ENGELMAXX — OAKS OF THE UNITED STATES. 385 



Over a year has past since the foregoing part of this paper was 

 published ; of the concluding seven pages only a small edition for 

 private distribution was then printed. Continued study of the 

 genus, aided by numerous kind communications of observations 

 as well as of specimens, have enabled me to make the following 

 corrections and additions. 



Pag. 374, 1. 22. Low shrubby forms of ^. stellata occur on 

 the southeastern seacoast, and of ^. macrocarpa on the north- 

 western plains. 



L. 2 from below ought to read : the only oak wood. — The wood 

 of ^. Prinus^ however, makes an exception, being more porous 

 than most other White-oak woods. 



A careful study of the numerous American oak woods dis- 

 played by the Agricultural Department and by different vStates, at 

 the Centennial Exhibition in Philadelphia, has revealed further 

 interesting facts. The Black-oaks grow, on an average, nearly 

 twice as fast as the White-oaks, and, if ^. nigra and the ever- 

 green Black-oaks be left aside, the disproportion will be found 

 still greater. In the average of 20 different White-oaks, from all 

 parts of the country, I find the growth nearly equal through the 

 first 40 vears ; in 14 specimens of Black-oaks the growth is more 

 rapid in the first 30 years than between the 30th and 40th. The 

 following little table will exhibit this more distinctly. 



AVERAGE WIDTH OF THE ANNUAL RINGS 



in 20 White-oaks : iti 14 Black-oaks : 



In the first 20 years 0.8 lines 1.6 lines. 



From the 2cth to the 30th year cS " 1.7 " 



" 30th " 40th " 0.7 " 1.2 " 



The heartwood of the White-oaks is always readily distinguish- 

 able from the sapwood by its darker color, varying between dark 

 gray and light brown, but in the Black-oaks the heartwood is 

 scarcely darker than the sap, and in some species or some indi- 

 viduals cannot be distinguished at all. Only in ^. nigra and the 

 curious ^. Emoryi is it often irregularly mottled with black. 



In the limited number of specimens which I could examine, 

 the sap turned into heartwood, 



in 19 White-oaks, in 8 Black-oaks, 



after 22 years, 17 years, 



having attained a thickness of. . 18 lines, 21 lines. 



Only in these 8 Black-oaks a line of demarcation was visible, 

 iii — 25 [Oct. IS, 1S77.] 



