124 



DERMAL SYSTEM 



according to Baumert, the thickened inner walls are shaped like 

 concave mirrors : they accordingly reflect a portion of the light that 

 falls upon the leaf and thus afford some protection against intense 

 illumination. In the case of the inner of the two cell-layers, all the 

 walls may he equally thickened (Fig. 33a), or else the outer walls may 

 be somewhat thicker than the rest. In the latter event the outer 

 walls of the inner layer and the inner walls of the outer layer collectively 

 form a remarkably thick and rigid composite membrane (Fig. 33 b). 



As this membrane is quite free from 

 cutin, it merely serves to increase 

 the mechanical strength of the der- 

 mal tissue ; the duty of controlling 

 transpiration, on the other hand, 

 is almost entirely relegated to the 

 cutinised outer walls of the super- 

 ficial layer. Thus two partial func- 

 tions which are both performed by 

 the thickened, and at the same time 

 cutinised, outer wall of an ordinary 

 epidermis are, in the leaves of the 

 Bromeliaceae, dissociated and dis- 

 tributed between two different cell- 

 layers. The water-storing function 

 is dropped altogether as far as the 

 epidermis is concerned, because the 

 leaves of Bromeliaceae are already 

 provided with a well-developed in- 

 ternal water-tissue. As a matter of 

 fact, however, it is the necessity for the storage of water which most 

 frequently leads to the development of a multiple epidermis. Those 

 cases in which a few scattered epidermal cells (Tradescantia zehrina) or 

 a larger number (e.g. lower side of the leaf in Passerina ericoides, stem 

 of Ephedra spp.) undergo a single tangential division, may be regarded 

 as examples of the first stages in the multiplication of the epidermis. 

 By a similar process of division the whole upper epidermis of the leaf 

 becomes two-layered in P&peromia arifolia, two- to four-layered in 

 Begonia manicata, Peperomia Manila, etc. The further development of 

 this tendency results in the formation of a typical many-layered peri- 

 pheral water-tissue. In these extreme cases, water-storage has become 

 the principal or even the sole function of the multiple epidermis, except 

 for its outermost layer ; in order to avoid needless repetition these 

 tissues will be discussed in connection with the storage-system 

 (Chap. VIII.). 



Fig. 33. 



Double epidermis of the BROMELIACEAE. 



A. Adaxial foliar epidermis of Bilbergia viridiflora. 



B. Adaxial foliar epidermis of Vriesea sp. 



