28 CHEMISTRY OF PLANTS. 



more constantly in cells presenting a low degree of vital activity, as those 

 of the wood and bark ; and those of early decaying excrescences, as galls : 

 but still it is found in many leaves, as those of the tea-plant, and of the 

 Ericacece; but here, perhaps, it only occurs in the bundles of vessels, or 

 less actively vital cells, of the leaf. Frequently the cells of the bark 

 have few or no contents ; and I may, perhaps, hazard the opinion that 

 the tannin is only found in the cell-wall, and perhaps as a product of the 

 commencing decomposition of the cellulose. If two equivalents of 

 cellulose, C 24 H20 O 20, take up 16 O from the air, and 12 HO 

 (water), and 6 CO 2 (carbonic acid) disappears, there will be left one 

 atom of tannin (CIS H 8 O 12). The formation of tannin may be 

 conveniently regarded as the commencing process of putrefaction 

 in the cell-membrane. According to Mulder's formula of cellulose, 

 C 24 H 21 O 21, only 4 O would be taken up ; and by the 'disappearance 

 of 13 HO, an equivalent of tannin would be formed. In the living 

 cells of plants, many substances are found which cannot exist with tannin, 

 such as mucus* (Schleim). 



6, Viscin (Birdlime), and Caoutchouc, have not been, up to this time, 

 sought after and examined, except in a few plants. Viscin is a clear, 

 very gelatinous, substance, and insoluble in water ; it is found in the 

 berries of the mistletoe ( Viscum album), in the fruits of Alractylis gum- 

 mifera, and in the milky juice of the green twigs of Ficus elastica. 

 Under this head we must also include the peculiar substance found in 

 the proscolla of the Orchidece, and which exists as a fibrous tissue be- 

 tween the pollen grains in the same plants ; likewise the fluid which 

 exudes from the glands on the stigma of the Asclepiadece ; and, lastly, 

 the product of the glands under the anthers of some Apocynece, as in 

 Nerium Oleander. Jf the history of the development of these parts is 

 examined, as well as the formation of the viscin in Viscum album, it will 

 be found that this substance is formed through the solution of existing 

 cells. It is well known, that in nearly all decompositions of cellulose, 

 carbon remains in excess ; and this agrees with the composition of 

 viscin, which contains, according to Macaire Prinsep, C 75*6 II 9*2 

 O15-2. 



Caoutchouc, or at least an essential element of it, appears to stand in 

 the same relation to viscin as gum to pectin. It belongs to the excretory 

 substances, and is found in the milky juice of plants, especially in the 

 three Jussieuean families, Urticece, Euphorbiacece, and Apocynece. The 

 milky juices of other plants are comparatively poor in this substance, 

 although it is absent in none of them. This substance, which defies all 

 chemical agents, swells up and diffuses itself (not dissolves) in ether, and 

 on dry distillation renders some remarkable chemical products (see 

 Himly de Kaoutschouk ej usque siccae Destillationis Productis ; Gottingen, 

 1835); has many peculiarities and unexplained properties ; and its rela- 

 tion to plants, its origin, &c., are at present almost entirely unknown. 

 In the milky juice of plants, it is found diffused emulsively in the form 

 of little globules. If the juice be allowed to stand, especially if diluted 

 with a little salt water, it collects on the surface in the form of a white 

 cream, which, when dried, is of a yellowish colour, and almost perfectly 

 transparent. Schulze, who, in all his views on milky juice and milk- 

 vessels, is dreamy, regards Caoutchouc as analogous to the fibrin of 

 blood. Any one who examines the milky juice of Siphonia elastica, 



* Endlicher and Unger say that the tannin is always dissolved in the cell-juice. How 

 comes it, then, that perfectly juiceless oak bark contains so much tannic acid ? 



