ELM. STEM. 69 



4. 1 Thick-walled masses of Sclerenchyma (hard 

 bast), which form an irregular broken ring (walls 

 brownish-red with Schulze's solution). 



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 : mucilage cells here and there. 



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), 

 separated from one another laterally by parenchy- 

 matous 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 

 to the centre and periphery of the vascular ring : these 

 are secondary medullary rays. 



Compare with the vascular arrangement of Helianthus. 



Cut transverse sections through the axis of a bud, or of a 

 young twig, during the process of extension in spring ; treat with 

 potash, and mount in glycerine. In these sections the vascular 

 system will be found to be much less developed, but even here 

 the primary bundles will not be found to be as clearly distinct 

 from one another as in the young stem of Helianthus. In 

 ligneous Dicotyledons the interfascicular cambium begins to be 

 active at an earlier period than in those which are herbaceous. 



} 4, 5, 6, 7, together form the vascular ring. 



