TISSUES AND SIMPLE ORGANS. 55 



outer portions of the radial walls furnishes the mechanical support. 

 In the indigenous foliage leaves which live only a few months this 

 character is correspondingly reduced (No. 1). 



As a rule, the greater portion of the epidermal cell-contents 

 is water (No. 3, p. 53). Eed and other coloring materials are 

 frequently found in solution. The absence of chlorophyll is a very 

 prominent anatomical characteristic of epidermal water-supplying 

 tissues. "We have now touched lightly upon the three functions 

 mentioned. 



In water-plants chlorophyll is very plentiful in the epidermal 

 cell-layer. This does not at all signify that a " tegument " or 

 "epidermis' 7 is wanting. An epidermal water-storing tissue is, 

 however, wanting, while we find well-marked evidence of epidermal 

 characteristics which find expression in a considerable thickness of 

 the outer wall, that is, mechanical characteristics, since that alone 

 is found to be useful. For example, in case of injuries water would 

 enter the air chambers in the interior and expel the much-needed 

 air (see Aeration). 



We speak of increase in the functional activity of epidermal 

 systems. As a rule, the amount of water in the epidermal water- 

 tissue increases with the depth of the single-layer epidermis and 

 with the number of layers in the many-layered epidermis. Most 

 of our indigenous plants have an epidermis of a single layer, with 

 various gradations in the thickness of this single layer. Leaves 

 which are exposed to considerable dry ness (Ficus, etc.) have several 

 layers. 



As a result of excessive loss of water the thinness of the radial 

 walls (Fig. 34) permits not only of shortening, but of wavy foldings. 

 The latter, according to our conception, is for the special protection 

 of green tissues. It can be clearly shown that epidermal and 

 internal water-tissue cells are the first to suffer from loss of water. 

 The assimilating cells may endure a much longer time without 

 visible signs of material loss. 



Certain leaves of Bromeliaceae show epidermal structures of 

 several layers thickness in the part functioning mechanically as well 

 as in the part functioning as a water-reservoir. Thin radial walls 

 are not always present. For example, in the epidermal structures of 

 xerophilous plants (desert plants, plants accustomed to excessive 

 dryness) we find thick radial walls ; these have, however, numerous 

 pores which facilitate the exchange of water in the water-tiseue. 



