332 CONSERVATIVE ORGANS. [LECT. VII. 



the longitudinal fibres or oblong cells ; c. c. c. the 

 vessels which in the Horse Chesnut are porous, 

 and b. b. b. the exterior ends of divergent layers. 

 In the alburnum, the walls of the concentric tubes 

 are tender and transparent ; but by the deposition 

 of ligneous matter in the membrane of which 

 they consist, and in the tubes themselves, they 

 become opaque and firm ; and according to the 

 degree of this, the wood is more or less dense, 

 hard, and tenacious. Other matters, also, are 

 deposited in this part of the woody texture ; such 

 for example as Guiaic in that of the Guiacum 

 officinale, colouring matter in the Logwood, 

 Haematoxylon Campechianum, and even silex, 

 which has been extracted from the Teak wood, 

 Tectona grandis, by Dr. Wollaston. The vessels 

 of the concentric layers are chiefly porous and an- 

 nular, and their section produces the openings ob- 

 served in the transverse section of any stem* ; but 

 besides these, in the circle of the wood of the first 

 year's growth, a circle of spiral vessels surrounds 

 the pith *f~. These are, however, justly regarded by 



* Vide Plate 6, fig. 2. h. and fig. 8. a. a. a. The tubular 

 nature of the oblong cells, forming the concentric layers, is 

 rendered evident in this section of the common Elder, fig. 8. at 

 b. b.; and the distinct character of the divergent layers at c. c. 

 Leuwenhock, in 1680, first delineated the porous vessels of the 

 wood. Vide his Epist. Physiolog. p. 14-. 19. 



f Vide Plate 6, fig. 7. d. e. 7, 7, 7. In this figure, also, the 

 first, or half-organized state of the alburnum, is represented 

 between b. c. ; and the vascular structure of the perfect wood 



