THE EPIDERMIS 129 



Structurally the value of the cuticle for purposes of identification or 

 as a basis of evolutionary speculations can easily be exaggerated. 

 There is little doubt that it shares with the general external form of 

 plants the shortcojning of unreliability, which results, as in the 

 case of formal characters, from close connection with physiological 

 necessities. 



The epidermis is not only a limiting membrane of plants, but is 

 likewise charged with the important office of bringing about 

 regulated interchanges between internal spaces and the outside 

 air. The mechanism which performs the function of facilitating 

 gaseous and other interchanges is the stoma. This structure con- 

 sists of a pair of guard cells variously organized to meet conditions of 

 environment and bounding a pore, the stoma proper. The guard 

 cells in cases where they are actually functional have their inner 

 and outer walls so thickened that the accumulation of osmotic 

 pressure within the cells leads to their divergence in the middle 

 line with the resultant opening of the pore. No matter what may 

 be the conditions in regard to the presence or absence of chloro- 

 plastids in the general epidermal cells, the guard cells are always 

 provided with green corpuscles and even under conditions of 

 starvation retain a modicum of starch after this substance has 

 disappeared in the rest of the cells of the leaf. In plants exposed to 

 a high degree of drought and insolation the guard cells of the stoma ta 

 become very much thickened and respond to the stimulation of 

 light and moisture only under extreme conditions. A further 

 safeguarding of the epidermal pores or stomata often results from 

 their being depressed below the surface of the leaf or sheltered 

 under a hairy protection. Plainly, under the last-described con- 

 ditions the loss of water from the stomata will be much less 

 than in the unsheltered condition. The guard mechanism in 

 plants exposed to extreme drought is sometimes throttled, as it 

 were, by a surrounding zone of cells which differ from the 

 ordinary epidermal elements in possessing chloroplastids. Cells 

 of this type are known as accessory cells. The accessory device 

 is variously related to the guard cells, the free movement of 

 which it serves to check. Very frequently the braking mechan- 

 ism consists of a collar surrounding the guard cells, while in a large 



