THE EPIDERMAL TISSUE. 
95 
is the plainer the more the part of tlie plant is exposed to air and light, under- 
ground and submerged parts showing it in a smaller degree ; in those destined 
to a longer term of life the formation of epidermis is usually also more perfect. 
The difference between epidermis and fundamental tissue can only be estab- 
lished by the outer layers of cells, whose morphological character is otherwise 
similar, becoming distinguished by the thickness and firmness of their cell-walls, 
and by having smaller cell-cavities than those which lie deeper. In this case 
there is usually no sharp boundaiy-line between the two tissues ; the charac- 
teristics gradually increase the more nearly the cell-layers approach the surface. 
This is usually the case, among Algae, with the Fucacea^ and larger Florideae, 
with many Lichens and the fructifications of Fungi ; even in the stem of Mosses 
the formation of epidermis is often indicated only in this manner. The con- 
trast between epidermal and inner tissue becomes most marked when, besides 
a sharp boundary between the two, a different morphological development dis- 
tinguishes the two kinds of tissue. In the sporogonia of Mosses and in all 
Vascular Plants at least one outer layer of cells may be distinguished in this sense 
as epidermal tissue, and is termed the Epidermis. In true roots and many root-like 
underground stems, as also in many submerged plants, the epidermis is only 
slightly different from the subjacent tissue ; but in most parts of stems and leaves it 
shows an altogether peculiar development of its cells, giving rise to stomata and 
trichomes of the most various kinds. In many leaves and parts of stems, the 
epidermis, after it has already become a recognisable tissue (during or after the 
bud- condition) undergoes cell- division tolerably late, by which it becomes divided 
into two or more layers. From this epidermis formed of several layers of cells 
(Pfitzer, /. c. p. 53) those layers of tissue may be conveniently distinguished as 
Hypoderma which lie beneath the simple, rarely beneath the multilamellar epidermis, 
and perform the physiological function of strengthening the epidermal tissue, without 
however belonging to it genetically, while they are strikingly distinct from the deeper 
lying fundamental tissue, although genetically a part of it. This hypoderma consists 
chiefly of layers of thick-walled sclerenchymatous cells, sometimes even of bast- like 
fibres. In Phanerogams, especially Dicotyledons, the hypoderma is mostly developed 
as Collejtchyma, the cell-walls being strongly thickened, and in a high degree capable 
of swelling at the longitudinal angles where three or four of them meet (Fig. 21, 
B, p. 24). 
In those parts of plants which live long and which increase greatly in thickness, 
the epidermal system attains a further development in the production of Cork. This 
originates in the epidermis itself or in the subjacent layers of tissue by subsequent 
cell-division, occurring often very late, and by the suberisation of the newly-formed 
cells. The formation of cork often continues for a very long period, or is renewed 
after interruption ; and when this occurs uniformly over the whole circumference, 
there arises a stratified cork-envelope, the Periderm, replacing the epidermis, which 
generally perishes, and surpassing it as a means of protection. But not unfre- 
quently the formation of cork penetrates much deeper; lamellae of cork arise deep 
within the stem as it increases in thickness ; parts of the fundamental tissue and 
of the fibro-vascular bundles, or of the tissue which afterwards proceeds from 
them, become, as it were, cut out by lamellae of cork. Since everything which 
