82 BOTANY. 



to the alkaloids. These are compounds of carbon, hydro- 

 gen, nitrogen, and generally oxygen, as follows : 



Nicotine (CioHuNj), found in tobacco. 

 Cinchonia (C^oHjiNsOa), found in Peruvian bark. 

 Morphia (OnHiaNOa), found in the opium-poppy. 

 Strychnia (C21H22N2O2), found in the seeds of Strychnos. 

 Caffeine (CoHioNjOa), found in coffee and tea. 



147. These and many others occur in plants in combina- 

 tion with organic acids, such as malic acid (C,H,0 J ; tar- 

 taric acid (C,HjO„); citric acid (C^HjO,); oxalic acid 

 (OjH^OJ; tannic acid (C,JI,„0,). These acids are proba- 

 bly formed by the oxidation of some of the sngary or starchy 

 substances in the plant, while the alkaloids with which they 

 are combined appear to have some relation to the nitro- 

 genous constituents of the protoplasm. 



148. From the fact that the alkaloids are formed more 

 abundantly in those tissues which have passed the period 

 of their greatest activity, it may be surmised that they are 

 either compounds of a lower grade than the ordinary albu- 

 minoids, or the first results of the incipient decay of the 

 cells. 



149. Results of Assimilation and Metabolism. — In the 

 preceding paragraphs we have found that chlorophyll-bear- 

 ing plants absorb carbon dioxide and exhale free oxygen, 

 the former being decomposed in the chloroplasts in sun- 

 light, and the oxygen being set free as a consequence. In 

 other words, the absorption of carbon dioxide and the ex- 

 halation of oxygen are essential parts of the process of car- 

 bon-assimilation. 



150. Now, it may be shown that oxygen is absorbed and 

 carbon dioxide evolved, as results of certain ~ metabolic 

 processes which take place in any tissues, whether possess- 

 ing chlorophyll or not, and independently of the presence 



