STEM-STRUCTURE 



339 



tains schizogenous resin-canals (r.), whose structure has already 

 been described (cf. p. 152). These resin-canals are particularly 

 characteristic of Conifers, and are found in all parts of the 

 plant ; they are entirely absent only from the Yew. 



The primary and secondary vascular tissues are not easily 

 distinguished from one another, particularly in the case of the 

 phloem, the broad zone of which is chiefly secondary in origin. 



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FIG. 196. Structure of the young stern of the Scotch Fir (Pinus sylvestris) 

 in transverse section. A, Entire section (diagrammatic). B, Small part 

 of the peripheral tissues enlarged. C, Single resin-canal enlarged, 

 showing the epithelium (ep.}. Ca., cork cambium ; e., epidermis ; 

 Ph., phloem ; r., resin-canals ; Xy., xylem. 



This tissue (Ph.) is strikingly uniform, its elements, which have 

 thick white shining walls, exhibiting a radial arrangement. The 

 rows of more or less empty-looking sieve-tubes, which have no 

 companion cells, are interspersed with a smaller number of 

 irregular files of phloem-parenchyma, whose cells contain dense 

 contents. In longitudinal sections the sieve-tubes appear as 

 long tapering elements bearing sieve-areas (Fig. 197, E, si.) upon 

 their sloping radial walls. 



The secondary wood (Fig. 196, Xy.), internal to the narrow 



