62 



PROTECTION FROM INJURIES AND LOSS OF WATER 



any time to make use of available materials and forces never 

 could have been wrought out. 



The Epidermis 



The Epidennis as a Protective Tissue.— The epidermis is 

 the first, and often the only, protective tissue formed, and since 

 it lies at the exterior it must bear the brunt of adverse conditions 



in the environment. It prepares itself 

 for its task mainly by modifications 

 of its outer wall, which it thickens and 

 waterproofs according to the demands 

 made upon it. The thickness of an 

 average outer epidermal wall under 

 normal conditions of moisture in the 

 B nnnr^rrSririr substratum and surrounding atmos- 

 " phere is not far from .0033 mm., or 



about one thirty-sixth the thickness 

 of this page; but as greater dryness in 

 the environment imposes severer con- 

 ditions, and where the epidermis per- 

 sists through several years with inci- 

 dent wear and tear, we find the outer 

 wall often increasing in thickness, 

 even up to .03 mm., or one-fourth 

 the thickness of this page, as in 

 the case of the mistletoe (Fig. 31). 

 Measured by ordinary standards, 

 the epidermal outer wall is in any 

 case extremely thin and would seem 

 poorly adapted to withstand any kind of mechanical attack; 

 but it must be remembered that the epidermal cells are very 

 minute, averaging about .03 mm. in tangential diameter, so 

 that the radial walls serving as a foundation on which the 

 outer wall rests, are that distance apart merely. Any stress 

 upon the outer wall is, therefore, not borne by it alone, but 

 by the radial walls also and the underlying tissues with which 



.36fe^ 



Fig. 31. — A, cross section 

 through upper half of leaf of 

 Pyrus Japonica, showing cutinized 

 layer of the outer wall at /, and 

 cellulose layer at g. B, the same 

 for the leaf of Russian olive; the 

 cutinized layer is thinner and the 

 cellulose layer thicker than in the 

 former instance. C, portion of 

 cross section of submerged stem of 

 Nymphsea odorata, where there is 

 no cutinized layer, and the cuticle 

 is a hardly distinguishable film. 



