TECHNICAL PROPERTIES OF WOOD. 



1. Wood-vessels are more or less narrow tubes closed at their 

 ends, which run longitudinally through the stem and branches 

 of trees. Their walls are thin when compared with their lumina, 

 or hollow interiors, and the latter appear as pores on transverse 

 sections of the wood. 



Each annual zone of broad-leaved trees contains more or less 

 numerous vessels, the distribution of which among the othei 

 elementary organs of the wood affords excellent characteristics 

 for distinguishing the different species. The pores may be 

 uniformly distributed throughout the annual zone, or arranged 

 in bands or wavy lines, in which their size usually decreases 

 towards the outer limit of the zone. In the case of many 

 broad-leaved trees the wood formed in the early-growing 

 season, or spring-wood, is rich in large vessels, and may be 

 termed ring-pored wood ; it contains less woody substance 

 than the summer- or autumn-wood of the same annual zone. 

 Coniferous wood possesses vessels, and consequently, pores, 

 only immediately around the pith. 



2. Wood-fibres are the chief constituents of wood ; they are 

 elongated, closed organs, a few millimeters long and pointed at 

 both ends, and their walls are more or less thickened, sometimes 

 so much so that their lumina are greatly contracted. There are 

 three kinds of wood-fibres : tracheids, with large lumina and 

 large bordered pits on their walls ; true wood-fibres, forming 

 sclerenchyma, or hard tissue, composed of thick-walled ele- 

 ments, svith small pits on their walls ; intermediate fibres, 

 resembling wood-fibres in shape, but containing protoplasm and 

 starch, &c. The two former kinds of fibres, as well as 

 wood-vessels, serve to convey air and watery sap throughout 

 the plant. 



Coniferous wood contains tracheids only, which are thin- 

 walled in the spring-wood, and have large lumina. Tracheids 

 become thicker-walled and more compressed, with smaller 

 lumina, towards the boundary of the annual zone in the 

 summer-wood. As the radial section of these organs is much 

 thinner than the tangential section, they are sometimes termed 

 broad fibres. 



Broad-leaved wood, on the contrary, often possesses several 

 kinds of wood-fibres, and then the tracheids and intermediate 



