INTRODUCTION 9 
10-20» tangentially, 174 radially, and are longitudinally 
extended to about 400». They are not only covered 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 epidermis 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 parenchyma inside this is composed of large cells 
30-100 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 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. 
Periderm. 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 
