120 GENERAL FEATURES OF SECONDARY THICKENING 



of secondary vasctdar elements between the primary xylem 

 and phloem, becomes more and more marked as the years go 

 by and its effects are most pronounced in the cortex. Moreover, 

 the steady increase in size of the woody core results in a gradual 

 enlargement of its circumference, so that the softer tissues beyond 

 become tangentially stretched. This tension can be readily 

 demonstrated by making an extended vertical incision through 

 the cortex of a three-year-old twig of the Ash. The edges of the 

 cut are seen to separate immediately owing to transverse con- 

 traction of the thin-walled tissues. In nature the tangential 

 tension is exemplified by the irregular longitudinal fissures which 

 are so marked a feature of the older bark of many trees. 



At certain points the segments cut off from the cambium on 

 either side develop into parenchymatous cells which differ from 

 the other elements of the wood and phloem in being radially and 

 not longitudinally elongated (Fig. 60, A, m.r.). In transverse 

 sections of a secondarily thickened stem these medullary rays 

 appear as a number of radiating streaks, one or more cells in 

 width (Fig. 58, pr. and Sr.}. Some of these rays extend from 

 cortex to pith (pr.) and, since they correspond in position to the 

 original rays between the vascular bundles, are called primary 

 medullary rays; others (the secondary medullary rays, Sr.), 

 however, though traversing the greater part of the secondary 

 phloem, penetrate only to a varying depth into the secondary 

 wood. In some plants (e.g. Aristolochia) the primary rays are 

 very wide, and practically the whole of the interf ascicular cambial 

 strips participate in their formation. 



The secondary wood is composed of four main types of ele- 

 ments, viz. vessels, tracheids, wood-fibres, and wood-parenchyma, 

 but transitions between the different types are not infrequent. 

 The component elements often exhibit a distinct radial arrange- 

 ment (e.g. Horse Chestnut), though this may be somewhat 

 obscured when the vessels are large and numerous (e.g. Lime, 

 Elm). The vessels of the secondary wood usually bear densely 

 crowded bordered pits (Fig. 60, V ; Fig. 17, E, b, p. 35), often 

 arranged in distinct vertical series, and not uncommonly show 

 additional reticulate or spiral thickenings deposited on the inner 

 surface of the wall. The septa are frequently placed obliquely 

 (Fig. 60, C, V) with reference to the radial plane, a feature which 



