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LECTURE XVII. 



The view was once held that it was ammonia salts particularly which, 

 being absorbed by the roots of plants, or perhaps indeed by their leaves in the dew 

 and rain water, yielded the nitrogen for the proteid compounds of the plant. But 

 the result of our water-culture proves, in the first place, that plants are able at 

 any rate to absorb the whole of their nitrogen in the form of compounds of nitric 

 acid ; but if, on the other hand, the attempt is made to replace the latter by ammonia 

 salts, experimental difficulties make their appearance which we will not here discuss 

 in detail. If, further, it is remembered that the ammonia produced in nature by 

 the rotting and decomposition of organic remains, especially in vegetable soil, is 

 easily transformed in the presence of potassium salts into compounds of nitric acid, 

 which may be detected everywhere in vegetable soils and waters, and that nitric 

 acid is contained in rain water, although in very small quantities, one comes to 

 the conclusion that (apart perhaps from certain special cases, and particularly apart 

 from parasites and Fungi) ordinary green plants obtain the nitrogen for the formation 

 of their proteid substances, and therefore of their protoplasm, from salts of nitric 

 acid. It may here be remarked in addition that the soil of gardens, wheat-fields, 

 vineyards, orchards, and so forth, is usually somewhat deficient in compounds of 

 nitric acid, and that on this account the production of vegetable substance, although 

 all other nutritive matters are present in the soil, proves to be relatively small; a 

 suitable addition of saltpetre is in such cases always calculated to increase the 

 vegetation to its utmost luxuriance, and if other nitrogenous manures act similarly, 

 we may assume that they give rise in the soil, sooner or later, to salts of nitric acid, 

 which are absorbed by the roots. 



Finally, the question remains as to the source of the carbon in our experimental 

 plants. If we suppose the plant, after driving off the water, to be dried at 

 ioo-i20°C., about half of the total dry substance consists of carbon. This 

 element in fact is contained in every organic compound, as well as in the cellu- 

 lose, protoplasm, and fatty bodies which play the most prominent part in plants. 

 Our plants, nourished with aqueous solutions of salts, could not absorb any 

 carbon compound, however, from this nutritive solution. Investigation shows, 

 on the contrary, that they continually excrete small quantities of a carbon com- 

 pound, viz. carbon dioxide ; and I shall subsequently show that it is entirely 

 erroneous to suppose that the carbon dioxide which usually abounds in the soil 

 is absorbed by the roots and conveyed to the leaves. The question for us now, 

 however, is whence comes this great mass of carbon which is gradually accu- 

 mulated in the organic compounds of our experimental plants? It is, at any 

 rate, not derived from the nutritive solution. There remains, therefore, only 

 one source, the atmosphere — half of the dry weight of the organic substance of 

 plants must be derived from the atmosphere. This fact now, after two hundred 

 years of scientific progress, follows with absolute certainty from every experi- 

 ment on nutrition with green-leaved plants. But the founders of the theory of 

 the nutrition of plants, Ingenhouss and De Saussure, discovered this fact — one 

 might almost say the most important fact of biological science — by very different 

 methods ; and in my ' History of Botany ' I have attempted to show how this 

 important natural process was gradually discovered by these men, and to what 

 misconceptions and foolish objections by incompetent minds, even this now in- 



