38 HOW CROPS GROW. 



ture, and which are produced from the elements carbon, 

 oxygen, hydrogen, nitrogen, sulphur, and phosphorus, by 

 chemical agency. The number of distinct substances 

 found in plants is practically unlimited. There are 

 already well known to chemists hundreds of oils, acids, 

 bitter principles, resins, coloring matters, etc. Almost 

 every plant contains some organic body peculiar to itself, 

 and usually the same plant in its different parts reveals 

 to the senses of taste and smell the presence of several 

 individual substances. In tea and coffee occurs an 

 intensely bitter " active principle," caffeine. From 

 tobacco an oily liquid of eminently narcotic and poison- 

 ous properties, nicotine, can be extracted. In the orange 

 are found no less than three oils ; one in the leaves, one 

 in the flowers, and a third in the rind of the fruit. 



Notwithstanding the great number of bodies thus 

 occurring in the vegetable kingdom, it is a few which 

 form the bulk of all plants, and especially of those which 

 have an agricultural importance as sources of food to 

 man and animals. These substances, into which any 

 plant may be resolved by simple, partly mechanical means, 

 are conveniently termed proximate principles, and we 

 shall notice them in some detail under eight principal 

 classes, viz.: 



1. WATER. 



2. The CARBHYDRATES. 



3. The VEGETABLE ACIDS. 



4. The FATS and OILS. 



5. The ALBUMINOIDS or PROTEIX BODIES and FER- 



MEXTS. 



6. The AMIDES. 



7. The ALKALOIDS. 



8. PHOSPHORIZED SUBSTANCES. 



i. Water, H 2 0, as already stated, is the most abund- 

 ant ingredient of plants. It is itself a compound of 

 oxygen and hydrogen, having the following centesimal 

 composition : 



