on the Geography of Plants. QQ^ 



has likewise mentioned these two facts, p. 1. Perhaps a third 

 may yet be adducecj, viz., that the pressure of the atmosphere, 

 at the same distance from the snow-line is different in different 

 latitudes. The author indeed admits this, as well in this treatise, 

 p. liv., as in his Essay, and Enoelhardt and Parrot hold 

 the same opinion *. On the contrary, Decandolle, Spren- 

 GEL, and Wahlenbero f, assert that the pressure of the at- 

 mosphere has not any influence upon vegetation. At any rate 

 the difference of altitude must have this effect, viz., that the 

 degree of humidity will vary according to the same. It is 

 stated in M. Humboldt's Essay, that the mean of Saussure's 

 hygrometer, 300 toises above the sea (250 toises lower than the 

 snow-line in Lapland,) is about 83°., but, at the height of 

 2,210 toises (250 toises lower than the snow-line in the equato- 

 rial mountains), it reaches only to about 54°. 



Another phsenomenon, closely connected with this, and 

 which is to be regarded as a continuation of the same sub- 

 ject, is that the difference in the distribution of plants, 

 which the difference of altitude occasions, does not stand 

 in the same relation with that produced by difference of 

 latitude %. If we thus first compare the distance of the limit 

 of trees from the surface of the sea at the equator, with the 

 distance of the snow-line from the same; and secondly, 

 the distance of the limit of trees above the table-land at the 

 equator, with the distance of those degrees of latitude where 

 the snow-line is contiguous to the sea, which is the case in the 

 northern hemisphere, particularly at 80° of latitude ; we shall 

 find that the limit of trees, in respect to latitude, is proportion- 

 ally much more extended towards the pole, than upon the equa- 

 torial mountains towards the snow-line. The proportion is 

 accordingly for the comparison of altitude, 2,460 toises: 1800 



• Reise in die Krimm und den Kaukasus. 



t FUrte Fran^aise. Tom. II., p. xi. Ban der Gewachse, p. 622. Flora 

 Carpathorum, p. cxvi. 



4 We might, perhaps, eall the former the vertical, the latter the iiori- 

 siONTAL distribution. 



VOL.X. T 



