192 THE FOOD AND NUTRITION OF PLANTS. 



renders it fit for the breathing and life of animals. That the foliage 

 of plants in sunshine is continually yielding oxygen gas to the sur- 

 rounding air has been familiarly known since the days of Ingenhouss 

 and Priestley, and may at any moment he verified by simple experi- 

 ment. The readiest way is, to expose a few freshly gathered leaves 

 to the sunshine in a glass vessel filled with water, and to collect the 

 air-bubbles which presently arise while the light falls upon them, 

 but which cease to appear when placed in shadow. This air, when 

 examined, proves to be free oxygen gas. In nature, diffused day- 

 light produces this effect ; but in our experiments, direct sunshine is 

 generally necessary to show it. What is the source of this oxygen 

 gas, which is given up to the air just in proportion to the vigor of 

 assimilation in the leafy plant, or, in other words, to the consumption 

 of crude sap ? 



347. This will be manifest on comparing the materials with the 

 general products of vegetation, — what the plant takes as its food, 

 with what it makes of it, in growth. Suppose the plant is assimi- 

 lating its food immediately into its fabric, viz. into Cellulose, or the 

 substance of which its tissue consists (27). This matter, when in a 

 pure state, and free from incrusting materials, has a perfectly uni- 

 form composition in all plants. It is composed of carbon, hydrogen, 

 and oxygen, the latter two existing in the same proportions as in 

 water.* It may therefore be said to consist of carbon and the ele- 

 ments of water. These materials are necessarily furnished by the 

 plant's food. The mineral food of the plant, from which its fabric 

 is made (329), is carbonic acid and water. If this be decomposed 

 in vegetation, and the carbonic acid give up its oxygen, carbon and 

 the elements of water remain, — the very composition of cellulose or 

 vegetable tissue. Doubtless, then, the oxygen which is rendered to 

 the air in vegetation comes from the carbonic acid which the plant 

 took from the air (328). 



348. This view may be confirmed by direct experiment. We 



imbibed by the roots bad absorbed from the air (326), and which passes off un- 

 altered fiom the leaves when this water is evaporated, or from nitiogen in the 

 air which the rootlets directly absorb. In the course of vegetation, no more 

 nitrogen is given out than what is thus taken in, and probably not so much. 

 So that the exhalation of nitrogen may be left out of the general view of the 

 changes which arc brought about in vegetation. 



* Cellulose is chemically composed of 12 equivalents of Carbon, 10 of Hy- 

 drogen, and 10 of Oxygen, viz. Cic, Hjo, O.o. 



