v 
EN 
Ith 
~» tissues (see Chap. V.). In plants with a long term of life, and which — 
K 
‘transformed into the so-called cuticle (see Fig. 21, p. 16), a thin 
which are everywhere closely contiguous. In the course of ag 
- growth some of these separate from one another, the deri- 
' ° ede 6 ; ° cd ' 
_ -vative cells that result from one or more divisions leavinga 
oe his "S Mactan? inn d Physiologica Bole a 
“rest: In some Mosses and in all the more highly organ | vin 
Soins at least one layer of cells specially defined and de- — a 
» to) 4 
. 
veloped, the efzdermis, exercises special functions. In true - 2 
roots, and in many root-like underground stems, as wellas ~ 
in many plants which grow submerged in water, it is less 
differentiated from the subjacent tissue than is the casein 
most stems and in leaves. 3 
The outer side of the outermost layer of epidermal cells is gradually © 
structureless membrane, but little sensitive to external and even to 
chemical agencies, insoluble, for example, in concentrated sulphuric 
acid or in potash. The cells of the epidermis usually contain clear or 
coloured fluids, occasionally crystals, rarely chlorophyll. Their form is 
very various, but usually tabular or conical, or sometimes even oF 
‘papileform, z.e. projecting on the outside in the form of knobs or ~~ 
bladders.. The peculiar appearance of many structures, such as the | 7 
velvety gloss of petals, is due to papillee of this nature (Fig. 84). In a 
many cases there lie beneath the epidermis peculiar layers or strings of 
_ cells (the AyZoderma), which are perhaps best included in the epidermal — : 
tissue. They consist either of thick-walled prosenchymatous bundles «| 
_which contribute to the greater firmness and elasticity of the epidermis, 
as in the aérial portion of the stem of Equiseta, and the acicular leaves 
of Pinus pinaster, or of collenchymatous tissue which tends to increase a 
the difference of tension between the epidermal and the subjacent .— 
Increase greatly in girth, cork is subsequently formed in the epidermal 
cells themselves or the subjacent layers of tissue, replacing the true 
epidermis, which has generally in the meantime perished, 2 
} 
‘The true epidermis consists, when young, of similar cells 
crevice or pore between them which 1s called a stoma (Fig. 
85) These are always! enclosed by two or four cells, the Ae 
guard-cells, which are crescent-shaped, and also smaller and 4 
with thinner walls than the rest of the cells of the epi- ii 
1 Exceptions, however, occur in the case of the Marchantieze and 
_ Bhizocarpeee. Piss 
