120 ANATOMY OF STEM, ROOT AND LEAF 



cells (f), and (2) a certain amount of thin-walled bast-parenchyma : 

 their cell-walls consist of ordinary cellulose. 



The bast-vessels are long thin-walled cells arranged end to end. 

 The transverse or end-walls which separate one vessel from 

 another, aire not completely absorbed as in the vessels of the 

 wood, but merely perforated by open pores through which the 

 contents of adjoining vessels are in continuous open communica- 

 tion : these transverse perforated walls are called sieve-plates. 



When mature the bast-vessels contain a thin lining of cytoplasm 

 but no nucleus : the rest of the cell-cavity is filled with an alkaline 

 slimy substance, rich in proteids, and frequently containing 

 starch-grains as well. 



The bast-vessels serve for the conduction of various complex 

 organic substances, but more especially for those of a proteid 

 character. 



The companion-cells are long narrow cells which lie alongside 

 the sieve-tubes : they are filled with granular cytoplasm in which 

 a nucleus is always present. Both the sieve-tube and its com- 

 panion-cell arise from the same mother-cell. 



{c) Cambium. — The cambium lies between the wood {c, Fig. 

 55) and the bast, and consists of a layer of thin-walled meris- 

 tematic cells, each of which has the form of a long, narrow, 

 rectangular prism with obliquely pointed ends. 



In young stems the cambium is confined within the vascular 

 bundles, but in older ones a new and exactly similar meris- 

 tematic tissue termed the interfascicular cambium arises in the 

 medullary rays, and extends across the latter, joining the 

 cambium of one bundle with that of the next {ic, Fig. 54). We 

 thus have in the older stems a thin complete cylinder of dividing- 

 cells which in transverse section appears as a narrow zone, spoken 

 of as the cambium-ring. 



The cambium-ring adds new elements to the wood and bast 

 of the stem in a manner explained below; but in short-lived 

 herbaceous dicotyledons this additional growth soon ceases, so 



