376] ENGELMANN—OAKS OF THE UNITED STATES. $ 
lata of the first, and nigra of the second group, both with 
densely tomentose, thick, young leaves. In other oaks, mostly 
such as have broader and more or less entire ieaves N , the young 
leaves imbricatively cover one another like those last mentioned, 
but are convex on the upper side, with the edges turned down or 
back. Such is the case in cinerea, myrtifolia, agrifolia, 
aquatica, chrysolepis , and, I believe, also in Jg. undulata, and 
in jg. Wislizeni; I find the same to be the case in the deeply 
lobed falcata. 
The narrow-leaved oaks of both sections have revolute young 
leaves, the halves being spirally rolled backwards towards the 
midrib, so that only the upper side of the leaf is exposed; the 
point of the young leaf is somewhat spreading so that the branch- 
let has a squarrose appearance, while in those with imbricative 
vernation it is compact. I find the revolute leaf in J|>. virens, 
pumila , Phellos , heterophylla , and imbricaria. In J|>. Catesbcei 
I observe an indexed vernation, the long bristle-pointed lobes of 
the nascent leaf being curved down over the still younger one. 
I believe that the characters of vernation will not only help to 
distinguish allied species or doubtful varieties, but will also assist 
in unravelling the intricate questions of hybridity. 
The young leaves of almost every oak are coated with a dense 
stellate down, which in some (J^. alba, rubra, etc.) is early de¬ 
ciduous, or it disappears later, or is entirely persistent. Besides 
these stellate one-celled hairs, several species, those with a clam¬ 
my feeling of the young leaf, have another kind of hair, single or 
a few stellately connected, consisting of several cells, obtuse or 
clavate, sometimes branched, and often colored, apparently glan¬ 
dular. I notice these articulate hairs, among the White-oaks, in 
j|>. stellata , and less conspicuously in jg. macrocarpa; among 
the Black-oaks, in jg. nigra, myrtifolia, cinerea, falcata, 
aquatica, and laurifolia; in jg. chrysolepis the characteristic 
“ golden scales” are no scales, but consist entirely of such articu¬ 
lated yellow hair, and the young jg. Catesbczi has the same rusty 
coating. 
The venation and more or less distinct reticulation of the 
leaves also present characters not to be neglected; by them, e.g. 
two easily confounded Californian oaks, agrifoliq, and Wisli¬ 
zeni can readily be distinguished even in sterile branchlets. 
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