MECHANICAL FUNCTION OF EPIDERMIS 109 



Attention must next lie directed to the mechanical significance of 

 the thickened condition of the outer epidermal wall. 51 It is obviously 

 essential that a dermal tissue should be endowed with a certain degree 

 of mechanical strength. As a matter of fact, cutinised epidermal walls 

 can actually withstand very considerable tensile stresses, the ultimate 

 resistance of the average epidermis ranging, according to Uamm, from 

 5 to 10 kg. per sq. mm. Thus isolated strips of the epidermal layer 

 of Aristolochia Sipho break under a load amounting on the average to 

 lO'l kg. per sq. mm. In three other species examined by Damm, 

 namely Acacia dcalbata, Aucaha japonica and Ilex aquifolium, the 

 breaking strengths are 9*2 kg., 7'7 kg. and 5'3 kg. respectively; in the 

 case of ordinary cellulose walls the corresponding value, according to 

 Schwendener, never exceeds 1 kg. It would evidently be a mistake to 

 suppose that the thickness and the cutinised condition of the outer epi- 

 dermal wall are valueless except as means of restricting transpiration ; 

 for both these features not infrequently serve in the first instance to 

 secure the requisite degree of mechanical strength. Thus, in the case 

 of many leathery-leaved tropical plants the epidermal wall is much 

 thicker than is necessary for the adequate restriction of transpiration 

 in a humid climate : here the thickness of the wall must be regarded 

 as a mechanical adaptation, which helps to preserve the leaf from 

 being injured by the violent rain that falls daily during the wet 

 season. 



The rigidity of the thickened epidermal wall entails a further 

 important advantage ; it minimises the distortion and compression of 

 the underlying tissues that are liable to result from the contraction 

 consequent upon any loss of water, and at the same time enables the 

 collapsed tissues to return to their former dimensions and relative 

 positions with ease and precision when water is reabsorbed. In this 

 respect the thickened outer walls of the epidermal cells may collectively 

 be said to form a rigid case in which the less resistant tissues are 

 safely stowed away. 



The rigidity of the outer wall is often increased by a network of 

 internally projecting flanges ; each flange represents a prolongation of 

 the cutinised layers, which extends more or less deeply into the 

 substance of a radial wall (Fig. 23 a). Every mesh of the network of 

 flanges is thus coextensive with the lateral outline of a single epidermal 

 cell. In transverse section each flange presents a wedge-shaped, or 

 more rarely a lanceolate, outline. 



The firm texture of the outer epidermal wall is important in yet 

 another way, because it affords more or less effective protection 

 against the attacks of animals. This interpretation applies more 

 particularly to those cases in which the hardness of the thickened 



