196 THE FOOD AND NUTRITION OF PLANTS. 



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

 age of plants in sunshine is continually yielding oxygen gas to the 

 surrounding air has been familiarly known since the days of Ingen- 

 houss and Priestley, and may at any moment be verified by sim- 

 ple experiments. 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 shad- 

 ow. This air, when examined, proves to be free oxygen gas. In 

 nature, diffused daylight produces this result ; but in our rude ex- 

 periments, direct sunshine is generally necessary. What is the 

 source of this oxygen gas, which is given up to the air just in pro- 

 portion to the vigor of assimilation in the leafy plant, or, in other 

 words, to the consumption of crude sap ? 



347. To take for illustration the case which exhibits the general 

 result (and whether this is actually attained at one operation, or 

 not, does not affect the view), and enables us directly to contrast 

 the materials with the principal product of vegetation, we will sup- 

 pose the plant is assimilating its food immediately 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 per- 

 fectly uniform composition in all plants. It is composed of carbon, 

 hydrogen, and oxygen, of which the latter two exist in the same 

 proportions as in water.* It may therefore be said to consist of 

 carbon and the elements of water. These materials are necessa- 

 rily furnished by the plant's food. But the universal food of the 

 plant, that which is only and absolutely essential to bare vegeta- 

 tion (324, 329), is carbonic acid and water. If this be decom- 

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

 there remain carbon and water, or rather the elements of water, 

 the very composition of cellulose or vegetable tissue. Doubt- 

 less, then, the oxygen which is rendered to the air in vegetation 

 comes from the carbonic acid which, as we have seen (328), the 

 plant took from the air. 



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

 have seen that many plants must, and all may, imbibe the whole or 

 a part of their food directly from the air into their leaves (132, 



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

 drogen, and 10 of Oxygen, viz. C 12 , H 10 , O'o. 



