EPIDERMIS AS A PROTECTIVE TISSUE 63 



these are connected. Added to this the outer wall commonly 

 curves outward, as it spans the space between the radial 

 walls (Fig. 14) and is thus made more resistant against stress 

 from without. The part which the radial walls play as props 

 for the outer wall will be better apprehended by an example. 

 Assuming that the tangential diameter of the epidermal cells is 

 .03 mm. and that the cells as seen from the surface are four-sided, 

 then if any object i sq. mm. in cross section were pressing upon 

 the epidermis there would be more than 2,000 radial walls imme- 

 diately beneath and sustaining that portion of the outer wall upon 

 which the pressure comes. Of course the smaller the object 

 which is pressing upon the surface the better chance it has of 

 piercing the outer wall. Since the point of an ordinary needle 

 would cover not more than one-quarter of the surface of our aver- 

 age epidermal cell it could be placed so as to avoid the radial 

 walls in piercing the surface. Experimenting on leaves grown 

 under average conditions, it has been found that when using such 

 a needle a pressure of .55 gm. to .8 gm. was necessary to break 

 through the outer wall of the upper epidermis. 



Incrustations and infiltrations of silica and calcium carbonate 

 often contribute to the defense of the outer wall, and this to a 

 very notable extent in Equisetum and many grasses where silica 

 is very abundant. 



The cutinization of the outer wall, which is primarily for 

 waterproofing, increases the power to resist tearing, experiments 

 having shown that the cutinized wall may be even ten times as 

 strong in this respect as ordinary cellulose walls. 



Turning from the anatomical and experimental details, we 

 find nature giving a broad and convincing answer regarding the 

 mechanical efficiency of the epidermis. Leaves, on the whole, 

 outride the storms and vicissitudes of three seasons practically 

 uninjured; and herbaceous plants run their course with nothing 

 but the epidermis to cover them. In the whole group of fleshy 

 fruits the epidermis has been found an adequate protection until 

 the time of ripening; and in many woody plants, such as species 

 of Acer, Rosa, Cornus, Acacia and Cinnamomum, the epidermis, 



