ORGANIC PARTS OF PLANTS. 245 



only is furnished by nature otherwise than in the greatest profu- 

 sion. Oxygen gas makes about one-fifth, and nitrogen or azote 

 about four-fifths of atmospheric air ; and pure water is a compound 

 of 8 parts of oxygen and 1 of hydrogen. Carbon in the form 

 of carbonic acid gas is universally present in the atmosphere, and in 

 variable proportions ; but usually (over land) making about 3 ^o3 

 only of the whole bulk. In weight, the proportion of carbonic acid 

 is 70*0 o f ^ ne atmosphere. Small as is this proportion, still, as it 

 is present in the air surrounding and in contact with all growing 

 plants, their supply might be deemed inexhaustible, provided they 

 possessed the power of attracting and arresting it, and taking up 

 and assimilating the carbon of the gas. But this power seems to 

 be not fully exerted under ordinary circumstances. The other 

 great elements, oxygen, hydrogen, and azote are in unlimited 

 quantity surrounding plants, as constituents of the atmosphere, or 

 entering and filling the bodies of plants as the constituents of wa- 

 ter. And as the atmosphere always contains, in large proportion, 

 water dissolved by heat, that is, the water itself being in gaseous 

 form, therefore the ordinary atmosphere alone offers to plants all 

 the four great elements required to constitute nearly their whole 

 substance. If then we suppose that the very small proportions of 

 necessary salts, found in the ashes of plants, are already in the 

 soil (as is generally the case), or, if not naturally present, to be 

 supplied by art, it is manifest that all cultivated plants, on all 

 soils and on the most barren not appreciably less than on the 

 richest have at hand unlimited supplies of all materials required 

 for their sustenance and growth. But the power to seize upon 

 these materials is either wanting, or possessed but to a strictly 

 limited extent. And it is in proportion to the power to use them, 

 and not to the abundance of the resources present, that the sup- 

 port and growth of plants are regulated. 



The proportions of the atmospheric constituents of each particu- 

 lar vegetable product (as gluten, starch, sugar, wax, &c.) seem to 

 be uniform ; and of each of the more compound products of a par- 

 ticular plant (as its seeds, flowers and leaves, bark, wood, &c., of 

 like age and kind), the constituents seem to approach uniformity 

 of proportions ; so that it may be inferred that the differences are 

 caused by differences of conditions, of wants and supplies; and that, 

 under like conditions, the constituents, organic and inorganic, would 

 be in like propositions. But the quantities of the simpler pro- 

 ducts of plants of like kind (as gluten or starch in wheat, sugar 

 in beets, &c.) vary greatly, and of course cause variation in the 

 proportions of elementary constituents of the entire plant. Es- 

 pecially does the proportion of azote vary in like plants, under 

 different circumstances of supply, even when the other constituents 

 vary but little. Boussingault found the following proportions in 



