PHELLOGEN 



141 



walled, tabular, meristematic cells with abundant protoplasmic 

 contents, which divide tangentially. Most often the outer of the pair 

 of daughter-cells produced at each division becomes a cork-cell, while 

 the inner remains an element of the phellogen. Occasionally, however, 

 the outer of the two daughter-cells remains a part of the phellogen, in 





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Fig. 41. 



Cork. A, B. T.S. of stems of Scutellaria aplendens showing epidermal origin of 

 phellogen. C. T.S. of twig of Ulmus suberosa. The cork is very wide-celled. In its 

 innermost layer (') living protoplasts are still present. The phellogen (ph) is in pro- 

 cess of division. The phelloderm (pd) contains abundance of starch. D. Periderm of 

 a yearling twig of Prunv.s Padus. (In all figures ph denotes phellogen.) 



which case the inner becomes an element of the secondary cortical 

 parenchyma or phelloderm. "Whereas, namely, the phellogen is mainly 

 concerned with the production of cork, it also frequently though not 

 invariably performs one or more subsidiary functions, by adding 

 secondary elements in the form of phelloderm cells to the cortical 

 parenchyma (Fig. 40, phd ; Fig. 41 c, pd), or by contributing scleren- 

 chymatous or collenchymatous cells to the mechanical system. 



The phellogen very often originates 84 either in the epidermis or 

 else in the outermost layer of cortical parenchyma. In such cases each 

 primary mother-cell usually undergoes two successive tangential 

 divisions giving rise to three superimposed daughter-cells of which 

 the middle one becomes an element of the phellogen. The so-called 

 superficial periderms produced in this way are epidermal in origin in all 



