30 



is absent. The compounds destitute of nitrogen may be divided into 

 those in which oxygen forms a 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 tne 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 atmo- 

 sphere; 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 iijiportant 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 sub- 

 stances 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, there- 

 fore, be employed in their nutrition. 



NECTAR OP 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 ofi the honey from any blossom it has visited, is merely 



