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A TEXTBOOK OF THEORETICAL BOTANY 



The same theory extends to the formation of interfascicular cambium 

 from fully differentiated parenchyma cells and perhaps also to the appearance 

 of the phellogen. Both these changes involve the alteration of cells with 

 lart^e vacuoles and little protoplasm into meristematic cells which have no 

 vacuoles and abundant protoplasm. The isoelectric point of the cell proteins 

 is the condition which would seem most favourable to such a change. The 

 cambium does in fact lie between acid xylem (pH 3-4 to 5-0) and alkaline 

 phloem (/)H 7-8), while the phellogen lies between the strongly acid cork 

 (pH 3-0) and the less acid cortex (pU 5-5 to 6-5). 



Protoplasm at its isoelectric point will lose water to vacuolating cells and 



Fusiform initial 



Medullary ray 

 initial 



Fig. 878. — Ulmus procero. Tangential view of the 

 cambium in a woody stem showing elongated 

 cambial elements and embryonic medullary rays. 



the condensed non-vacuolate condition produced is that most favourable to 

 protein synthesis. 



When cambium is viewed in tangential longitudinal section, two types of 

 cell are seen, the long, pointed fusiform cells which give rise to the vascular 

 elements, and the small, isodiametric cells in groups, which produce the 

 medullary rays (Fig. 878). These latter are much more numerous than in 

 Conifers and may account for up to 55 per cent, of the cells in a cambial 

 ring. The long, fusiform elements are thin walled and without pits except 

 in the dormant season. They are often flexed into curves as they pass round 

 the groups of ray cells. Their divisions are mostly longitudinal-tangential, 

 and it is doubtful whether they divide radially. More probably the appear- 

 ance of radial division is created by transverse walls which become more 

 and more inclined in a tangential direction until they are vertical, thus 



