3S6 TRANS. ST. LOUIS ACAD. SCIENCE. 



In a geographical point of view, we notice that the Black-oaks 

 of the present day are confined to America, and are principally 

 developed in the Atlantic part of North America. We have there 

 15 species, including a single abnormal type, while west of the 

 Great Plains and on the Pacific slope only 5 species occur, 3 of 

 them receding from the normal type. Numerous Black-oaks are 

 found in Mexico and Central America ; in DeCandolle's Prodro- 

 mus 20 are enumerated with known maturation, and of iS, of 

 which the fructification is not ascertained, some may also belong 

 here. In the tertiary period the Black-oaks, it seems, extended 

 into the old world, just as many other recent North American 

 types did. 



The White-oaks are more uniformly distributed over the tem- 

 perate parts of the northern hemisphere. We have on the Atlan- 

 tic slope S species and 9 in the western half of the continent, 

 only 2 of the latter abnormal. 



Thus we have in our Flora nearly as many White-oaks as 

 Black-oaks ; but while the former are nearly equally distributed 

 between east and west, the latter predominate eastward. 



Pa g- 375- Vernation — A too hasty examination, partly of 

 specimens too far advanced, has led me into several errors. The 

 vernation is conduplicate only in part of the White-oaks (in the 

 European type of the genus, Q. Robur ; in our Q. alba, macro- 

 carpa, Garryana, and in all the species of the Prinus group ; pro- 

 bably in lyrata, Douglasii 'and lobata). The imbricate vernation 

 is the rule for the balance of the White-oaks (Q. stellata, undu- 

 lata, dumosa, and chrysolepis) and for all the Black-oaks, with 

 the exception of the. few species (p. 376) with rcvolute vernation. 

 In the oaks with imbricate vernation the outer leaves are always 

 imbricate, but the inner ones are imbricate or fiat or even 

 slightly revolute on the margins in the species with thicker, 

 firmer leaves {undulaia, dumosa, chrysolepis ; Emoryi, agrifo- 

 lia, nigra, aquatica, laurifolia, cincrca, myrtifolid). In the 

 others, with broader and more lobed leaves (coccinca, rubra 

 palustris, falcata. etc.) the 2 or 3 innermost leaves are con- 

 duplicate. The indexed vernation, mentioned as occurring in j£. 

 Catcsbcei, is also occasionally seen in falcata, evidently in oaks 

 with slender-lobed leaves, but it is by no means constant and of 

 no specific value. 



