240 
MORPHOLOGY 
Epidermis. The dermatogen passes into the epidermis, which is 
usually a single layer of protective cells closely interlocked and with 
relatively impervious walls, but 
pierced by stomata. In case the 
stem increases in diameter, as in 
dicotyledonous trees, the epidermis 
is usually ruptured and destroyed, 
and another protective layer is 
developed by the cortex, as de- 
scribed below. 
Cortex. The periblem pro- 
duces the cortex (figs. 539, 541), 
which is a hollow cylinder of 
living cells. It is exceedingly 
variable in structure, but is char- 
acterized by containing chloro- 
FlG. 538. Stem tip of Hippuris (a di- 
cotyledon), showing dermatogen (outermost 
layer), periblem (the five layers beneath 
the dermatogen), and the plerome (central 
region of more elongated cells). After 
DEBARY. 
phyll tissue abutting against the 
epidermis; and if it is thick enough, there is also a deeper region of 
the cortex free from chlorophyll. The layer of cortical cells abutting 
against the stele often forms a very distinct bounding layer, like an 
inner epidermis, and is called the endodermis (fig. 379). In the cortex, 
strands of fibrous cells may be de- 
veloped, and cavities or canals of 
various kinds may occur. 
In case the stem increases in 
diameter, the cortex develops a 
meristematic layer known as the 
phellogen or cork cambium (fig. 
539), which forms cork cells, a very 
impervious kind of cell (see p. 318). 
Continuous activity of the phellogen 
within results in an increasing thick- 
ness of the sheet of cork cells with- 
out, and such sheets form a most 
efficient protection. The chloro- FlG ^ _ Transverse section of outer 
phyll tissue beneath the cork cells portion of cortex of a geranium, showing 
maintains connection with the air the cork cambium or phellogen (c) cutting 
. off layers of cork cells (four such layers 
for a time through special structures, beneath epidermis) . below are chlorophy ii. 
called lenticels (fig. 540) , interrupting containing cells of the cortex. 
