464 Comparative Physiology. 



being decomposed into carbonic acid and dissolved in the water 

 of tbe soil. It is found that those soils which contain the most 

 steady and equable supply of carbonic acid are the most favour- 

 able to vegetable growth. It is only within a short period that 

 the dependence of the activity of vegetation on a due supply of 

 nitrogen has been ascertained. Nitrogen in the form of ammonia 

 is regarded by Liebig as the most powerful stimulant to the 

 vegetative processes, when the other requisite conditions are 

 supplied. It is interesting to remark that none of the elements 

 of vegetables are introduced in a simple or uncombined state 

 into the plants ; oxygen and hydrogen are combined in the 

 state of water, oxygen and carbon in the state of carbonic acid, 

 and hydrogen and nitrogen in the state of ammonia. The only 

 class of plants that seems dependent for support on matter al- 

 ready organised is that of Fungi ; a group of peculiar interest 

 from their rapidity of growth, their universality of diffusion as 

 dormant germs ready to be developed, and the importance of 

 the offices they perform, like insects being denominated the 

 scavengers of nature. The large quantity of carbonic acid 

 which their absorbent system furnishes prevents the necessity 

 of deriving any additional supply from the atmosphere. The 

 proportion of nitrogen contained in their tissues is much greater 

 than in any other vegetable, so that fungin, a proximate prin- 

 ciple which may be obtained from them, is as highly azotised as 

 animal flesh. Vegetables seem to constitute the intermediate 

 link, in the scale of creation, between the Inorganic World and 

 the Animal Kingdom ; the latter being altogether dependent for 

 its support, and even existence, on the Vegetable Kingdom." 



According to Liebig, the food of animals consists principally 

 of substances which are similar in composition to the tissues of 

 the body. The vegetable fibrine or gluten, albumen and caseine, 

 are similar to the same substances found in animals. They are 

 also similar to each other, differing only in the quantity of 

 saline earthy matters they contain, and have all for their basis 

 the same proportions of carbon, hydrogen, oxygen, and nitrogen. 

 This basis he reckons a separate substance, and it has been deno- 

 minated proteine by its discoverer Mulder, who found that it 

 was separated by potash from vegetable iibrine, albumen, and 

 caseine, in the same way as from the same substances in ani- 

 mals. This substance, proteine, united to earthy saline sub- 

 stances, he says, is the basis of all the animal tissues, with the 

 exception of the nervous, and requires only to be dissolved in 

 the blood, and assimilated by each organ in its own way ; che- 

 mical decomposition and recomposition not being required ex- 

 cept for the formation of such as nervous matter. The non- 

 azotised substances of the food, he says, are formed into bile 

 along with the waste of the body, to carry on respiration, what 



