378] ENGELMANN— OAKS OF THE UNITED STATES. 
7 
sessile, or 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 stig¬ 
mas 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 j£>. Robur and Jg. pedunculata is based on such dif¬ 
ferences, and we have an analogous difference in our Jg. 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 «^. 
stellaia , while Jg. bicolor always bears them on long peduncles. 
The acorns mature either in one season or in two, and gener¬ 
ally speaking we find the annual maturation among the White- 
oaks and the biennial maturation in the Black-oaks, but the ex¬ 
ceptions 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, Jg. chrysolepis , with biennial fruit, 
and three Black-oaks with annual maturation, pumila of the 
-east, and agrifolia and J^. hypoleuca of the west. 
The biennial maturation is easily recognized in the oaks with 
deciduous leaves ; the tree is never without younger or older fruit, 
Eastern Asia, the best known representatives of which are jfh Cerris , Pseudo-Suber, and 
^>. occidentalism differs from all these by their patulous or recurved styles bearing Jigulate, 
racutish stigmas. 
Botanic 
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