3S TROPICAL AGRICULTURE 



CHAP, 



solved in water, for neither the root nor any other part of 

 the plant can take up solid food. Sugar, salt and other such 

 solid bodies, are easily dissolved in water ; that is to say, 

 the substances disappear in the water, but they still exist in 

 a different form, as may be known by the taste. Now the 

 water in the soil aided by carbonic acid and the acid fluid 

 of the root-hairs, dissolves the various soluble substances 

 inexactly the same way as water dissolves sugar. Ammonia, 

 carbonic acid and compounds of lime, potash, &c,, are always 

 present in a soluble form in a fertile soil, and thus they can 

 be taken up by the roots and used for the nutrition and 

 growth of plants. The carbonic acid and ammonia in the 

 soil comes from the rain (wliich brought them down from 

 the atmosphere) and from the decomposition or putrefac- 

 tion of the animal and vegetable matter in the earth, but 

 the inorganic compounds are derived from the soil itself ; 

 which, as we have seen, has been formed out of the hard 

 rocks. 



The Composition of Plants. — The whole process of 



the nutrition of plants will be better understood after a 



knowledge of their composition is gained. If a plant be 



The ashes of taken with its roots, stem and leaves, and destroyed by fire, 



plants. nothing will be left but a small quantity of ashes. About 95 



parts have been burnt off, and only about 5 parts remain. 



The 95 parts have disappeared in the form of gas or vapour ; 



they are styled the oi-ganic or volatile portions, and they 



have come principally from the atmosphere. The ashes 



which are left are called the inorganic or mineral portions, 



and they have come from the soil. Sometimes the ashes 



are in larger or smaller proportions than 5 per cent., the 



limits being from i to 11 per cent., but 5 per cent, may be 



taken as the average quantity. 



The organic The volatile portions of the plant are of two kinds, one 



volatile^ ^^^ containing all the organic elements except nitrogen — and 



called, therefore, 7ion-7iilrogenous, and the other containing 



