THE EPIDERMAL TISSUE. 



9^ 



is the plainer the more the part of the 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 boundary-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 Fucacese and larger Floridese, 

 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 becomics 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 

 slighily 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 Collenchyma, 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 



Ienerally perishes, and surpassing it as a means of protection. But not unfre- 

 uently the formation of cork penetrates much deeper; lamellae of cork arise deep 

 ithin the stem as it increases in thickness; parts of the fundamental tissue and 

 f 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 



