64 PROTECTION FROM INJURIES AND LOSS OF WATER 



continually renewing the outer wall as it wears away, remains 

 as the sole outer covering for many years. 



The epidermis is, however, not proof against all attacks that 

 plants are subject to; insects seem to have little difficulty in 

 gnawing through or piercing the outer wall, and many parasites 

 excrete ferments that render it soluble, and storms sometimes 

 overtax its strength. But such things are on the whole not 

 sufficiently severe and widespread, at least in the course of a 

 single growing season, to make a serious demand for reenforce- 

 ment of the epidermis. 



The Epidermis as a Waterproof Covering. — The chief value 

 of the epidermis lies in the protection which it gives against a too 

 rapid evaporation of water. All terrestrial plants of any size 

 would lose water faster than they can absorb it, but for the pro- 

 tection which the epidermis affords. As stated in the previous 

 chapter, the waterproofing of the epidermis lies in the cuticle 

 and cutinized layer of the outer wall. The cutin which gives 

 these their peculiar character is waxy in its nature, and when 

 present in abundance it makes the wall practically impervious to 

 water. The cuticle seems to be nearly pure cutin, while the 

 cutinized layer appears to contain a certain percentage of un- 

 altered cellulose. In many cases the cutinized layer is absent 

 and then the outer wall consists of cellulose bounded externally 

 by the water-proof cuticle. We adopt the scheme of plants 

 when we pour paraffin over jelly to keep it from drying out and 

 molding, and when we coat paper with paraffin for waterproofing 

 purposes. 



The efficiency of the epidermis in preventing loss of water is 

 seen by comparing the amount of loss where the epidermis is 

 removed in some cases and left intact in others. For instance, 

 two apples were hung up in a dry atmosphere, one pared and the 

 other uninjured, and after forty-eight hours the former had lost 

 T,T, per cent, of its original weight and the latter i per cent. 

 An Aloe leaf, according to Heberlandt, had in twenty-four 

 hours lost 15.6 times more water where the epidermis was re- 

 moved than where it was left on. 



