RINGS AND FALSE RINGS. 37 



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 

 distinctness 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 nume- 

 rous large vessels and that with fewer or smaller ones : or, as in 

 Birch, Maple, Horse-chestnut, etc., to the fibres being smaller 

 across and thicker-walled in one part of each ring, whilst the 

 vessels may be evenly dispersed through the whole wood. Woods 

 differ widely as to the circularity of their rings. In not a few 

 cases they are distinctly wavy ; and, whilst in Beech and Horn- 

 beam the crests of the waves as seen in a cross-section bend 

 inwards at the primary pith-rays, in the Barberry they bend 

 outwards. In evergreens, to which type belong the bulk of 

 tropical broad-leaved timbers, where there is not the check to 

 physiological activity produced by the "fall of the leaf," we do 

 not, as a rule, find such well-marked annual rings. Sometimes, 

 however, the annual rings are replaced by less completely con- 

 centric zones, often stretching as wavy, pale, bar-like markings 

 from one primary pith-ray to another, and sometimes running 

 into one another. These "false rings," as they have been 

 termed, which are seen in the wood of Figs, She-oaks (Casuarina\ 

 Padouk (CalophyUurri), etc., will be found on microscopic examina- 

 tion to be mainly produced by zones of wood parenchyma. 



The grouping of the vessels also affords some useful distinctive 

 characters. Thus in Box and in Quince they usually occur singly ; 

 in Hazel and Holly in groups of from 5 to 1 2 ; in Hornbeam in 

 long sinuous radial lines between the pith-rays; in Elms in 

 concentric bands like false rings ; and in Oaks, Chestnut and 

 Buckthorn, from 20 to 50 together, in flame-like groups 

 (Fig. 28). 



The elements of the wood are generally parallel in direction 

 to the axis of the stem or limb in which they occur i.e. the 

 wood is straight-grained ; but they may be spirally twisted round 

 the stem, or oblique, in which latter case if successive layers lie 



