GENERAL FEATURES OF SECONDARY THICKENING 119 



The secondary wood thus added on the inside forms a larger 

 and larger core each year (Fig. 58). Since it is composed of 

 hard persistent tissue, there is practically no compression of the 

 wood, which progressively accumulates, so that the increase in 

 girth of the stem serves as a rough measure of the amount of 

 tissue added. To this enlargement the secondary phloem con- 



V. 



FIG. 58. Diagram showing the arrangement of tissues in a transverse 

 section of a woody trunk about twenty-four years old. The large 

 vessels of the spring wood of each annual ring are shown as black 

 dots, b., bark ; c., region of cambium ; p., pith ; Ph., secondary 

 phloem ; pr., primary ray ; Sr., secondary ray. 



tributes but little, since this tissue is mainly thin-walled, and 

 the outer earlier-formed elements become compressed more and 

 more, as a consequence of the increasing pressure resulting from 

 the growth of the wood and the annual formation of intercalated 

 phloem. The cambium keeps pace with the enlargement of the 

 circumference of the secondary wood by tangential stretching and 

 occasional radial divisions in its cells. 



The pressure on the outer tissues, due to the interpolation 



