CH. rr. 



COURSE OF THE SAP. 



75 



different names ; but if we give the name of cuticle to 

 the outer covering of the hving bark, it will be found, 

 "with its green under layer oi parenchyma^ — the green, 

 porous, spongy layer, which is called the ' herbaceous 

 envelope,' — to extend over a much larger space of our 

 forest-trees than De Candolle assigns to it. There is 

 nothing 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, 

 with a green under layer, may be found on oak, ash, 

 beech, Spanish and horse-chestnut, sycamore, poplar, 

 &c., &c., on parts varying from twenty to fifty years 

 in age. And in the plane-tree, whose bark scales off 

 as it dies, and thus admits light and air to the under 

 layer, or herbaceous envelope, this may always be found 

 green on any part of the stem or branches. On roots 

 also from twenty to fifty years in age may be found a 

 fine silvery cuticle which tears off like paper ; though 

 in roots, under ordinary circumstances, the under layer, 

 or herbaceous envelope, is white, not green. 



De Candolle states it as a distinctive character- 

 istic 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 the hyacinths 

 grown in transparent glasses do not turn green. This 



