392 ABOUT THE OAKS OF THE UNITED STATES. 
which retain the greater part of their old leaves at Jeast 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 Q. 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 Q. 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 Q. 
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 distinguish 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 (Q. agrifolia and Wislizent) 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 sessile, or [378 (7)] 
rarely (and that sometimes in the same species) borne on short, more or less erect, styles ; 
in the black-oaks we always find them on longer, patulous, or recurved styles! As the stigmas are 
measurably persistent, we often recognize this difference even in the mature fruit. 
The Fruit exhibits the most important characters in the period of its maturation, first noticed 
by Michaux, and especially in the position of the abortive ovules, the beautiful discovery of A. De 
Candolle. But before I speak of these I must allude to the position of the fruit on the branch. It 
is single or clustered in the axils of the leaves or their scars, sessile, or more or less peduncled. In 
the black-oaks the peduncle is short or missing, but in the white-oaks it is sometimes several inches 
in length ; its presence, however, is of very little specific value, as in many species either sessile or 
peduncled acorns are found. In some oaks this feature is connected with slight differences in the 
length of the petiole or the shape of the leaf; the distinction between the European Q. Robur and 
Q. pedunculata is based on such differences, and we have an analogous difference in our Q. alba, 
where, at least here in the Mississippi Valley, the form with deeply pinnatifid leaves has usually 
peduncles as long or little shorter than the acorn, and the other form with more broadly-lobed leaves 
has shorter peduncles or sessile fruit ; but sometimes we find sessile and peduncled fruit on the same 
tree. Some white-oaks have always sessile or nearly sessile acorns, as Q. stellata, while Q. bicolor 
always bears them on long peduncles. 
The acorns mature either in one season or in two, and generally speaking we find the annual 
maturation among the white-oaks and the biennial maturation in the black-oaks, but the exceptions 
to this rule prove that this peculiarity is not necessarily connected with the essential characters of 
the two groups. We have one western white-oak, Q. chrysolepis, with biennial fruit, and three black- 
oaks with annual maturation, Q. pumila of the east, and Q. agrifolia and Q. hypoleuca ~ the west. 
The biennial maturation is easily recognized in the oaks with deciduous leaves ; the t 
is never without younger or older fruit, or, from May to September, with both ; the ae [379 (8)] 
acorns are then seen on the older, leafless part of the branchlet, and the young apes ones 
on the younger, leafy part. In oaks with persistent leaves some difficulty may arise from the pecu- 
1 A group of white-oaks with biennial fructification, peculiar to southern Europe and eastern Asia, the best known 
representatives of which are Q. Cerris, Q. Pseudo-Suber, and Q. occidentalis, differs from all these by their patulous or recurved 
styles bearing ligulate, acutish stigmas. 
