30 



OF WOOD IN GENERAL 



broad-leaved trees no such regularity of radial arrangement of 

 elements as characterizes the simple wood of conifers. In the 

 cambium region, it is true, owing to the repeated regular tan- 

 gential divisions, the cells not only appear rectangular m a trans- 

 verse section, but are also in regular radial rows ; but in the xylem 

 itself this regularity is disturbed by the different diameters 

 attained by the various elements as they become fully formed. 

 In Oak, for example, the aimual rings are marked in a cross- 

 section by the large and conspicuous pores, or sections of the 

 vessels, which occupy the greater part of the spring wood of each 

 ring (Fig. 27). On a radial section the layers appear as parallel 





P£G 26 —Transverse section of Beech (Fagus syhahca) Magnified 100 times 

 a, narrow pitli ray , &, broad pith-ray , c, boundary of an annual ring Tho largo 

 pores are transverse sections of vessels (tracliese) Tlie tlnck-walled elements with 

 narrow lumma aie wood-fibres ; those with thinner walls and wider lumin-x, wood 

 parenchyma or trachexds (From Hartig s Timbers and Jwiv to know them, by permission 

 of Dr Somorville and Mi David Douglas ) 



stripes, and on a tangential one as broader and less parallel stripes ; 

 but, whilst in coniferous woods the dark bands were denser summer 

 wood, in this case the darker parts are produced by the vessels 

 in the spring wood, the more uniform fibres of the summer wood 

 appearing lighter. Vessels, tracheids, and fibres formed in spring 

 have larger diameters and thinner walls than those formed in 

 autumn, which fact produces much of the distmctness of the 

 annual rings. In timbers with well-marked rings the distinctness 

 of these rings may either be due, as in Oak, Ash, Teak, etc., to the 

 contrast between wood with numerous large vessels and that with 

 fewer or smaller ones ; or, as in Birch, Maple, Horse-chestnut, etc.. 



