ELM. STEM. 77 



2. The Cork is composed of square cells arranged in 

 radial rows, which are continuous through the cork- 

 cambium to the periderm, the latter presenting much 

 the same appearance as in transverse sections. 



3. The Cortical tissue, which is parenchymatous 

 throughout, also appears much the same as in transverse 

 sections. 



4. The Hard bast consists of long fibres, with thick 

 walls, and very small cell-cavity: they are distributed 

 in irregular groups among 



5. The Soft bast, characterised by thin walls and 

 protoplasmic contents, and composed of 



a. Sieve-tubes, which are best seen in the part of 

 the phloem nearest to the cambium. They resemble, 

 in the main, those of Cucurbita (p. 84), but are not so 

 wide ; the sieve-plates are oblique, and face the radial 

 planes. This is the usual arrangement of sieve-plates in 

 secondary phloem ; but their structure is often more 

 complicated, e.g. in Vitis, Tilia. The sieve-tubes may 

 easily be recognised in stems cut in autumn by the 

 masses of callus which surround the sieve-plates : this 

 stains brown with Schulze's solution. For the reactions 

 of the callus, see p. 31. Companion cells are not easily 

 seen. 



I. Bast-parenchyma: oblong cells with cellulose 

 walls, some contain protoplasm and starch. (More or 

 less of the latter according to the season.) Others 

 contain crystals : note the medullary rays as before. 



Passing inwards the differentiation of tissues of the 

 phloem is lost in 



6. The Cambium, which appears here as a narrow 

 band of cells with thin walls, and abundant protoplasmic 



