TISSUE SYSTEMS 



Fig. 723. Outer portion of cortex of 

 yoimg stem of Lime. ph. Phellogeu. 

 per. Cork. 



cells thick, of brick-shaped cells without interspaces [fig. 723, 

 per^. The cell-walls remain thin, but generally become com- 

 pletely suberised. The merismatic layer of cells is known as 

 the ■phellogen (fig. 723, ph.) ; it is classed as a secondary 

 nieristem, the cells regaining 

 the power of dividing after 

 ha^'ing assumed the condition 

 of permanent tissue. 



Sometimes it is not the 

 outer layer of the hypoderma 

 Avhich becomes phellogen, 

 but one deeper in the cortex. 

 The depth varies in different 

 steins : it may be seated so far inwards as to be withui the 

 limits of the stele. It never extends within the wood of the 

 vascular bundles. Sometimes many pheUogens are developed 

 in succession, each of which gives rise to a cork layer. 



Whatever be its origin, it gives rise to the same forms of 

 tissue, externally corlt, and internally often to a secondary cortex 

 known as phelloderiii. The phellogen and the tissues which it 

 fornas both internally and externally, have been called periderm 

 by some authors ; by others the term peridermis restricted to the 

 layers formed on the out- 

 side only of the phellogen, '" 

 the phelloderm and the 

 phellogen being distin- 

 guished from it. 



A variety of ordinary 

 peridermis formed in some 

 cases by strata of thich- 

 ened or ]iardened cells 

 oecurring-amoiigthe thin- 

 walled suberised ones. 

 This is the case when 

 the activity of the same phellogen is maintained for several years. 

 In roots, cork is found less frequently' in the hypoderma ; gene- 

 rally the phellogen arises in the external layer of the stele, known 

 as the pericycle. It may or may not be accompanied by a de- 

 Yelopment of phelloderm. In leaves the occurrence of cork is 

 rare. It is found in the scales of the winter-bud of the Horse- 

 chestnut, and in the petioles of Hoya carnosa and a few other 

 plants. 



Cork is usually formed as a covering over any portion of the 



Fi'j. 724. Section of Leiiticcl (/. ). 

 2}er. Periderm. 



