EPIDERMIS 



95 



formed by the division of the dermatogen (Ficus elastica), or by 

 differentiation from the underlying periblem (as in the Holly, 

 Fig. 43, A, Hy.). The cells of such a hypoderm (cf. also p. in) 

 resemble the epidermal cells in the possession of large vacuoles 

 and the absence of chloroplasts, but usually have thinner 

 walls. It will be realised that in all cases leaf and stem are 

 surrounded by a water-jacket tending to diminish the heating 

 effect of the sun. 



Another device serving for water-storage consists in the 

 development of thick mucilaginous internal walls (cf. p. 38) 

 by the epidermal cells (e.g. Hollyhock, Sycamore, etc.). These 

 are inconspicuous in sections mounted dry, but swell up con- 

 siderably, often projecting deeply into the assimilatory tissues 

 (e.g. Hollyhock), when placed in water. This device tends to 

 retard the evaporation of water during periods of drought. 



The pronounced thickening and cuticularisation of the outer 

 walls of the epidermal cells, and the absence of intercellular 

 spaces between them, endues the epidermis as a whole with 

 considerable strength. It hence serves as a mechanical protec- 

 tion, but also (in combination with the hypoderm, when present) 

 as a light-screen for the underlying chlorophyll. Moreover, the 

 polished cuticle, especially characteristic of tropical plants, 

 serves to reflect a large part of the light and heat rays falling 

 upon it. Most important of all, it checks evaporation from the 

 general surface of the plant, and indeed, where the cuticle is 

 thick, transpiration is almost entirely restricted to the special 

 apertures or stomata which will be considered below. 



Certain modifications of the epidermis are associated with 

 special physiological functions. Thus in many shade-loving 

 plants the outer walls of the epidermal cells are markedly convex 

 (e.g. Wood Sorrel; Moschatel, Adoxa, Fig. 45). Where this is 

 the case, each cell functions as a plano-convex lens focussing a 

 localised patch of bright light at its base. It has been suggested 

 that this acts as a mechanism for the orientation of the leaf, 

 since, when the direction of illumination is altered, the position 

 of the patch of light likewise changes. The protoplasm upon 

 which this light falls is assumed to be sensitive, so that an 

 adjustment of the leaf follows until the patch of light is again 

 located in the normal position. But quite apart from any value 



