10 



INTRODUCTION 



that it is often nearly impossible to find the lumina or to 

 distinguish one cell from another ; the layer may be two 

 cells in thickness, or it may be only half a cell in thickness, 

 only one side of the cell having taken on a thick wall. The 

 new phellogen then forms a few layers of cork cells and 

 a fresh phelloderm. 



This process is repeated year by year, so that a large 

 number of cork layers are formed, all in the same radial 

 rows with the rest, except where phellogen cells have 



divided tangentially, and 

 without encroaching on the 

 inner cortex, which remains 

 intact. 



In special cases, how- 

 ever, e. g. where the cortex 

 has become injured, a fresh 

 phellogen may be formed 



FIG. 5. Diagram showing method 

 of formation of periderm : c., cortex; 

 c,, first-year cork ; c 2 , second-year bark trees. 



i j i i i ill* n 1 



through the cortex or outer 



-,. i 



, as 111 Ordinary scale 



cork; c 3 , third -year cork; l.l., lignified 

 layer ; jnn, phelloderm ; pn, phellogen. 



A longitudinal section 

 shows at the top of each leaf 

 cushion a leaf scar where the abscission layer has cut off 

 the leaf. A poorly developed cork layer is formed under the 

 abscission layer, and the cells beneath this become suberized. 

 The cork layer is joined on to the primary cork layer of the 

 stem on all sides. These leaf scars would appear to be weak 

 points in the defensive armour of the stem. At the same 

 time they may serve for the interchange of gases, since no 

 lenticels are present in the cork until the second yen r. 



Inner cortex. The parenchyma inside the cork layer 

 remains alive, and the cells contain chloroplasts. The cells 

 are at first nearly circular in all sections, but later become 

 extended tangentially to keep pace with the increasing 

 girth of the stem. These cells usually divide by radial 

 walls, and each elongated cell is replaced by two or three 

 circular ones. This continued vitality of the cortex is 

 rendered necessary by the peculiar form of periderm 



