CHAPTER II. 



CHEMICAL PROCESSES IN THE PLANT. 



Sect. 4. — The Elementary Constituents of the Food of Plants ^ If we 



dry at a temperature of 100° or 1 10° C. any fresh vegetable structure so as to expel all 

 the water which it contains, a friable residue will be left which no longer loses weight. 

 In ripe seeds this usually amounts to about f of the weight ; in seedlings after the 

 supply of reserve-material has been consumed, to generally less than ^q, increasing 

 in subsequent stages of vegetation to i or ^; in submerged water-plants and 

 Fungi it often amounts to less than yV? and sometimes even to only Jg-. These 

 proportions, which are only roughly estimated, vary within wide limits according to 

 the nature and age of the plant and of the particular organ. 



If the dried residue of the plant is further exposed to a red heat in the 

 presence of oxygen, by far the greater part of it is consumed and disappears in 

 the form of products of combustion, chiefly carbon dioxide and aqueous vapour. 

 The residue which now remains behind, usually a fine white powder, is the Ask, 

 constituting generally only a small percentage of the dried substance, a proportion 

 which is again subject to great variations with the specific nature of the plant and 

 the kind and age of the particular organ. 



Chemical analysis of the combustible part of the dried substance shows that it 

 consists in all plants of Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur ; the 

 latter remains behind after combustion in the form of sulphuric acid in combination 

 with the bases of the ash. 



In the ash are invariably found in addition Potassium, Calcium, Magnesium, 

 Iron, and Phosphorus, and generally Sodium (Lithium?), Manganese, Silicon, and 

 Chlorine ; in marine plants also Iodine and Bromine. With these constituents 

 there are sometimes associated, in rare cases and under special circumstances, 

 very small quantities of Aluminium, Copper, Zinc, Cobalt, Nickel, Strontium, and 

 Barium. The presence of very small quantities of Fluorine in plants is also 

 inferred from the presence of calcium fluoride in the bones of animals which obtain 

 the whole of their food directly or indirectly from plants. 



^ For a preliminary acquaintance with the very copious literature, my Handbuch der Experi- 

 mental-Physiologic (Sects. 5 and 6) will be sufficient. A study of Th. de Saussure's Recherches 

 chimiques sur la vegetation, Paris 1804, is also indispensable to any one who wishes to form an 

 independent judgment for himself. A detailed description of the theory of nutrition is contained in, 

 among other works, Mayer's Lehrbruch der Agriculturchemie, 1876. A variety of fundamental 

 researches will also be found in Boussingault's Agronomic et Physiologic vegetale. E. Wolffs 

 Aschenanalyse fiir landwirthschaftliche Prod. &c., Berlin 1871, is also very valuable; as well as 

 his Vegetationsversuche in wasserigen Losungen ihrer Nahrstoffe (Hohenheimer Jubilaumsschrift, 

 1862). 



