THE TEEE ITS SHOOT-SYSTEM 117 



layer maybe called the initial layer. This layer behaves 

 essentially like the cambium of a fibro-vascular bundle, 

 except that its daughter-cells become cork and phello- 

 derm instead of phloem and xylem. 



The first event to notice is that each of the initial 

 cells grows radially, and divides by a tangential wall 

 into an inner cell nearest the axis of the branch and an 

 outer cell nearer the epidermis ; the outer cell becomes 

 forthwith a cork-cell i.e. its contents die and mostly 

 disappear, and the cellulose cell-wall becomes suberised 

 the inner cell remains capable of repeating the pro- 

 cess. But this is not the only case. After the division, 

 as before, of the initial cell, it may happen that the inner 

 cell becomes transformed into a collenchymatous cortical 

 cell containing chlorophyll, and it is the outer of the 

 daughter-cells which retains the meristem character 

 and acts again as a phellogen cell, cutting off daughter- 

 cells sometimes on one side and at others on the other. 

 Thus, in the oak, the phellogen gives rise to permanent 

 tissue on both sides of the initial layer : those cells which 

 lie on the inside become phelloderm (cortical cells), those 

 on the outside become transformed into phellem (cork). 

 The three tissues, phelloderm, phellogen, and phellem, 

 are called the periderm. 



It is obvious that the cork-cambium, by thus adding 

 to the cortical parenchyma, is gradually driven radially 

 outwards from the centre of the stem. This means that 

 it obtains room to extend tangentially, and it does this 

 by its cells occasionally dividing by walls perpendicular 



