162 THE PLANT IN ACTION, [LESSON 26. 



the fabric and a part of the general products of the plant are made. 

 Water and carbonic acid are mineral matters : in the plant, mainly 

 in the foliage, they are changed into organic matters. This is 



464. The Plant's proper Work, Assimilation, viz. the conversion by the 



vegetable of foreign, dead, mineral matter into its own living sub- 

 stance, or into organic matter capable of becoming living substance. 

 To do this is, as we have said, the peculiar office of the plant. How 

 and where is it done ? 



465. It is done in the green parts of plants alone, and only when 

 these are acted upon by the light of the sun. The sun in some way 

 supplies a power which enables the living plant to originate these 

 peculiar chemical combinations, to organize matter into forms 

 which are alone capable of being endowed with life. The proof of 

 this proposition is simple ; and it shows at the same time, in the 

 simplest way, what the plant does with the water and carbonic acid 

 it consumes. Namely, 1st, it is only in sunshine or bright daylight 

 that the green parts of plants give out oxygen gas, then they do ; 

 and 2d, the giving out of this oxygen gas is just what is required to 

 render the chemical composition of water and carbonic acid the same 

 as that of cellulose (454), that is, of the plant's fabric. This shows 

 why plants spread out so large a surface of foliage. 



466. In plants growing or placed under water we may see bubbles 

 of air rising from the foliage ; we may collect enough of this air to 

 test it by a candle's burning brighter in it ; which shows it to be 

 oxygen gas. Now if the plant is making cellulose or plant-substance, 

 that is, is making the very materials of its fabric and growth, as 

 must generally be the case, all this oxygen gas given off by the 

 leaves comes from the decomposition of carbonic acid taken in by 

 the plant. 



467. This must be so, because cellulose is composed of 10 parts of 

 oxygen and 10 of hydrogen to 12 of carbon (454) : here the first 

 two are just in the same proportions as in water, which consists of 

 one part of oxygen and one of hydrogen, so that 10 parts of water 

 and 12 of carbon represent one of cellulose or plant-fabric ; and to 

 make it out of water and carbonic acid, the latter (which is composed 

 of carbon and oxygen) has only to give up all its oxygen. In other 

 words, the plant, in its foliage under sunshine, decomposes carbonic 

 acid gas, and turns the carbon together with water into cellulose, at 

 the same time giving the oxygen off into the air. 



468. And we can readily prove that it is so, namely, that plants 



