8o MORPHOLOGY OF TISSUES. 



which consist of a thick mass of tissue. In general the contrast of the two is the 

 plainer the more the part of the plant concerned is exposed to air and light, 

 underground and submerged parts showing it in a smaller degree ; in those des- 

 tined 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 established 

 by the outer layers of cells, whose morphological character is otherwise the same, 

 becoming distinguished by the thickness and firmness of their cell-walls, and hence 

 usually by being smaller than those which lie deeper inside. In this case a 

 sharp boundary of the two tissues does not usually occur; the distinctions gradu- 

 ally increase the more nearly the cell-layers approach the upper surface. This is 

 usually the case, among Algae, with the Fucaceae and larger Floridese, with many 

 Lichens and the fructification of Fungi ; even in the stem of Mosses the formation 

 of epidermis is often indicated only in this manner. A further development of the 

 contrast between epidermal and inner tissue arises when not only a sharp boundary 

 lies between the two, but when an essentially different morphological development 

 also distinguishes the epidermal from the inner tissue. In many Mosses and all 

 Vascular Plants at least one outer layer of cells is to be distinguished in this sense as 

 epidermal structure, and is here termed Epidermis. In true roots and many root- 

 like underground parts of stems, as also in many submerged plants, it is generally 

 only slightly different from the tissue lying beneath ; but in most parts of stems 

 and leaves it shows an altogether peculiar development of its cells, giving rise to 

 stomata and hair-formations of the most various kinds. In many leaves and parts 

 of stems, the epidermis, after it has already become recognisable as a tissue of a 

 peculiar kind (during or after the bud-condition of the organs concerned) 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, /. <r. p. 53) may be 

 conveniently distinguished as Hypoderma^ such layers of tissue as lie very com- 

 monly beneath the simple, rarely beneath the many-layered epidermis, and per- 

 form the physiological function of strengthening the epidermal tissue, without how- 

 ever belonging to it genetically; while they are strikingly distinct from the deeper 

 lying fundamental tissue, although, according to their development, they are a part of 

 it. This hypoderma consists chiefly of layers or bundles of thick-walled scleren- 

 chyma-cells, sometimes even of bast-like fibres. In Phanerogams, especially Dico- 

 tyledons, the hypoderma is mostly developed as Collenchyma, the cell-walls of 

 which are 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 the parts of plants which live long and are endowed with vigorous growth in 

 thickness, the epidermal system attains a further development in the production of 

 Cork; this originates by subsequent cell-division in the epidermis itself or in the 

 subjacent layers of tissue, occurring often very late, and by the transformation into 

 cork of the newly-formed cells. The formation of cork is very frequently continuous, 

 or is renewed with interruption; and when this occurs uniformly over the whole 



I prefer the word Hypoderma proposed by Kraus and adopted by Pfitzer to the expression 

 previously used by me ' sub-epidermal layers.' Cf. besides sect. 1 7 (c). 



