^ .:-':;•«";: THE chemistry o:^ the j^arm 



If a branch of a tree is burnt, the greater part is 

 consumed and passes away in the form of gas ; but 

 there is left behind a small quantity of white ash. 

 The same happens if any other part of a plant is 

 burnt. The constituents which form the dry matter 

 of plants may be thus conveniently divided into two 

 classes — the combustible and the incombustible. 



The covihustible part of plants is made up of 

 six chemical elements — carbon, oxygen, hydrogen, 

 nitrogen, and sulphur, with a little phosphorus ; 

 without these no plant is ever produced. Carbon 

 generally forms about one-half of the dry combustible 

 matter of plants. Nitrogen seldom exceeds 4 per 

 cent, of the dry matter, and is generally present in 

 much smaller amount. Sulphur and phosphorus are 

 still smaller in quantity. The remainder is oxygen 

 and hydrogen. 



The carbon, hydrogen, and oxygen form the cellu- 

 lose, lignin, pectin, gummy matters, starch, dextrin, 

 sugar, fat, and vegetable acids which plants contain. 

 The same elements united with nitrogen form the 

 amides and alkaloids ; and further united with sulphur 

 the still more important albuminoids, which are 

 essential constituents of all plants. Nuclein and 

 lecithin also contain phosphorus. 



The incombustible, or ash constituents, form gener- 

 ally but a small part of the plant. The timber of 

 freely-growing trees contains but OS— 04 of ash con- 

 stituents in 100 of dry matter. In seeds free from 

 husk the ash is generally 2 — 5 per cent, of the dry 

 matter. In the straw of cereals 4 — 7 per cent. In 

 roots and tubers 4—8 per cent. In hay 5—9 per 

 cent. It is in leaves, and especially old leaves, that 



