SECTION 16.] PLANT FOOD AND ASSIMILATION. 147 



(uitric acid and nitrates) plants appropriate it with avidity. And several 

 natural processes are going on in which nitrogen of the air is so combined 

 and supplied to the soil in forms directly available to the plant. The most 

 efficient is nitrification, the formation of nitre (nitrate of potash) in the soil, 

 especially in all fertile soils, through the action of a bacterial ferment. 



450. Assimilation in plants is the conversion of these inorganic sub- 

 stances essentially, water, carbonic acid, and some form of combined or 

 combinable nitrogen into vegetable matter. This most dilute food the 

 living plant concentrates and assimilates to itself. Only plants are capable 

 of converting these mineral into organizable matters ; and this all-important 

 work is done by them (so far as all ordinary vegetation is concerned) only 



451. Under the light of the sun, acting upon green parts or foliage, that 

 is, upon the chlorophyll, or upon what answers to chlorophyll, which these 

 parts contain. The sun in some way supplies a power which enables the 

 living plant to originate these peculiar chemical combinations, to organ- 

 ize 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 a 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 regularly do so ; 

 and 3d, the giving out of this oxygen gas is required to render the chemical 

 composition of water and carbonic acid the same as that of cellulose, that 

 is, of the plant's permanent fabric. This shows why plants spread out so 

 large a surface of foliage. Leaves are so many workshops, full of ma- 

 chinery worked by sun-power. The emission of oxygen gas from any 

 sun-lit foliage is seen by placing some of this under water, or by using an 

 aquatic plant, by collecting the air bubbles which rise, and by noting that 

 a taper burns brighter in this air. Or a leafy plant in a glass globe may 

 be supplied with a certain small percentage of carbonic acid gas, and after 

 proper exposure to sunshine, the air on being tested will be found to con- 

 tain less carbonic acid and just so much the more oxygen gas. 



452. Now if the plant is making cellulose or any equivalent 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. For cellu- 

 lose, and also starch, dextrine, sugar, and the like are composed of carbon 

 along with oxygen and hydrogen in just the proportions to form water. 

 And the carbonic acid and water taken in, less the oxygen which the carbon 

 brought with it as carbonic acid, and which is given off from the foliage in 

 sunshine, just represents the manufactured article, cellulose. 



453. It comes to the same if the first product of assimilation is sugar, 

 or dextrine which is a sort of soluble starch, or starch itself. And in the 

 plant all these forms are readily changed into one another. In the tiny 

 seedling, as fast as this assimilated matter is formed it is used in growth, 

 that is, in the formation of cell-walls. After a time some or much 01 



