PITH OF BROAD-LEAVED TREES 



25 



In the last-mentioned case the projections of the primary xylem 

 into the pith may give the pith a wavy or crenate outer margin, as 

 in Hawthorn, Rowan, Laburnum, Horse-Chestnut, or Elder ; or 

 this margin may appear even, as in Elm, Hazel, and Dogwood 

 (Cornus). In the Walnuts the pith has an interrupted or chambered 

 structure : in the Elder it soon dies and disintegrates, leaving the 

 stem hollow ; whilst in young stems of Elm the inner portion of it 



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/; 



Fig. 21.— Tangential longitudinal section of Oak, magnified 50 diameters, showing 

 transverse sections of pith-rays. (After Mtiller, from The Oak, by permission of Prof. 

 Marshall Ward and Messrs. Kegan Paul, Trench, Triibner & Co.) 



\ 



has thin walls and loses its protoplasm, whilst the outer part 

 becomes thick- walled but retains its cell-contents. ^ 



The pith-rays of broad-leaved trees are in general far more 

 conspicuous than those of conifers. In Oak the large primary 

 pith-rays extending from pith to cortex are often twenty or more 

 cells jn width, appearing as long, clearly defined, greyish hues in 

 a transverse section of the stem (Fig. 19). The secondary pith- 

 rays are much narrower as well as shorter (Fig. 20). In a tan- 



