INTRODUCTION 9 



10-20 /x taiigcntially, 17/x radially, and are longitudinally 

 extended to about 400 ix. They are not only eovered by 

 a thick cuticle, but the outer and radial walls are strongly 

 cuticularized, and a cellulose wall is superposed on this 

 cuticularized wall, leaving a lumen with a diameter about 

 half that of the original cell. Owing to the formation of 

 the cork layer, this and the outer cortical layers are killed 

 before the lapse of the first year. 



Outer cortex. Next the epidermi'Pr are one to three layers 

 of collenchyma, the walls of which give both the chlor-zinc- 

 iodine reaction for cellulose and the phloroglucol and 

 hydrochloric acid reaction for lignified walls. The walls 

 are thick, with well-marked secondary and tertiary layers, 

 and there are small intercellular spaces at the corners. 



The jiarenchyma inside this is composed of large cells 

 30-100 ju, in diameter, and nearly isodiametric in all three 

 planes. The resin ducts of the cushions are formed in 

 this tissue, and may either touch the collenchyma or be 

 separated from it by one or two layers of parenchyma. 

 They are up to 170 /u, in breadth, and are lined by an epi- 

 thelium protected by small parenchymatous cells on the 

 outside. There is no further mechanical tissue to prevent 

 their being crushed. These, with all the parenchyma in 

 the cushions, are cut off by the primary cork layer, and are 

 thus functional for less than one year. 



Peridertn. The phellogen is first formed in the layer of 

 parenchyma immediately inside the collenchyma at the 

 furrows, and forms a circle by cutting across the parenchyma 

 of the leaf cushions (see fig. 2). The phellogen forms 1-3 

 layers of cork cells on the outside, and as many layers of 

 phelloderm cells on the inside. 



A new cork layer is formed each year in the following 

 way. The innermost phelloderm layer becomes meri- 

 stematic, and forms a new phellogen. The outer phello- 

 derm cells and the phellogen of the previous year develop 

 thick walls, become lignified, and take on the appearance of 

 stone cells. In this way layers of stone cells, laterally 

 welded together, are formed, and the walls become so thick 



