6 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



commenced with, i.e. to examine a so-called transverse 

 section. 



The microscope will show us a figure like that in 

 the woodcut (Fig. 3). There are to be seen certain 

 angular openings, which are the cross sections of the 

 long elements technically called tracheides^ shown in 

 elevation in Fig. 4. It will be noticed that whereas 

 along some parts of the section these openings are 

 large, and as broad in one direction as in the other, in 

 other parts of the section the openings are much 

 smaller, and considerably elongated in one direction 

 as compared with the other. The band of small 

 openings naturally looks more crowded and therefore 

 darker than the band of larger openings, and it is to 

 this that the differences in the shading of the annual 

 rings in Fig. 2 are due. But it is not simply in having 

 larger lumina or openings that the dark band of 

 tracheides is distinguished from the lighter one : the 

 walls of the tracheides are often also relatively thicker, 

 and obviously a cubic millimetre of such wood will 

 be denser and contain more solid substance than a 

 cubic millimetre of wood consisting only of the larger 

 thin-walled tracheides. It is equally obvious that a 

 large block of wood in which the proportion of these 

 thick-walled tracheides with small lumina is greater 

 (with reference to the bands of thin-walled tracheides) 



