380] ENGELMANN—OAKS OF THE UNITED STATES. 
thirds up. DeCandolle has observed the same in the Cork-oak 
of Europe and in some Mexican White-oaks. The Black-oaks 
with annual fructification have these ovules always suspended 
near the tip of the seed, and are in this respect undistinguishable 
from the regularly biennial Black-oaks. 
It is well known that in the southeastern Live-oak both cotyle¬ 
dons are united into one mass—a singular but isolated fact which 
has no systematic significance. 
In the foregoing pages I have purposely left aside the very 
peculiar Californian densijiora , which is in every respect dif- 
erent from the other oaks, and thus far the sole representative of 
a peculiar group named by DeCandolle Androgyne . In many 
respects it is more a chestnut than an oak, for it has, just like the 
chestnuts, the same dense-flowered, erect male spikes, 10 stamens 
to each flower, very small anthers on long filiform filaments, with 
very small pollen-grains (0.017 mm. in diam., not much more 
than half as large as in other oaks), and in the female flowers 
slender, terete, pointed stigmas, grooved above. In place of the 
spiny involucre of the chestnut our plant has a spiny cup, and 
is thus made an oak and not a chestnut. The maturation is 
biennial. The shell of the nut is thicker and harder than in any 
other of our oaks, the inside thickly tomentose, and the abortive 
ovules are found near the top of the seed. The wood is brittle 
and worthless. 
It results from these investigations that our oaks, leaving again 
aside the one last mentioned, arrange; themselves into two great 
groups, often alluded to above as the White-oaks and the Black- 
oaks. 
The White-oaks are characterized by paler, often scaly, bark, 
tougher and denser wood, and sessile or subsessile stigmas, and 
bear the abortive ovules at the base or rarely on the side of the 
perfect seed. Besides this, the leaves and their lobes or teeth are 
obtuse, never bristle-pointed, though sometimes spinous-tipped; 
their stamens are more numerous, the scales of the cup more or 
less knobby at base, the inner surface of the nut glabrous or 
(rarely) pubescent; the fruit generally matures in the first year. 
The Black-oaks have dark, furrowed bark, brittle and porous 
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