25 



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 lite 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 

 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 merelv taking with it the fee or reward provided hv nature 

 for that special service. 



Sometimes thrown off as superfluous. 



There are, however, occasions when considerable quantities of 

 such matter are thrown off or exuded by the leaves, which effect is 

 taken to indicate an abnormal or unhealthy condition of the plant. 

 At pages 106 and 107 of Liebig's book (speaking of an experiment 

 made to induce the rising sap of a maple-tree to dissolve raw sugar 

 applied through a hole cut in the bark) he shows (in a passage 

 already quoted at page 86) that, — 



When a sufficient quantity of nitrogen is not present to aid in the 

 assimilation of the substances destitute of it, these substances will be 

 separated as excrements from the bark, roots, leaves, and branches 



