NATURE OF FOOD. 65 



directly to the means of fertility, of securing greater abundance 

 in future harvests. 



3. The kind of food must be such as the vital forces of the 

 plant can assimilate ; such as lis peculiar constitution requires. 

 The nature* of the food of plants has been a subject of much 

 conjecture and controversy. Lord Bacon believed it to be 

 water ; Tull and Du YidJueX, pulverized, earth ; Hunter, oil and 

 salt. But the investigations of modern chemists, have thrown 

 much light on this subject, although some things are not yet 

 settled. It now appears, that the food of vegetables, like that 

 of animals, consists of several substances ; that it is derived 

 from numerous sources. The principal substances regarded 

 as food,t are carbonic acid,| ammonia, water, and several or- 

 ganic substances which form the constituents of vegetable 

 mould, and alkalies, alkaline earths, metallic oxides and seve- 

 ral salts. 



The vegetable mould, according to the analysis of Ber- 

 zelius, consists of several compounds of carbon, oxygen, hy- 

 drogen and nitrogen, called humin, humic, crenic and apocre- 

 nic acids. The humic acid has been called geine. These 

 substances are combined in the soil, in part, with alkalies and 

 oxides, and constitute the principal food which plants derive 

 from that source. 



But different species of plants require different kinds of 

 food, or require it in different quantities. Plants which 

 contain a large quantity of nitrogen, must be supplied with 



* The full consideration of this subject will be deferred to a future 

 section, on the source and assimilation of the simple bodies which en- 

 ter into the composition of plants. 



t Nutritive matters are, correctly speaking, those substances which 

 when presented from without, are capable of sustaining the life, and 

 all the functions of an organism, by furnishing to the different parts^ 

 of plants, the materials for the production of their peculiar constituents. 

 — Liebig. 



X For a description of these bodies, see second and third chapters. 



6 



