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acorns are heavy and disseminated mainly by water, mammals, birds, 

 and gravity on slopes. The fruit of some Oaks matures in one season, 

 while others require two seasons. At the end of the first season the 

 latter appear as immature acorns. The mature fruit of the annual 

 fruiting Oaks is attached to the growth of the season, while that of 

 the biennial fruiting Oaks is attached to last season's growth. Dur- 

 ing the winter season, immature acorns of the biennial fruiting Oaks 

 are found on the growth of the previous season. The Oaks of Penn- 

 sylvania may be classified in two groups: 



A. Acorns mature in one season; leaves with rounded lobes, not bristle-pointed; shell 



of nut usually smooth inside; kernel usually sweet; bark pale often scaly WHITE 

 OAKS, ANNUAL OAKS, LEPIDOBALANUS. 



B. Acorns mature in two seasons; leaves or their lobes bristle-pointed; shell or nut usually 



pubescent inside; bark dark usually furrowed BLACK OAKS, BIENNIAL OAKS, 

 ERYTHROBALANUS. 



The subjoined list shows the respective groups to which the sev- 

 eral Oaks of Pennsylvania belong. 



White Oak group: 



1. White Oak, Quercus alba, 



2. Swamp Oak, Quercus bicolor. 



3. Post Oak, Querous stellata. 



4. Chestnut Oak, Quercus Prinus. 



5. Yellow Oak, Querous Muhlenbergii. 



6. Bur Oak, Quercus macrocarpa. 



7. Chinquapin Oak, Quercus prinoides. 



Black Oak group: 



8. Black Oak Quercus velutina. 



9. Red Oak, Quercus rubra. 



10. Scarlet Oak, Quercus cocciuea. 



11. Pin Oak Quercus palustris. 



12. Spanish Oak, Quercus falcata. 



13. Scrub Oak Quercus ilicifolia. 



14. Black Jack Oak, Quercus marilandica. 



15. Laurel Oak Quercus imbricaria. 



16. Willow Oak, Quercus phellos. 



Of the 300 species of Oaks which are known, about 55 species are 

 native to North America, and 16 species to the State of Pennsylva- 

 nia. Of the 16 species native to Pennsylvania, 7 belong to the White 

 Oak group and 9 to the Black Oak group. The centre of distribu- 

 tion of this genus is in the mountains of Central America and Mexico. 

 A few species are found in Europe. The subjoined keys will identify 

 the species native to Pennsylvania. 



