26 



THE ANATOMY OF WOODY PLANTS 



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walls in this position numerous pores may be distinguished, in 

 profile, forming the principal means of communication between 

 tracheid and tracheid and the sole one between tracheid and ray. 

 To the right of the figure are to be seen the radial c and tangential 

 d views of a summer tracheid. The narrower lumen and thicker 



walls are conspicuous. In the case 

 of the pitting the most marked fea- 

 ture of contrast with the spring fibers 

 is the greater number of bordered 

 pits occurring on the tangential walls. 

 The situation in Pinus may 

 now be advantageously considered. 

 Figs. 2ia and b reproduce the spring 

 tracheary elements of this genus from 

 the same aspects as represented in 

 the case of Sequoia. Beginning with 

 the spring element on the left, it is 

 clear that the pits in relation to the 

 rays show a considerable degree of 

 differentiation, since they consist of 

 two categories namely, small, dis- 

 tinctly bordered pores which form 

 the intermediary between marginal 

 ray cells and tracheids, and large, 

 angular, scarcely bordered apertures 

 uniting the central ray cells with the 

 tracheids. The more pointed tan- 

 gential contour of the tracheid in b 

 surrounds an area entirely free from 

 pits, a situation nearly universal for 

 the spring elements of coniferous 

 woods. In the lateral walls of b 

 numerous radial pits are seen in profile. On the right of the figure 

 are shown the corresponding views of the summer tracheids in Pinus. 

 The narrower diameter and thicker walls, as well as the numerous 

 tangential pores, clearly differentiate elements terminating the 

 annual growth from those formed at the beginning of the year. 



FIG. 21. Tracheids of the pine. 

 Explanation in the text. 



