74 



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 fe m 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 pro- 

 ducts 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, alu- 

 minium, &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 sub- 

 stance 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 noimal functions of the plant, taking place at the season 

 when it is desirable to attract the visits of insects for he 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 taking with it the 

 fee or reward provided by nature for that special serx'ice. 



