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UPWARD GROWTH OF THE HEAD, AND [Part II. 
Living external bark, with a green under layer, 
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. 
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 of 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 charac¬ 
teristic of roots, as compared with the stem, that 
u 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 hyacinths grown in 
transparent glasses do not turn green. This is 
true of them, and also of the silver ends of 
woody roots; but it must be recollected that to 
neither of these can the air be admitted when 
they grow in water, or light when they grow in 
earth. It is however, I believe, true of these 
