PERIDERM 129 



by the pressure exerted by the growth of the wood, and their 

 place is taken by totally new tissues which arise by division of 

 a meristem known as \he phellogen or cork-cambium (a, Fig. 59). 



This phellogen may arise in the epidermis itself, in the cortex 

 or even in the pericycle within the vascular cylinder. The 

 divisions of its cells take place in a manner similar to those of 

 the ordinary cambium, but instead of producing wood and bast 

 tissue it gives rise on its inside to phelloderm or secondary cortical 

 tissue (b) and on its outside to cork {c). To the phellogen and 

 the products of its growth the term periderm is applied. 



In most aerial stems little or no phelloderm is formed : when 

 present its cells have thin walls, and protoplasmic contents ; 

 chloroplasts are generally present in the tissue when it is 

 developed near the surface of the stem. 



The cork-tissue formed by the phellogen shields and protects 

 the interior of the stem from mechanical injuries and prevents 

 the stem from losing water by transpiration. 



Cork is also a bad conductor of heat and efficiently protects 

 the delicate phellogen and cambium from excessive heat in 

 summer and frost in winter. 



It consists of a number of layers of cells which fit closely 

 together in regular radial rows (f). The cells soon die and 

 generally become filled with air only ; their walls are mostly thin, 

 often brownish in colour and impermeable to water and gases. 



' Corks ' for bottles are cut from the extensive cork-tissue of 

 the Cork Oak {Quercus Suber L.). 



When the phellogen originates in a deep layer of cortical 

 cells or in the pericycle, all the tissues outside it become cut 

 off from water and food supply by the cork which is formed : 

 these tissues dry up in consequence, and, together with the 

 cork constitute what is sometimes spoken of as bark by botanists, 

 although in popular language the term bark is applied to all 

 tissues which are external to the cambium of a stem. 



Scattered over the outer surface of the periderm of most woody 



I 



