SECTION 16.] PLANT FOOD AND ASSIMILATION. 147 



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

 natural processes are going on in whicli nitrogen of the air is so combiued 

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

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

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



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

 stances — essentially, water, carbonic acid, and some form of comi)iued or 

 combinubL' 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, anting upon green parts or fulic/ge, 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 cliemical combinations, — to organ- 

 ize matter into forms which are alo'ie capable of being endowed with life. 

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

 t!i3 simplest way, what a plant does with the water and carbonic acid it 

 consumes. Namely, 1st, it is only in sunshine or bright dayhght that the 

 green parts of plants give out oxygen gas, — then they regularly do so; 

 and 2J, 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 

 lar^e a surface of foliage. Leaves are so many workshops, full of ma- 

 chinery worked by suu-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 tiiat 

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

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

 proper exposure to sunshine, the air ou 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 th3 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 wiiich the carbon 

 l).-ought with it as carbonic acid, and which is given off from the foliage in 

 sunshine, just represen's the manufactured article, cellulose. 



453. It comes to t'.ic same if the first product of assimilation is sugar, 

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

 plant all those forms arc rradily 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 of 



