[22 



DERMAL SYSTEM 



numbers in the outer walls of the epidermal cells. Reference may 

 lastly be made to the tactile pits of the outer epidermal wall which 

 occur in the tendrils of Cucurbitaceae, and in certain other organs 

 that are sensitive to contact. The protoplasm which lines such pits is 

 concerned with the perception of contact-stimuli ; the detailed dis- 

 cussion of these structures must therefore be deferred until a later 

 occasion (Chap. XII.). 



B. THE MULTIPLE EPIDERMIS. 



Sometimes the primary dermal covering of a plant consists of more 

 than a single layer of cells. This exceptional condition is in every 

 case attributable to an increase in the demands that are made upon 

 the protective capacity of the epidermis. The increased demand may 



Fig. 31. 

 Development of cuticular epithelium in an old branch of Viscum allium. (T. S.) 



relate either to the control which the epidermis exercises over transpira- 

 tion, or to the mechanical role of that layer, or, finally, to its water- 

 storing function. These various possibilities may be illustrated by a 

 few typical cases. 



Damm 70 has shown that the epidermis of the stem of Viscum album 

 and other Viscoideae not only persists for a number of years, but is 

 gradually reinforced by cutinised layers, which develop in the outer 

 walls of the underlying cortical cells. To begin with, the guard-cells 

 of the stomata and certain of the ordinary epidermal elements lose the 

 power of adding further cutinised layers to their outer walls. As the 

 neighbouring cells continue to thicken their walls, those which cease to 

 do so soon sink below the general level of the epidermis : at the same 

 time cutinised layers begin to appear in the outer walls of the hypo- 

 dermal cells. In this way the inactive epidermal elements become 

 conpletely enveloped in cutinised layers and consequently die. This 

 process gradually extends inwards" until it involves a number of cortical 

 layers ; in this way the older branches become invested by a multiple 



