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 ; J^. pumila and laurifolia 

 on the eastern and ^. 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 

 J^. 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 ^. 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 J^. nigra I have seen 

 abortive pistils with prominent filiform styles — singularly enough 

 always 2, where we might have expected 3. In flowers of j^. 

 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 (.^. 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 



