CONDITIONS ESSENTIAL TO NUTRITION. 103 



tion between them would be a condition necessary for their 

 production. 



In the buds and young leaves, we find salts with alkaline bases ; 

 we find also the azotized constituents invariably accompanied by 

 salts of phosphoric acid : we must, therefore, suppose that these 

 substances execute some functions necessary to the support of 

 the vital processes of plants. We may suppose that, in the ab- 

 sence of certain constituents of the soil, the compounds of plants 

 containing nitrogen and sulphur could not be formed, and that 

 without the presence of such compounds and of alkaline bases, 

 carbonic acid could not be taken up and decomposed. 



According to this view, the assimilation of the substances 

 generate* m the leaves will (cceteris paribus) depend on the 

 quantity f nitrogen contained in the food. When a sufficient 

 quantity if 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. The 

 exudations of mannite, gum, and sugar, in strong and healthy 

 plants, cannot be ascribed to any other cause.* 



Analogous phenomena are presented by the process of diges- 

 tion in the human organism. In order to restore the loss sus- 

 tained by every part of the body in the processes of respiration 

 and perspiration, the organs of digestion require to be supplied 

 with food, consisting of substances containing nitrogen and of 

 others destitute of it, in definite proportion, and also with certain 

 mineral substances to effect their transformation into blood. If 

 the substances d< stitute of nitrogen preponderate, either they 

 irill be expended in the formation of fat, or they will pass un- 

 manged through the organism. This is particularly observed 



* M. Trapp, in Giessen, possesses a Clerodendron fragrans growing in 

 the house ; it exudes on the surface of its leaves, in September, large 

 colorless drops, which form regular crystals of sugar-candy upon drying ; — 

 I am not aware whether the juice of this plant contains sugar. Langlois 

 has lately observed, during the dry summer in 1842, that the leaves of the 

 linden-tree became covered with a thick and sweet liquid, in such quan- 

 tity, that for several hours of the day it ran off the leaves like drops of 

 rain. Many kilogrammes might have been collected from a moderately- 

 sized linden-tree. This sweet juice contained principally grape sugar and 

 mannite. (Annales de Chimie et Physique, iii. Serie, torn, vii., p. 34& x 



