22 LEAVES 



Description. The full-grown tea leaf is from 5 to 10 cm. long, 

 dark green in colour, lanceolate or elliptical in outline and blunt 

 at the apex, tapering at the base into a short stalk ; the margin is 

 distantly and shortly serrate, the serrations terminating in charac- 

 teristic, glandular teeth which readily break off and are often absent 

 from mature leaves. When quite young the leaves are covered with 

 silky hairs, but as they mature these are lost, and the surface becomes 

 glabrous. This difference is readily observed in commercial tea, the 

 bud still bearing numerous hairs (' flowery ' Pekoe), while the larger 

 leaves are glabrous or nearly so (Congou). 



Microscopical characters. Fragments of the leaves may be readily recognised 

 under the microscope by the small cells of the upper epidermis, those 

 of the lower epidermis being rather larger and accompanied by numerous large 

 stomata and long, thick- walled, one-celled simple hairs bent near the base ; 

 in the mesophyll, especially of older leaves, large, elongated, branching, thick - 

 walled, sclerenchymatous idioblasts occur. 1 



Constituents. The principal constituents of tea are caffeine and 

 tannin. It contains in addition traces of theobromine, theophylline 

 and volatile oil. 



Caffeine or trimethylxanthine, C 5 H(CH 3 ) 3 N 4 O 2 ,H 2 O, an alkaloid obtain- 

 able in colourless silky crystals melting at 237, was isolated from coffee by 

 Runge in 1820 and from tea by Oudry in 1827. It occurs also in mate (the 

 leaves of Ilex paraguayensis, Lambert, which are largely used in the Argentine 

 Republic as tea is in this country), in cola seeds, and in guarana. Tea contains 

 from 1 to 5 per cent, (usually 3 to 4) of it apparently in combination with tannin, 

 the combination being decomposed by water and the caffeine liberated. The 

 tannin varies from 10 to 24 per cent. But the commercial value of tea is not 

 determined by the percentage of caffeine or of tannin contained in it, but by 

 a combined consideration of several factors, such as the appearance, the size 

 of the leaves as indicating their age, the presence of ' tip ' (unexpanded leaf- 

 bud), and the taste of the infusion. 



Theobromine, or dimethylxanthine, C 5 H 2 (CH 3 ) 2 N 4 O 2 , and its isomer theo- 

 phylline are closely allied to caffeine, but occur in small proportion only. 



Caffeine, theobromine, and theophylline are derivatives of xanthin, C 5 H 4 N 4 2 , 

 which, with a number of other substances, is regarded as containing the purin 

 group, C 5 H 4 N 4 . Tea, coffee, and other drugs containing caffeine, theobromine, 

 or a similar purin derivative form therefore a natural group the members of 

 which are allied by the similarity of their chief constituents. 



The following are the most important drugs belonging to the purin group : 



Tea, the leaves of C. Thea, containing caffeine (1 to 5 per cent.), theobromine, 

 and theophylline. 



Coffee, the seeds of Coffea arabica, Linne (N.O. Ruibiacece), containing caffeine 

 (0-4 to 1-5 per cent.). 



Cola Seeds, the seeds of Cola vera, Schumann (N.O. Sterculiacece), containing 

 caffeine (2 to 2-5 per cent.) and traces of theobromine. 



1 Compare Greenish, Foods and Drugs, p. 122 ; Greenish and Collin, Anatomical 

 Atlas, p. 89. 



