6S ELEMENTS OF BIOLOGY. 



elements, carbon, hydrogen, oxygen, and nitrogen, alone. 

 Other good authorities deny the existence of any sucl; base 

 as proteine. Nevertheless, it is a common and often a very 

 convenient practice to speak of the various albuminoid 

 substances of animals or vegetables as " proteids," or " pro- 

 teine compounds." 



CHEMISTRY OF VEGETABLES. 



The organic substances which compose the tissues of 

 plants, as in the case of those of animals, may be divided 

 into a non-nitrogenous and a nitrogenous group, according 

 as they consist of carbon, hydrogen, and oxygen alone, or 

 contain nitrogen in addition to these three elements. The 

 chief difference to be noted between animals and vegetables, 

 as regards their chemical composition, concerns the pro- 

 portion borne by the nitrogenous substances to the non- 

 nitrogenous. In both kingdoms we find " proximate prin- 

 ciples " which, if not actually identical, at any rate represent 

 each other ; but there is a considerable distinction in the 

 relative amount of the two groups of compounds in a plant 

 as compared with an animal. Animal bodies exhibit a 

 marked predominance of albuminoid or nitrogenous com- 

 pounds over the fatty or non-nitrogenous compounds. 

 Plants, on the other hand, are mainly composed of non- 

 nitrogenous compounds, and they are, comparatively speak- 

 ing, poor in albuminous or nitrogenous matter. 



The chief 7ion-nitrogenous principles of plants are starch, 

 cellulose, and sugar, all of which differ from the fatty com- 

 pounds of animals in the fact that the oxygen is present in 

 sufficient quantity to form water with the hydrogen. Plants, 

 however, are by no means destitute of non-nitrogenous sub- 

 stances in which the proportion 'of oxygen is less than this, 

 or in which, indeed, oxygen is wholly absent. 



Starch is composed of Carbon, Hydrogen, and Oxygen, 

 with the formula CigHj^Oio- It occurs plentifully in vege- 

 table tissues, especially in seeds, fruits, stems, and roots; and 



