PHELLODERM.—PERIDERM. 91 
The liber-cells of which it is essentially composed are either 
placed side by side in a parallel direction, and thus form by 
their union a continuous layer, as in the Horsechestnut tree ; 
or far more frequently they present a wavy outline, and only 
touch each other at certain points, so that numerous interspaces 
are left between their sides, in which the medullary rays con- 
necting the cellular layers of bark and the pith may be observed. 
From this circumstance the inner bark when macerated in water 
commonly presents a netted appearance, as may be especially 
seen in that of the Lace-bark tree (Lagetta lintearia) of Jamaica, 
_and of other plants belonging to the same natural order. 
b. The Cellular Envelope, Green Layer, or Phelloderm (fig. 
190, c, and 185, g, g).—This layer lies between the liber and 
corky layer, and hence the name middle layer which is also applied 
to it. It is connected on its inner surface with the medullary 
rays. It consists of thin-sided, usually angular or prismatic, 
parenchymatous cells (jig. 190, c), which are loosely connected, 
and thus leave between their walls a number of interspaces. 
The cells of which it is composed contain an abundance of 
chlorophyll, which gives the green colour to young bark, and 
hence the name of geen layer, by which it is also commonly 
distinguished. It is also sometimes known under the name of 
phelloderm. This layer and the next belong to the fundamental 
tissue, and form together the parenchymatous system of the bark. 
ce. Corky or Suberous Layer (figs. 190, b, and 185, h, h).— 
This is the outer layer of the bark, and is invested by the 
epidermis (jig. 190, a). It has also received 
the name of periderm ; this term is, however, 
sometimes used in a general sense to indicate 
the dead portion of the bark, or that which 
has ceased to perform any active part in the 
life of the plant ; which is commonly the case, 
as we shall presently see, in a few years with 
the two outer layers (see page 92). In this 
sense the periderm may consist of the corky 
layer alone, or of phelloderm chiefly, or of por- 
tions of both, or even in some cases ofa portion 
of the phloém also. Those botanists who 
adopt this nomenclature commonly apply the 
term derm to the inner living portion of the 
bark. Other botanists also use the term peri- 
derm to indicate the inner portion of the corky 
layer, and which consists of cells with thicker _,,. 
wells and less elastic than true cork cells. ae ia 
The corky layer consists of one or more /, /. Lenticels. ¢, ¢. 
layers of tabular cells (fig. 190, b), elon- 22% 
gated more or less in a horizontal direction, and which in 
most cases ultimately become dried up and filled with air, and 
form by their union a compact tissue, or one without inter- 
Bre. 1915 
