698 CHEMICAL PROCESSES IN THE PLANT. 



Only a very small portion of the hydrogen contained in the nitrogenous vegetable 

 substances can be supplied to the plant in the form of ammonia. 



Oxygen is always present in organic compounds in smaller quantities than 

 would be sufficient to oxidise the hydrogen and carbon present in them into 

 water and carbon dioxide, because organic compounds are produced from carbon 

 dioxide and water with the elimination of a part of their oxygen. The proportion 

 of oxygen in vegetable substances is moreover very variable; and some even contain 

 none at all of this element. But the total quantity of oxygen forms, next to carbon, 

 the largest proportion of the weight of the dried substance. Oxygen is intro- 

 duced into the plant in the form of water, carbon dioxide, and oxy-salts, in larger 

 quantities than any other element ; while extraordinarily large quantities of oxygen 

 are set free into the air by the process of assimilation in the green organs. All the 

 other organs of the plant also absorb atmospheric oxygen, and thus slowly re- 

 produce carbon dioxide and water at the expense of the assimilated substances. 

 Together with the process of deoxidation which is very active in the cells containing 

 chlorophyll, another process of oxidation is proceeding comparable to that of the 

 respiration of animals, but not generally very active, by which a part of the assimi- 

 lated substance is again decomposed. 



Nitrogen^ an essential constituent of the albuminoids which form protoplasm, of 

 vegetable alkaloids, and of asparagine,, always forms only a small fraction of the 

 weight of the dried substance of plants, — often less than i, seldom more than 

 3 p. c. The nitrogen contained in the chemical compounds just mentioned is 

 obtained from compounds of ammonia and nitric acid ^ ; parasites and saprophytes 

 perhaps also absorb organic nitrogen-compounds from without. It is on the other 

 hand certain from a great number of experiments on vegetation, especially those of 

 Boussingault, that plants have no power of using the free nitrogen of the atmosphere 

 for the production of their nitrogenous compounds ^. If plants are artificially sup- 

 plied with all other food-materials, but it is rendered impossible for them to absorb 

 ammonia or compounds of nitric acid as their source of nitrogen, no increase takes 

 place of the albuminoids or of the nitrogenous substances generally, although the 

 nitrogen of the atmosphere is at the command of the plant in such great quantities, 

 filling up the intercellular spaces and diffusing through the fluids of the tissue ^ 



Sulphur, a constituent of albuminoids, of allyl, and of the essential oil of 

 mustard, is taken up in the form of soluble salts of sulphuric acid, and chiefly 



* [Although plants containing chlorophyll usually absorb their nitrogen in the form of ammonia 

 or nitrates, they can nevertheless absorb it also in the form of organic compounds such as urea, 

 leucin, tyrosin, creatin, hippuric acid, and similar bodies; they cannot absorb it in the form of 

 alkaloids.] 



^ [The important researches of Lawes, Gilbert, and Pugh on the sources of the nitrogen of 

 vegetation (Phil. Trans. 1861, pt. 2, and Journ. Chem. Soc. 1863, p. 100) should be carefully studied 

 on this point,] 



^ [Adolf Mayer has recently carried out a number of experiments to determine whether the 

 aerial parts of plants have the power of absorbing ammonia or not. Preventing access of ammonia 

 through the roots, he subjected the leaves to the influence of ammonium carbonate both in the 

 gaseous and dissolved state, and found that it was absorbed in appreciable quantities, although 

 the plants did not appear to thrive when access of ammonia through the roots was entirely 

 prevented. Similar results have also been obtained by T. Schloesing (see Comptes Rendus, vol. 

 LXXVIII. p. 1700).] 



