52 



in which even the same sort of trees differ 

 more than in this respect. According to 

 growth, soil, exposure, &c., the cuticle 

 exists to a very indefinite period ; and it 

 would be hard to say where cuticle ceased 

 and epidermis began. Living external 

 bark may be found on oak, ash, beech, 

 Spanish and horse chesnut, sycamore, 

 poplar, &c., &c., on parts varying from 

 twenty to fifty years in age. On roots 

 also of the same age may be found a fine 

 silvery cuticle which tears off like paper ; 

 though in roots, under ordinary circum- 

 stances, the under layer is white, not 

 green. 



De Candolle states it as a distinctive 

 characteristic of roots, as compared with 

 the stem, that " they do not become 

 green even when they are exposed to 

 the air and light." And this opinion is 

 universally held by physiologists ; but it 

 is an error. De Candolle, in proof of 

 the opinion, states that the roots of 



