8 ENGELMANN—OAKS OF THE UNITED STATES. \_Zl9 
or, from May to September, with both ; the older acorns are then 
seen on the older, leafless part of the branchlet, and the young, 
incipient ones on the younger, leafy part. In oaks with per¬ 
sistent leaves some difficulty may arise from the peculiarity 
that the branchlets which had flowered the previous year, and 
are now maturing the fruit, often in the second year do not 
elongate or make new leaves or new wood—in short, do not per¬ 
form any function but the maturation of the fruit. In this case 
the fruit is found near the end of the branchlet, absolutely as if it 
were an annual fruit; but the appearance of the leaves as well 
as of the epidermis of the branch proves them to be over a year 
old, and wherever a new shoot of the present year can be discov¬ 
ered, the difference between this and those of the last year easily 
solves any doubts. In Jg. chrysolepis this peculiarity is quite 
striking; very rarely (at least in the herbarium specimens exam¬ 
ined by me) the fruit-bearing branchlets elongate and again bear 
flowers, which is the rule in our deciduous biennial oaks. 
The cup of the acorn, an involucral organ, is in all our species 
covered with imbricated scales, appendicular organs which simu¬ 
late bud-scales, and even occasionally seem to assume a pseudo- 
phyllotactic arrangement. In the Black-oaks these scales are 
membranaceous and never thickened at base ; in the White-oaks, 
on the contrary,, they sometimes, have herbaceous tips and, at 
least the outer and lower ones, are always more or less thick¬ 
ened, inflated, or knobby at base ; they are very thick, e.g. in jg. 
alba and lob at a , and very slightly thickened in jg. stellata and 
Garryana ; in macrocarfa they are herbaceously tipped. 
The shell of the nut or acorn is thinner in the White-oaks and 
thicker in the Black-oaks; a much more important and striking 
character is, that in the former its inside is dark, smooth, and even 
shining, or rarely pubescent, and in the latter densely silky-tomen- 
tose, a difference which, I believe, is constant. 
Only one of the 6 ovules of the oak-ovary is developed, while 
the 5 others persist as small but distinctly recognizable oval, 
dark colored, pendulous bodies, outside of the seed-coat, in the 
White-oaks at the base of the perfect seed, in the Black-oaks 
just below its tip. Only in one of our species, Jg. chrysolepis , 
are they intermediate or lateral, in some acorns almost basal, and 
in others scattered over the side from near the base to two- 
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