EPIDERMIS. 



73 



surface, the slit is therefore surrounded by a series of radiate bands. In Milde's 

 E. cryptopora (at least E. hiemale) the striated wall is traversed about half way 

 between the slit and the convex outer border by a narrow oblique furrow, which is 

 almost parallel to the slit, and this divides two concentric series of radial bands, one 

 of these next the slit, the other on the side opposite to it (Fig. 24). 



What has been said of the walls of the epidermal cells holds in the main for 

 those of hair-structures. Those walls which separate the cells of multicellular hairs 

 resemble on the whole the lateral and inner walls of the epidermis. Their details of 

 structure are, if possible, more various than their modifications of form. Pro- 

 jections of the outer surface, in form of ridges, warts, or even of those sharp 

 prickles represented in Fig. 2 1 5, appear in hairs more commonly than in the epi- 

 dermal cells. The stiffness of the hairs depends upon the thickening of the walls, 

 which may proceed till the lumen is obliterated. A hard, rigid hair or shaggy hair, 

 is called a bristle or seta. If it is also conical and sharp one can prick oneself 

 with it, as with the horizontal hard bristles of Malpighia urens, or the rigid hairs of 

 the Borragineae and Cucurbitacese. In this property, so disagreeable to men, lies 

 the ground for the often-asserted similarity of the hairs of the Malpighiacae to the 

 stinging hairs, and of puncturing bristles to prickles. 



Sect. 15. The cell-walls of the epidermis are cellulose membranes; a number 

 of other bodies are embedded in, or superposed on this : Cuticular-substance or 

 cuit'n, wax, resin, volatile oils, gum and lassorin, compounds of silicon, and 

 lime salts, bodies with whose presence remarkable peculiarities of structure are 

 connected. 



Sect, i 6. Of the relatively pure layers of cellulose of epidermal membranes it 

 may be said that in the majority of cases, especially in herbaceous parts, they appear 

 similar to that watery highly refractive modification of cellulose membranes which 

 is charaicteristically developed in the CoUenchyma, to be described later. The 

 detailed investigations necessary for an exact statement on this point have not 

 been made. 



Allied to these watery cellulose layers are the parts of the membrane of 

 epidermal cells, which have been altered to vegetable mucilage and bassorin ; 

 Radlkofer ^ has lately drawn attention to the frequent occurrence of these substances 

 in foliage leaves. The thickened inner wall of these epidermal cells consists, espe- 

 cially in their inner layers, in the mature condition, of a vegetable mucilage, which 

 swells in water till its identity is lost, like the mucilage of Linseed, &c. These 

 layers of mucilage are developed especially strongly on the leathery leaves of the 

 Cape Diosmese (Diosma alba, Agathosma spec, leaves of Buku''), where they are 

 found on the upper side of the leaf, which has no stomata, and in the parts of the 

 under surface where stomata are absent. The cells in these parts are of a great 

 height, their outer half has the usual structure of tough cuticularised epidermis. The 

 whole of the inner half, which is often large, is filled with the stratified mass of 

 mucilage, which is limited on the exterior by a level surface. This body swells on 

 addition of water or glycerine to such an extent that it lifts the whole outer parts of 



■ Monogr. d. Gattung Serjania, p. 100 (1875). 



" Compare Fluckiger, Schweizerische Wochenschrift f. Pharmacie, Dec. 1873. 



