372 
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_ possessing as a rule large pits on the side that faces the ey 
medullary rays. The characteristic position of the pits, — 
which in the cells of the stem are usually arranged inone, in. 
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Fic. 478.—Longitudinal section through the medullary sheath of the spruce-fir ; a-e ao 
spiral vessels or vascular cells ; 7 transitional forms into the ordinary wood- 
fibres; # normal wood-fibre ; 7 cells of the pith (x 400). NG 
those of the root often in several rows, as well as that of the ~ 
resin-passages, which are especially abundant in Coniferze, — 
form an excellent diagnosis to distinguish the different kinds — 
of wood (Fig. 479). The structure of the swollen ster 
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