974 DR. HOOKER'S MISSION TO INDIA. 
has a winter fully as rigorous as at St. Petersburgh, though seven de- 
grees farther south; yet the heat.of summer, at this very inland place, 
is scarcely above that of Brussels, barely one degree nearer the equator 
than itself, and at about the same elevation. At Irkutsk, in lat. 52° 
16’, and the easternmost place in the series selected for illustration, the 
subsoil must be very little above the point of congelation, as the mean 
temperature of the air is somewhat below it ; and allowing 5° Fahr. for 
its en of 1342 feet, the mean heat of Irkutsk would still be only 
36° 80’, or below thatof St. Petersburgh, and this is probably too high 
an ae May not the well known poverty of the Siberian Flora, 
commencing eastward of the Uralian mountains to Dahuria and the Sea 
of Ochotsk, in hardwood or deciduous trees, (nearly all those indige- 
nous to Europe failing beyond that chain) be less owing to the mere 
rigour of winter than to the extreme low temperature of the subsoil, to 
which the roots of the larger trees must descend ; whilst those of the 
various species of Caragana, Halimodendron, and other shrubs so charac- 
teristic of the Siberian Flora, penetrate only to a depth at which the 
sun’s rays can be effectual in raising the temperature sufficiently for 
vigorous vegetation during some months of the year. The Conifere 
alone, protected by their resinous secretion, seem endowed with power 
to resist a continued low temperature, and to strike their roots into a 
perpetually frozen soil. We observe the same phenomenon in America 
as in northern Asia: there, around Hudson's Bay, in Labrador and 
north of the Saskatchewan, where the mean' annual temperature of the 
earth sinks to 32°, nearly every kind of hard-wood tree, common a few 
degrees farther south, has disappeared; and pines, with a few poplars, 
(Populus trepida and balsamifera) willows, and under-shrubs, are the 
sole representatives of arhoreous vegetation. 
Extracts from the Private Letters of Dr. J. D. HookER, written during 
a Botanical Mission to INDIA. 
(Continued from p. 233.) 
CALCUTTA TO DARJEELING IN SIKKIM-HIMALAya. 
Patna is the strong-hold of Mahomedanism, and from its central 
position, its command of the Ganges, and its proximity to Nepaul, 
(which latter has been aptly compared to a drawn dagger, pointed to 
