M. Humboldt cm Isothermal Lines ^ 
In all places whose mean temperature is below 62°. 6, the re- 
vival of nature takes place in spring, in that month whose mean 
temperature reaches 42°. 8 or 46°. 4. When a month rises to, 
41^.9, the VeeLoh-ixee ( Amygdalus Persica) flowers. 
46®. 8, the Plum-tree (Prunus domestica) flowers. 
51®. 8, the Birch-tree * (Betula alba) pushes out its leaves. 
At Rome, it is the month of March, at Paris the beginning 
of May, and at Upsal the beginning of June, that reaches the 
mean temperature of 51®. 8. Near the Hospice of St Gothard, 
the birch , cannot vegetate, as the warmest . month of the year 
there scarcely reaches 46®. 5. Barley, in order to be cultivated 
advantageously, requires -f-, during ninety days, a mean tempe- 
rature of from 47®. 3 to 48®. 2. By adding the mean tempera- 
tures of the months above 51®. 8, that is, the temperatures of 
those in which trees vegetate that lose their foliage, we shall 
have a sufficiently exact mean of the strength and continuance of 
vegetation. As we advance towards the north, vegetable life is 
confined to a shorter interval. In the south of France, there 
are 270 days of the year in which the mean temperature exceeds 
51®.8 ; that is to say, the temperature which the birch requires 
to put forth its first leaves. At St Petersburgh, the number of 
these days is only 120. These two cycles of vegetation^ so un- 
equal, have a mean temperature which does not difler more than 
5®.4 ; and even this want of heat* is compensated by the effects 
of the direct light, which acts on the parenchyma of plants in 
proportion to the length of the days. If we compare, in the 
following Table^ Eastern Asia, Europe, and America, we shall 
discover, by the increase of heat during the cycle of vegetation, 
the points where the isothermal lines have their concave sum- 
mits. The exact knowledge of these cycles, will throw more 
light on the problem of Agricultural Geography, than the exa- 
mination of the single temperatures of summer, 
• Cotte, Meteorologie, p. 448. — Wahlenberg, Flor. Lap. Pi. 31. 
*}• Playfair, Ediii. Trans, vol. v, p. 202.— Wahlenberg in Gilbert’s Annalen^ 
torn. xli. p, 282. 
