ELM STEM 89 



cells : it will be more strongly developed in older twigs, 

 while it is completely absent in very young twigs : its 

 development is studied below, pp. 91-93. Here and 

 there a lenticel may have been cut through, in which 

 case it will appear as a lateral extension of the band 

 of cork. 



3. Cortical tissue : parenchyma with chlorophyll, 

 having cellulose walls, and intercellular spaces ; here and 

 there are large transparent areas : these are cells with 

 swollen mucilaginous walls. 



4. 1 Thick- walled masses of sclerenchyma (hard 

 bast), which form an irregular broken ring : the walls 

 stain brownish-red with chlor-zinc-iodine. 



5. Soft bast : a transparent tissue with cellulose 

 walls, and plentiful protoplasm. 



6. Cambium : a misty layer of thin-walled tissue 

 with plentiful protoplasm : cells in radial rows. 



7. Xylem : a broad band of thick-walled lignified 

 tissue, with crenated inner margin ; centrally lies 



8. The pith or medulla : round-celled parenchyma, 

 with thin pitted walls : here and there are large, trans- 

 parent mucilage cells. 



The crenated appearance of the inner margin of the 

 xylem is due to the presence of the wedges of primary 

 xylem, forming the so-called medullary sheath : the 

 wedges are separated from one another laterally by 

 parenchymatous bands, which may be followed outwards 

 in a radial direction through the whole thickness of the 

 vascular ring : these are the primary medullary rays : 

 other rays will also be seen following a similar course, 

 but extending only part of the way from the cambium 



1 4, 5, 6, 7, together form the vaSClllar ring. 



