57 
ment’s thought will show even the novice that plants thrive on the 
_pafticular spots on which they grow according to the climate at 
that particular spot, and therefore the Larrea must grow high u 
on a hot slope and will not grow along a stream where the climate 
is more humid and where cool breezes from above follow down 
the canon. So these life zone tongues will be found to dovetail 
into one another wherever there are mountains and canons or large 
streams. But when their origin is once understood, they need. 
not cause confusion. 
t almost never occurs that any zonal plant wil! stop off sud- 
denly at the upper and lower limit of its own zone. And we must 
The juniper makes an excellent zonal plant for the majority 
of the Great Plateau, since it begins suddenly near the limit of 
the upper limit of the Larrea. Whenever either of these two 
plants is seen growing, the traveller may know that he is enter- 
ing the Lower Temperate life zone. In southern Arizona, how- 
ever, the juniper does not occur sufficiently to be ot value, but its 
place is taken by the live oaks, which are as characteristic of 
the Lower Temperate life zone there as the juniper is in the Gr t 
Basi The upper limit of the Lower Temperate life zone is im- 
mediately recognized by the disappearance of the live oaks in Ari- 
zona and by the disappearance’ of the juniper and ihe appearance 
of the deciduous oak from Colorado to California. So that wher- 
ever the deciduous oak is absent, the limit of the juuiper 1s a g00 
index of the limit of the life zone. The two junipers which must 
California juniper, and they must nc 
cedar, which is a juniper, or the thick- 
upper limits of the Lower Temperate t : 
ular line skirting around the base of the Rocky Mountains from 
Cheyenne, Wyoming, to New Mexico, 
we co the Snake River and eastern Oregon, 
vestward to t forming islan ds on the Lower Tem- 
