6 ENGELMANN-OAKS OF THE UNITED STATES. [377 
The persistence of the leaves is a good character in some spe¬ 
cies, while in others it is unreliable; Jg. fiumila and laurifolia 
on the eastern and jg. agrifolia on the western coast sometimes 
retain their leaves until the new ones are fully developed, and 
other specimens, even in the same neighborhood, lose them be¬ 
fore the buds swell; some have deciduous leaves northward and 
partly persistent ones southward. The broad-leaved forms of 
undulata are decidedly deciduous, while those with small, 
coriaceous, spiny.toothed leaves retain them through part of the 
winter, or, towards their southwestern limit, even into summer. 
Only such oaks ought to be called evergreen which retain the 
greater part of their old leaves at least until the new ones are fully 
grown ; the leaves of some oaks persist even into the third year. 
The male flowers are important for the diagnosis of some 
species, and to some extent even for the grouping of them. I 
pass by the form and pubescence of the bracts and of the calyx 
lobes as well as the pubescence of the anthers (among all our oaks 
only observed in J^. stellata and virens) ; even the sometimes 
present cusp or point of the anthers seems to be of lesser value, 
because variable in some species. Of greater importance is the 
size and the number of the anthers. The smaller and more 
numerous (usually from 5 to 8 or even 10, rarely only 4) occur 
in the White-oaks, while in the Black-oaks the anthers are 
usually larger and fewer, as a rule only 4, in some species as 
many as 5 or 6 ; only in J^>. agrifolia , which also shows other 
abnormal characters, 6-8 stamens are the rule, and sometimes 10 
are found. The pollen-grains of both groups have a diameter of 
about 0.03-0.04 mm. 
In numerous flowers of a certain tree of nigra I have seen 
abortive pistils with prominent filiform styles—singularly enough 
always 2, where we might have expected 3. In flowers of 
agrifolia the connective of the anthers was seen to elongate, the 
cells to dwindle down and finally to disappear. 
The female flowers furnish valuable characters to distin¬ 
guish the principal groups of our oaks. The pistil consists 
normally of 3 carpels and 3 stigmas ; not rarely 4 occur, and in 
some Californian species (jg. agrifolia and Wislizeni) I have 
repeatedly seen as many as 5. The stigmas in our species are 
dilated, retuse, or emarginate; in the White-oak group they are 
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