374] ENGELMANN—OAKS OF THE UNITED STATES. 
dance of material must shake our confidence in our discrimina¬ 
tion: within the compass of a few hundred yards we find not 
only the forms above distinguished, but numbers of others which 
are neither the one nor the other, but which are intermediate 
between them and clearly unite them all as forms of one single 
extremely polymorphous species. 
If one oak behaves thus, why not others? Thrown into a sea 
of doubt, what can guide us to a correct knowledge ? 
Though oaks are so common and such well-studied plants, I 
venture in the following pages to repeat old observations in order 
to combine with them some which I think are new, and which 
will help to throw a little more light on the subject. 
The trunk —its bark as well as its wood —is what we first 
contemplate, and this at once takes us to one of the principal 
points I wish to discuss. 
That the trunk is that of a large, sometimes one of the largest, 
or of a middle-sized tree, or occasionally that of a shrub, even a 
very low one, is well known. On the Atlantic slope of the con¬ 
tinent most species of oaks make trees and only a few are known 
as shrubs; I can now recall not more than one species, the live- 
oak of the south, which occurs in both forms: usually an im¬ 
mense tree, it occasionally bears a rich harvest of fruit as one of 
the smallest bushes. But it is different on the Pacific slope; 
there we find many oaks as trees in the lower countries, and as 
shrubs, usually with smaller foliage and smaller fruit, in the moun¬ 
tains. The lesser number of oaks seem to occur solely in one or 
in the other of these forms. 
Examining the bark , we at once become aware of the fact 
that the popular distinction of “White-oaks” and “Black-oaks” is 
based on correct observation. The paler, ashy-gray bark of the 
former and the darker, often nearly black, color of the latter cor¬ 
responds, as will be shown, with other essential characters, and 
well marks the two principal groups of our American Oaks. The 
bark of the White-oaks is inclined to be scaly or flaky, that of the 
Black-oaks is usually rougher and deeply cracked and furrowed. 
The wood of the White-oaks is tougher, heavier, and more com¬ 
pact—the only^wood which is fit to be used by the wheelwright 
or cooper, and is for their purposes unsurpassed. The wood of 
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