A NATURAL GROUP OF UNUSUAL BLACK OAKS. 



(Plates II to IV.) 



By WILLIAM TRELEASE. 

 {Read Jpril 21, 1921.) 



Though most oaks bear only one or two acorns in a leaf axil, it 

 is recognized generally that even when essentially sessile these fruits 

 and the flowers that produce them really pertain to a reduced pis- 

 tillate catkin which is comparable with the staminate catkin. Some- 

 times the fertile flowers, though close together, are raised on a rather 

 long peduncle, a condition well shown by our swamp white oak 

 {Oitcrcits hicolor) and the English oak (Q. pcdnnculata, or Q. 

 Rohitr pcdnnculata). Several groups of Mexican and Central Amer- 

 ican white oaks have similarly long-peduncled acorns, e.g., those 

 centering about Q. macrophylla, Q. peditncularis and Q. reticulata. 

 In some of these, as in an exceptional ally of our own southern live 

 oak 0. z'irginiana, occasional acorns occur along the peduncle; and 

 in Q. dccipicns of the eastern Sierra Madre these are frequent 

 enough and the rachis is long enough to make the inflorescence in 

 fact a sort of loose spike or catkin. 



All of these belong to the section of white oaks, Leucobalanus, 

 which are characterized technically by their short broad stigmas, 

 basal abortive ovules, and the glabrate interior of the acorn shell. 



The purpose of this paper is to make known three black oaks of 

 the southern Mexican mountains which are quite unique in their 

 section of the genus in bearing their fruit in racemes — or more 

 properly spike-like clusters. They possess the technical characters 

 of the black or red oak section, Erythrobalanus: elongated spatulate 

 stigmas, subapical abortive ovules, and acorn shells tomentose 

 within ; but they dififer from most black oaks and agree with all 

 white oaks in maturing their fruit in the course of the season of 



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PROC. AMER. PHIL. SOC, VOL. LX, C, JULY 2$, I92I. 



