CHAPTER VIII 



THE EPIDERMIS 



THE surface of the shoot is protected by a skin, or epidermis, 

 composed of one layer of living cells which possess certain marked 

 characteristics. In transverse sections the cells usually appear 

 somewhat flattened with slightly convex outer walls, whilst the 

 lateral and inner ones are generally straight ; moreover, they fit 

 closely together without intercellular spaces (Figs. 19 and 43,^.). 

 The epidermal cells have living contents, usually contain plastids, 

 and possess large vacuoles filled with watery, generally colour- 

 less, sap (Fig. 43, Ep.). Chloroplasts are not developed in the 

 cells in well-illuminated situations, but are often present in the 

 epidermis of submerged aquatics or of land-plants when growing 

 in the shade (cf. p. 169). 



The outer epidermal walls in most cases are more strongly 

 thickened than the others, and so changed (cuticularised) as to 

 render them more or less impermeable to water vapour and 

 gases. The exact nature of the modification is not known, but 

 it appears to consist essentially in impregnation with substances 

 of a fatty or waxy character; these are most abundant in the 

 outermost region of the external walls, which forms a continuous, 

 relatively impermeable, layer over the whole epidermis (except for 

 the stomata) known as the cuticle (Figs. 43 and 46, Cu.) . Where the 

 walls are strongly thickened an intervening zone (the cuticular- 

 ised layers), in which cuticularisation has not progressed to the 

 same extent, can sometimes be distinguished between the cuticle 

 and the unaltered cellulose on the inside. The cuticle is often 

 yellowish in colour, whilst the cuticularised layers, when present, 

 appear faintly yellow and less transparent than the colourless 

 cellulose. The cuticle may be quite smooth, but in some cases 

 (e.g. Helleborus fcetidus) it is provided with numerous minute 



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