FUNCTIONS OF THE LEAF. 



55 



Nature, and by Plants only is it accomplished. By 

 changing inorganic material into their own substance, 

 they convert it into food for men and animals. They 

 stand at the basis of all Life, being, in fact, the only 

 Nourishers of the world ; for men and animals only con- 

 sume what they so bountifully provide. 



306. In the process of Vegetable Digestion, another 

 result, almost equally wonderful, is also accomplished; 

 for oxygen is liberated by means which Chemistry, as 

 yet, in vain attempts to imitate. Oxygen is the most 

 important atmospheric principle for the support of animal 

 life. Carbonic acid gas is the vital principle of the 

 atmosphere for the support of vegetable life ; but at the 

 same time it is so deadly to animals, that if the air is 

 greatly infected by it, it becomes noxious. Plants 

 absorb this gas continually, giving in return free oxygen 

 gas that vital element, without which not a single ani- 

 mal could live or breathe, being liberated in the very act 

 of vegetable digestion. Thus one vital office is made to 

 counterbalance the other, and the whole is harmonized. 

 The respiration of animals, and other causes which infect 

 the atmosphere, give food to plants, which, by assimilat- 

 ting these crude and otherwise noxious substances, con- 

 vert them into organism ; and thus while they are puri- 

 fying the air, they are also elaborating nourishment and 

 support for the animal world. And these beautiful rela- 

 tions do the two great organic kingdoms ever maintain 

 with each other. Each consumes only what the other 

 rejects, and furnishes what the other demands. Plants, 

 then, may be considered as the great providers or caterers 

 of the world. They are the only Producers; for the 

 whole animal world are only consumers of the nutritive 

 elements which they alone have power to elaborate. 



307. EXHALATION is that process by which the super- 

 abundant or hurtful elements are thrown off. It is to 

 be distinguished from evaporation, which depends solely 

 on the heat and condition of the atmosphere, and which, 

 as you have seen, is almost wholly restrained by the 

 epidermis of plants; and it has a strong analogy to 

 perspiration, in the animal system. Exhalation is 

 maintained chiefly, if not entirely, by the action of the 

 stomata ; and as these are only open under the influence 

 of light, it follows that a plant can support this important 

 function only in the daytime, or in the presence of light. 

 These facts are shown in several ways. If, under the 

 influence of a bright sunshine, and a still, warm air, a 



cold plate of glass be held before the under surface of 

 any leaf whose exhaling power is great, as the Annual 

 Sunflower, or Hydrangea, it will soon be covered with 

 dew ; but if held before the upper surface, it will remain 

 dry. The stomata, you will remember, are chiefly in the 

 lower surface of the leaf; and this shows that there is 

 an absolute connection between them and the property of 

 Exhalation, as also does the following. If the light be 

 suddenly excluded from an actively growing plant, exha- 

 lation will immediately cease ; while the stomata, if ex- 

 amined directly on its readmission, will be found closed. 



308. The amount of liquid exhaled by plants is 

 frequently enormous. By various experiments, it has 

 been shown that they often perspire from eight to sixteen 

 times as much as the same extent of surface in the human 

 body. A plucked leaf of the Sunflower, with its petiole 

 immersed in water, absorbed and exhaled its own weight 

 in six hours. 



309. Exhalation by the leaves must always, in a 

 healthy state, maintain a certain proportion to the ab- 

 sorption of the roots. If the former exceed the latter, 

 there is a waste of vital power, and consequent exhaus- 

 tion. In the spring, before the leaves appear, and while 

 the roots are most vitally active, the absorption by far 

 exceeds exhalation, and the stem is gorged with sap, 

 which will flow readily from an incision. A large portion 

 of this is expended in the production of leaves, after 

 which a continuous supply is demanded to support the 

 growth of the plant. Then the two forces are nearly in 

 equilibrium. Later in the season the vitality of the 

 leaves is impaired, and in autumn they fall and perish. 

 But still the roots remain active for a time; and the 

 excess of nutriment, beyond what is required to maintain 

 a feeble circulation, is again accumulated in the system 

 for future supplies. This may be seen in the swelling of 

 buds during a period of warm weather late in winter. 

 And thus the processes of vegetable life go on, with their 

 continual round of changes, while acquisition, develop- 

 ment, growth and rest, are happily maintained. 



310. RESPIRATION. This has been considered by 

 some authors as a function of the vegetable being. But 

 the whole process that has been so termed, must be re- 

 ferred back to that which has just been considered under 

 the head of Digestion. The theory was founded chiefly 

 on the assumption that plants uniformly evolve carbonic 

 acid gas in the absence of light. This, in all cases of 



What are the Nourishers of the world ? What the consumers ? What other 

 wonderful result? Next function, define. From what distinguished ? What 

 analogy ? By what maintained ? W hat effect of the epidermis ? When is Ex- 

 halation supported? Why? Stomata. What effect of light? How shown? The 



experiment, its philosophy. Liquid exhaled by plants quantity. Sun-flower. 

 To what should Exhalation always correspond? How is it in the spring 

 What becomes of the excess ? How later in the season ? In autumn. What does 

 swelling of buds show ? Do plants respire 1 On what was tlie theory founded 



