphere, — affording a beautiful 

 example of the wonderful and most effect- 

 ual works of nature, a chemical laboratory 

 in the minutest leaf, exceeding in beauty 

 and design the most elaborate and ingenious 

 of the works of art. Thus, to form our 

 compound in the leaves, we derive the wa- 

 ter, and inorganic constituents of plants* 

 from the soil — the carbon and the nitrogen 

 from the atmosphere. 



The carbonic acid of the atmosphere is 

 supposed to amount to, from a 1000th ta 

 2000th part of its bulk. The atmosphere 

 being composed of four parts of nitrogen 

 and one part of oxygen, — having the small 

 portion of carbonic acid gas already stated ; 

 carbonic acid gas, as the reader is no doubt 

 aware, being a compound of one part of 

 carbon to two parts of oxygen. Now the 

 stoviata or pores on the leaf, being open, 

 inhale or receive the carbonic acid of the 

 atmosphere ; and as it is a peculiar pro- 

 perty of the solar ray, acting upon green 

 vegetable matter, to decompose carbonic 

 acid gas, this chemical decomposition is 

 effected in the stomata, the carbon perme- 

 ates the sides of the vessels, is taken into, 



