FUNCTIONS OF TIIK LEAF. 



57 



growing plants, must be very slight ; for the amount of 

 growth in any plant may be correctly estimated by its 

 evolution of oxygen, and consumption of carbonic acid, 

 since only by these processes can it form the material of 

 its tissues. And that plants no grow in the night is 

 absolutely certain, as we see in the case of Mushrooms, 

 and many other plants ; and by just so much as they 

 actually grow, they consume and appropriate carbon. The 

 textures, however, which are produced in the long-con- 

 tinued absence of light, differ, in wanting the firm consist- 

 ence and green color, from tissues which are formed under 

 its invigorating influence, as you may see in the familiar 

 fact of Potato sprouts growing in the cellar, the texture 

 of which is always white and spongy. 



311. It is quite possible that -when the stmiuli of 

 heat and light in the solar rays, are withdrawn, the vital 

 powers being passive, the chemical forces assert their 

 sway, and thus a partial decomposition of the forming 

 tissues occurs, in which case there would be necessarily 

 a slight evolution of carbonic acid gas, which is always a 

 result of decomposition. 



312. But it is still more probable that the unappro- 

 priated carbon which has last been taken up, in the ab- 

 sence of light, and consequent relaxation of the system, 

 cannot be retained, and thus is permitted to escape. It 

 is not known that darkness has any effect on the ahsorp- 

 tion of carbon, and probably it has not. But it can, at 

 least, be said, that what has been termed Vegetable Kespi- 

 ration is not, like that of the animal, established on uniform 

 laws, and essential to the life and health of the plant. 



313. There is one remarkable difference between the 

 constitutions of the Vegetable and Animal, which has an 

 important bearing on this part of the subject. The ani- 

 mal tissues, in their whole substance, retain their vitality 

 after completion, and continue to live dv/ring the life of 

 the animal. Hence, in order to repair the waste and 

 wear of vital action, the substance of these tissues must 

 be continually renewed by nutriment, deposited inter- 

 stitially through their whole substance ; while the 

 worn-out particles which the new materials displace, are 

 consumed and thrown off in respiration, by means of 

 which a cotnbustion and excretion of the decomposed 

 substances of the old fabric are constantly maintained. 

 Breathing, let it be here understood, is nothing but a 

 burning up and casting out of exhausted particles in the 



How may the amount of growth in a plant be measured ? What gas do 

 growing plants consume — what evolve — in what measure? How are plants 

 grown in the dark f What familiar example ? What theory at 811 ? What 

 more probable ? What may be said in regard to Vegetable Kespiration ? What 

 tissues retain their vitality after completion — what do not ? How are the ani- 

 mal tissues — how deposited — how consumed and thrown off — what effects 

 S 



animal body. On the other hand, but a very small pro- 

 portion of the vegetable tissues is ever in a really living 

 state at any given time. They begin to lose their vitality 

 almost as soon as they are completed (33), and become 

 concrete substances. 



314. There is, in the Vegetable constitution, nothing 

 like the decomposition and recomposition, which constitute 

 the fundamental law of life in the animal body. We con- 

 clude, then, by the analogy of Use, which in the works 

 of the Supreme Intelligence everywhere prevails, that 

 there can be no such function as Respiration in the 

 Vegetable system, simply because there is no use for it, 

 nothing for it to do ; and more, there is no superfluous 

 material for it to act upon. It would be charging the 

 Creator with want of wisdom and economy, and at the 

 same time involve an absurdity, to suppose that He 

 would set a piece of organism to work without reason, or 

 could sustain it without means. 



DUKATION OF LEAVES. 



315. The different periods of fall in the leaf are dis- 

 tinguished by particular names. Leaves are 



(1) Deciduous, when they last but one season, as in 

 most of our trees and shrubs ; 



(2) EuGACEOUs, when they fall very early, or before 

 other parts of the plant, as in Spiranthes ; and 



(3) Persistent, when they remain through the 

 periodical drought of the tropics, and the cold season of 

 temperate climes, or until their loss is supplied by a new 

 growth, so that the tree is never without leaves, as in 

 Evergreens. 



316. The leaves of deciduous trees are developed about 

 the same period, and they also perish nearly at the same 

 time. The leaves of Pines, Firs, and some other ever- 

 greens, are only partially renewed from year to year, by 

 the protrusion of a certain portion of new leaves, and 

 the fall of others, so that there are often on the tree at 

 the same time the leaves of from two to eight or ten 

 successive years. 



317. Decay and Fall of the Leaf. — It will be 

 remembered that in the progress of growth, the older 

 tissues often have their cells coated internally with for- 

 eign substances, which have been termed the Secondary 

 Deposit (38), and that thus their walls become greatly 

 thickened, and their vitality impaired. 



318. There is a tendency in all living bodies to cast 



produced? Vegetable Tissues — explain the difference. Why may we suppose 

 that Plants do not Eespire? Why shovld they not? Why can they not? 

 Duration of Leaves — when they last one season — many seasons — fall very soon 

 How are the leaves of our forest trees — of Pines ? Decay and Fall of the Leaf- 

 Secondary Deposit— what effect What tendency in all living bodies ? 



