PERSISTENT EPIDERMIS 



111 



and as a consequence every drop of water that falls upon the velvety 

 surface spreads out at once, owing to the action of capillarity, and is 

 thus caused to evaporate rapidly. Possibly the network of delicate 

 external ridges which is so frequently developed on epidermal surfaces 

 usually owing to corrugation of the cuticle produces a similar 

 effect. 



This account of the structure and properties of the outer epidermal 

 wall may conclude with a reference to the changes which the epidermal 

 surface may undergo with age, partly owing to the natural weathering 

 of the surface, and partly as a result of the growth in thickness of the 

 organ to which the epidermis belongs. In this connection it will only 

 be necessary to consider the so-called persistent . 



epidermal layers, which act as the sole dermal 

 covering for a number of years. Such a persistent 

 epidermis is by no means rare ; it occurs, for in- 



^r^ 



r* 



Fig. 25. 



A. Epidermis of a year- 

 ling twig of Acer striatum, ; 

 r, r, cracks in the cuticle 

 and cutinised layers, x 370. 

 B. Epidermis of a six-year- 

 old branch of the same tree. 



stance, in a number of Lauraceae (Cinnamomum )C_j(_3CJ) 

 officinarum, Lauriis nobilis), Eosaceae (Bosa alpina, 

 B. canina, B. multiflora, Kerria japonica), Legu- 

 minosae (spp. of Acacia, Sophora japonica), Aqui- 

 foliaceae (spp. of Ilex), Ace race ae {Acer striatum, 

 A. palmatum, A. Negundo), Cornaceae (Cornus 

 alternifolia, Aucuba japonica), Oleaceae (spp. of 

 Jasminum), etc. There is nothing surprising in 

 the fact that the outer wall attains a relatively 

 enormous thickness in epidermal layers of this 

 type. But it is interesting to note that this wall 

 undergoes a process of continual regeneration ; its outermost layers 

 peel off or crumble away, while new layers are deposited on its 

 inner side. Acer striatum affords a very instructive illustration of the 

 crumbling process. On yearling branches of this tree the epidermis is 

 quite smooth (Fig. 2 5 a). The cutinised layers, which are thick and 

 impregnated with wax, are sharply delimited from the thin sinuous 

 cellulose layer. The cuticle is very delicate, and is covered with a thin 

 crust of wax. Late in the autumn of the first year numerous radial 

 fissures develop in the outer wall ; these appear over the radial walls 

 of the epidermal cells, and extend inwards about half-way through the 

 cutinised layers. This cracking is the first visible consequence of the 

 passive tangential extension to which the outer epidermal wall is 

 subjected, owing to the growth in thickness of the twig. The epidermis 

 of a six-year-old branch (Fig. 25 b) shows a whole series of characteristic 

 changes. The thickness of the outer wall or rather that of the 

 cutinised layers has increased considerably. The cellulose layers are 

 no longer sinuous, but have become flattened out. This fact, as well 



