1012 FOOD OF PLANTS. 



culiar ferment, to effect the conversion of the former into new 

 substances at the proper season. But the action of ferments is 

 a department of chemistry still in its infancy. 



Food of Plants. With the exception of the provision for 

 the first growth of the young plant which exists in its seed, 

 the food of plants appear to be exclusively inorganic. M. 

 Liebig has ably shown that the humus or decayed vegetable 

 matter which exists in soils is not absorbed and assimilated 

 by plants the extremely sparing solubility of that substance 

 being manifestly incompatible with its absorption in any con- 

 siderable quantity, while even if humus did enter plants, the 

 presumption is that like a solution of gum or sugar absorbed 

 by the roots, it would pass through the plants unchanged, and 

 be excreted by the leaves. The admitted value of humus in 

 soil appears to depend almost exclusively upon its decomposi- 

 tion by the atmosphere, which is greatly assisted by tillage, and 

 the formation of carbonic acid, which gas dissolved in water is 

 taken up by the spongioles of the roots, and supplies the plant 

 with carbon. 



The ultimate constituents of all plants are oxygen and 

 hydrogen, carbon, nitrogen, with a small portion of mineral 

 acids and bases in the form of salts ; and the condition in which 

 the first mentioned substances enter the plant, adopting the 

 conclusions of M. Liebig, are all the hydrogen, and most of 

 the oxygen, as water, the carbon as carbonic acid, and the 

 nitrogen as ammonia. All these matters are derived from the 

 atmosphere. 



Water, or its elements in the proportions of water, enters 

 largely into the constitution of vegetable matter, forming 50 

 per cent of lignin, and an equally large proportion of the other 

 neutral principles, starch, gum, sugar, &c. Certain hydro- 

 genated compounds are also found in plants produced by 

 the fixation of the hydrogen of water without its oxygen, which 

 are employed by the plant for accessory purposes. They form 

 the volatile oils which serve as its defence against the ravages 

 of insects ; the fixed oils, or fats, which envelope the seed, 

 and which serve to develope heat by burning at the period 

 of germination ; and the wax with which the leaves and fruit 

 are coated to render them impermeable to water. 



Caroonic Acid is found as a constituent of air in a proportion 



