Limits of Plants in North-West Yunnan. 343 
In the middle of May, at an altitude of 16,000 feet, we found a 
south-facing slope, consisting of scree material, dry on the surface 
and clear of snow, where one had to look carefully to find scattered 
plants—a Ranunculus, one or two Cruciferae and Umbellifene, a 
Corydalis, a “cushion-plant” belonging to the Caryophyllaceae, and 
a few others (PI. 7, Fig. 1). 
Crossing the crest of the ridge and attaining the north slope we 
immediately found quite a different state of affairs. 
Here a thick carpet of dwarf shrubs, chiefly Rhododendron and 
Cassiope, with a shrubby saxifrage, grew in a comparatively rich 
loamy soil, and large patches of melting snow still remained, the 
water from which kept the entire slope moist. The transition was 
abrupt, the contrast extraordinary. A bitterly cold wind came 
slashing up the screes from the south, but immediately below the 
crest of the ridge, which was only a few feet wide, protection was 
guaranteed to the vegetation and genial warmth prevailed. 
The bare screes commenced at an altitude of about 15,500 feet, 
almost immediately succeeding the limit of trees in the lower part 
of the valley, and the limit of shrubs in the upper part; they 
extended another thousand feet higher, to the peak dominating the 
valley-head. 
Looked at from a little distance, the limit of plants on such a 
south-facing slope is practically a straight line, the belt of dwarf 
shrubs ending abruptly on the screes at an average altitude of 
16,000 feet. This l call the Conditional Limit of Plants, and it may 
be defined as the altitude at which the rate of transpiration due to 
full exposure to the south wind is equal to the rate of absorption by 
the roots from an increasingly drier soil. 
It will be noted that this limit is always marked by a belt of 
dwarf shrub vegetation, and that it is the alpine grassland belt 
which is lacking. True, careful search will, as indicated, reveal 
scattered scree-plants above the assigned limit, but the chief point 
is that the complete sequence of belts is interfered with, the dwarf 
shrub belt being immediately succeeded by desert, the scattered 
vegetation of which is highly specialised to meet the changed 
conditions. 
In October, as previously stated, we crossed the Riin-tsi-Ia, a 
pass between the Mekong and the Yangtze, not much less than 
18,000 feet above sea-level, and the ferocious wind which bit through 
everything was alone sufficient to blast all vegetation from the ridge 
at that altitude. 
