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is absent. The compounds destitute of nitrogen may be divided into those in which oxygen forms & constituent (starch, lignine, &c.) and those into which it does not enter (oils of turpentine, lemon, &c.)
And, at page 141, that-
Sugar and starch do not contain nitrogen; they exist in the plants in a free state, and are never combined with salts or with alkaline bases. They are compounds formed from the carbon of the carbonic acid and the elements of water (oxygen and hydrogen).
Sir Humphrey Davy had already stated that, ‘‘according to the latest experiments of Gay Lussac and Thenard, sugar consists of 42:47 per cent. of carbon and 57:23 per cent. of water and its constituents.” Now, Liebig in several parts of his work shows that the carbon in sugar and all vegetable products is obtained from carbonic acid in the atmosphere; and that ‘‘plants do not exhaust the carbon of the soil in the normal condition of their growth; on the contrary, they add to its quantity.’’
DERIVED FROM THE ATMOSPHERE AND RAIN-WATER.
The same authority shows that the oxygen and hydrogen in these products are derived from the atmosphere and from rain-water; and that it is only the products containing nitrogen (such as gluten or albumen in the seeds or grains), and those containing mineral matter (silex, lime, aluminium, &c.), which take away from the soil those substances that are required to be returned to it in the shape of manures. The saccharine matter, once it is secreted by the plant and separated from it, is even useless as a manure. Liebig says on this head, page 21,-
The most important function in the life of plants, or, in other words, in their assimilation of carbon, is the separation—we might almost say the generation—of oxygen. No matter can be considered as nutritious or as necessary to the growth of plants which possesses a composition either similar to or identical with theirs, because the assimilation of such a substance could be effected without the exercise of this function. The reverse is the case in the nutrition of animals. Hence such substances as sugar, starch, and gum, themselves the products of plants, cannot be adapted for assimilation; and this is rendered certain by the experiments of vegetable physiologists, who have shown that aqueous solutions of these bodies are imbibed by the roots of plants and carried to all parts of their structure, but are not assimilated; they cannot, therefore, be employed in their nutrition.
NECTAR OF PLANTS INTENDED TO ATTRACT INSECTS.
The secretion of saccharine matter in the nectaries of flowers is shown to be one of the normal functions of the plant, taking place at the season when it is desirable to attract the visits of insects for the purposes of its fertilisation. It may, then, be fairly asserted that the insect, when it carries off the honey from any blossom it has visited, is merely
