324 THE OAK-VEGETATION OF AMERICA, 
some species, whose leaves become blood-red towards the fall, thereby 
imparting to the forests in autumn, a magnificent appearance; among 
these are Q. coccinea and rubra. 1 now go onto sketch out the Oak- 
vegetation of America, proceeding from north to south. 
The belt within which the Oak makes its appearance in America, 
may be placed at between 50° and 2° N.; consequently it does not ap- 
proach, much less pass the equator, but disappears with three species 
in the Cordilleras of New Granada. The northern limit is at 50° N. 
on the western side of Nootka Sound (according to Luis Neé). In 
the interior of British North America, Oaks occur, as far as the 
southern shore of Lake Winnipeg, where the mean summer heat is 
+ 19° C., and the mean winter cold —10°C. On the eastern side they 
hardly ascend to the north of Quebec in 47° north. It appears, hence, 
that they follow nearly that line (isotherm) whose mean heat is + 5° C. 
It is interesting to notice that, while the Oak in Europe, on the western 
side of Norway, extends up to Molde and Christianssund (65° N.), its 
north boundary descends considerably southwards in the interior of the 
continent, and closely following the isotherm quoted, + 5° C., which 
touches Christiania, Stockholm (60°), and Moscow, continues through 
Siberia and Mongolia, and ends on the coast of North China, where 
it sinks down to 50° N. 
Tn Canada the most northern kind is Q. alba, a large tree, sixty feet 
high, producing very good timber. Q. tinctoria exceeds this in size, 
attaining sixty to eighty feet in height, and six to ten feet in diameter ; 
it spreads considerably over North America, as far down as Georgia, 
furnishing the best timber for building purposes in the Northern States. 
Q. rubra extends to the Alleghanies, and reaches a height of as far as 
ninety or a hundred feet. Q. obfusiloba is spread along the east side of 
North America quite down to Florida; it is a tree of the size of Q. alba. 
There is finally a shrubby sort (Q. Banisteri) which reaches down to 
Virginia. 
` The area of the United States is of such immense magnitude, and 
. the climate is so much diversified, that it is expedient to separate 
. the Oak-forms of the Eastern and Western States, of the Northern 
States with temperate climate, and the Southern with subtropical cli- 
: . mate. We may observe, generally, that the species of Oaks in the 
.. United States increase steadily towards the south, and, stretching into 
.. the interior from the eastern side, cross the Apalachians and Alleghanies, 
