170 WOODS. 



SECTION VI. 



POWDERED WOODS. 



The structural elements of which the official woods are com- 

 posed are limited in number, and also in the extent to which 

 they vary in size, shape, etc. ; in studying them, therefore, it is 

 necessary to pay particular attention to the minute details of 

 each of these elements. Diagnostic characters must be sought 

 not only in the shape and size of the wood-fibres, vessels, wood 

 parenchyma, and medullary rays, but also in the thickness of 

 the walls of the various elements, in the nature and distribution 

 of the pits, and in the nature of the cell contents. 



Wood Fibres. 



These, together with the vessels, make up the bulk of the 

 official woods, and are therefore the most numerous elements 

 met with in the powder ; the chief differences they exhibit are 

 in the thickness of the wall, the number and nature of the pits, 

 and the nature of the contents. They exhibit their length to 

 the observer far more often than their transverse section, and 

 are usually more or less broken by the pulverisation to which 

 they have been subjected, fragments from the central part of 

 the fibre being from this cause not un frequently lacerated or 

 frayed at the broken ends. Sometimes they are isolated, but 

 more frequently they are in groups associated with vessels, 

 wood parenchyma, or portions of medullary rays. They may 

 also differ in colour, those of guaiacum, for instance, are of a 

 greenish-grey or brown colour, whilst those of red sanders wood 

 are red, and those of quassia are colourless. The last-named 

 contain only the remains of protoplasm, whilst the fibres of 

 guaiacum contain resin, and those of yellow sandal wood 

 contain oil. 



