ELM- STEM 53 



and there are large transparent areas : these are cells with 

 swollen mucilaginous walls. 



4. Thick-walled masses of sclercnchyma (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 plenti- 

 ful 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, transparent 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 to the 

 centre and periphery of the vascular ring : these are secondary 

 medullary rays. 



Examine the several tissues, above enumerated, in detail 

 with a high power : 



1. Epidermis : a single layer of cells, with the outer wall 

 thickened and circularized or corky (test with the usual re- 

 agents, see p. 26) : stomata will be found in a normal position 

 in young twigs, in older ones they are found at the apices of 

 the lenticels (see below, p. 54). Note the form of the conical 

 hairs, the walls of which are silicified. 



2. The periderm (when present) lies immediately below the 

 epidermis : its cells are arranged, in radial rows, without 

 intercellular spaces. Select a thin part of the section for 

 special study of ther.e radial rows, and note in each the following 

 succession of tissues, passing from without inwards : 



a. A series of cork-cells as above described : walls stained 

 yellowish brown with chlor-zinc-iodine. 



