Limits of Plants in North-West Yunnan. 345 
the limit, and this I call the Absolute Limit of Plants. It is 
important, I think, to distinguish between these two limits, as the 
following consideration will shew. 
It is as easy for a plant to obtain protection from the wind at 
19,000 feet as it is at 16,000 feet, so that evidently a new factor 
comes into play and determines the absolute limit; this I take to 
be the decreasing temperature of the soil, which, as the snow-line 
is approached, entirely checks absorption by the roots. The Absolute 
Limit of Plants then, may be defined as the altitude at which the 
rate of absorption by the roots from an increasingly colder but 
nevertheless moist soil, is balanced by the rate of normal tran¬ 
spiration at this altitude. 
On the Mekong-Salween divide the greater rainfall and conse¬ 
quent lowering of the snow-line makes the conditional limit of plants 
coincident with the absolute limit, since now the controlling factor 
of the one, namely, an abnormal rate of transpiration due to wind 
action, becomes effective at about the same altitude as the controlling 
factor of the other, namely, an abnormal decrease in the rate of 
absorption due to the coldness of the soil. Hence the distinction 
cannot here be maintained ; in fact, the distinction itself is 
characteristic of highly abnormal conditions. 
That the mere fall of temperature alone is not responsible for 
the final cessation of plant-life, is suggested by the results obtained 
when taking the temperatures of certain “ cushion-plants.” 
Plunging the bulb of the thermometer into different species of 
“cushion-plants” 1 obtained the following readings:—59°, 64-4° 
(twice), 59-9"(twice), 52-8"F., the corresponding soil temperatures 
being 53-6°, 60-35°, 52-7", 54-5" (twice), 49T°. The “cushion-plant” 
thus maintains an average temperature 6-98"F. in excess of the 
average soil temperature; so that simply by assuming the “cushion” 
habit a plant is able both to reduce transpiration to a minimum 
and to keep up its own temperature. 
One further point calls for consideration, but can only be 
touched on here, since it opens up a wide field for investigation. 
There is good reason for believing that the Yangtze-Mekong 
divide was, in times geologically recent, seamed with glaciers 
reaching down to about 14,000 feet; and, granting their former 
existence, their disappearance is more likely due to an amelioration 
of climate or a change in the seasonal distribution of rainfall than 
to a general depression of the ridge, since we should have to 
postulate an average lowering of several thousand feet. 
