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 oxj^gen. 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.] 
2 [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).] 
