The chemical processes involved in vegetable life are only comparable to those concerning the animal organism, and are of the same general order. Fed upon w ater, the atmospheric constituents, and the matters present in soil, the list of vegetable products is wide and wonderful. In addition to the cellulosic compounds constituting the cells and vessels, there are the wide range of colouring matters, the various ilkaloidal bodies, such as aconitine, quinine, and morphine, a range of carbohydrates, including starches and sugars, a still wider range of essential oils and other volatile principles, and innumerable chemical products ha, ing medicinal or commercial value, all of which are built up synthetically by the agency of the nitrogenous protoplasm. Vegetation has an important bearing upon the health of communities as, apart from the capacity of plant life for absorbing carbon dioxide from the an and restoring oxygen thereto, trees absorb from the soil a great deal of water, which for the most part is subsequently evaporated into the air from the surfaces of the leaves, so that many malarious districts can be made drier and more healthy by means of plantations. Again, many plants and trees give off essential oils into the air and thus exercise a beneficial effect due to their balsamic character and the subsequent production of hydrogen dioxide by the action of the air and moisture upon these oils. Vegetation also assists in the purification of polluted waters and soils by utilizing their organic constituents. As a rule, plants obtain what nitrogen they requixe from the ammonia and other soluble nitrogen compounds that are present in the air and soil, the ammonia for the most part coming from products of organic decay; but some plants of the leguminous order, such as peas and beans, assimilate nitrogen direct from the air by some action of bacteria situate in nodules which form on the rootlets of these plants.
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