WOODS. 171 



Vessels. 



All the official woods contain vessels, but as these elements 

 are comparatively fragile, pieces of them only are to be found in 

 the powder. Isolated fragments of vessels are always small, but 

 larger portions are often to be found surrounded or supported 

 by wood parenchyma or fibres. 



Vessels may vary considerably in size in one and the same 

 wood, but the structure generally remains constant for the 

 same species. The thickness of the wall, and more particularly 

 the nature of the pits are often characteristic features, thus the 

 vessels of guaiacum wood are easily distinguished from those of 

 red sanders wood, or yellow sandal wood by their very numerous 

 small pits. If the vessels are large, small fragments showing 

 the transverse section and the thickness of the wall are some- 

 times to be found. 



Wood Parenchyma. 



Wood parenchyma is found in all the official woods. In 

 width the cells of this tissue closely approach the wood-fibres, 

 but in longitudinal sections they are seen to be much shorter 

 than the fibres. They also differ in having square ends and 

 thinner walls with numerous, simple, circular pits, the pits 

 of the wood-fibres being usually few in number, cleft, and 

 arranged in a left spiral. 



Medullary Rays. 



These are also present in all official woods. In the powder 

 they may present their transverse, tangential, or radial section. 

 In the first of these which is comparatively rare the cells are 

 rectangular, elongated, and arranged in one, two, or three rows. 

 In radial section the appearance is somewhat similar, but the 

 rows of cells are generally attached to wood-fibres, wood 

 parenchyma, or vessels. In tangential section they form oval 

 groups of cells inserted usually between the wood-fibres, the 

 groups varying in size according to the number of cells in the 

 height and width of the ray. In the powder fragments only of 

 these groups are usually to be found. The contents of the 

 cells and the pits on the walls may furnish useful information 

 as to the identity of the powder. 



Calcium Oxalate. 



As in all drug powders the presence or absence of calcium 

 oxalate crystals, their shape, size, and distribution must be 

 taken into account. In some cases they are irregularly scattered 

 through the cells of the medullary rays or wood parenchyma 

 in others they occur in regularly superposed rows of cells. 



