Limits of Plants in North-West Yunnan. 337 
Throughout the vegetative season a strong wind blows through 
these funnels from the south, and, sweeping up the side ravines 
which gash the dividing ridges, spreads out, becoming less regular 
and less violent, though still maintaining its character, that of a 
warm dry wind carrying little moisture; so that on the high 
plateaux of the Yangtze-Mekong divide to which the following 
remarks principally refer, I have noticed this desiccating south wind 
blowing chiefly during the mornings. 
The effect of such a prevailing wind is very clearly marked in 
the case of valleys and slopes facing south, and therefore freely 
exposed to it, though this effect is considerably more marked in the 
case of the latter, for reasons which will appear later (PI. 7, Fig. 2). 
Before comparing ridge with ridge and valley with valley, 
however, it will be convenient to consider the complete sequence of 
formations on the Yangtze-Mekong divide, where five well-marked 
belts may be distinguished, as follows:— 
(1.) The forest-belt with Abies, Quercus, Rhododendron , and 
deciduous-leaved trees such as Larix , Betula, Acer, Pyrus, with 
other familiar north temperate genera: (2) the shrub-belt with 
Rhododendron, Rosa, Potentilla, Cotoneaster, Berberis and Salix 
conspicuous: (3) the belt of dwarf Rhododendrons ; and finally the 
alpine grassland belt (4) marked by a wealth of Primulas, Saxifrages, 
Gentians and many other plants, gradually giving place to (5) an 
open scree-formation, of which more anon (see Section, p. 338). 
The limiting altitudes of these belts cannot be set down with 
any degree of accuracy, and indeed the object of this paper is to 
point out how, and, if possible, why they vary within wide limits; 
so much so that in one locality the tree-limit stands at about 14,000 
feet and is succeeded by a well-developed shrub-formation, while in 
another locality it stands at over 15,000 feet and dwindles directly 
into alpine grassland, thus skipping two distinct formations which 
elsewhere are conspicuous enough. 
The plateau valleys of the Yangtze-Mekong divide are of course 
exposed to the desiccating winds from both main valleys, but on their 
upper limits towards the water-parting it will often be found that 
one up-valley wind prevails, and thus becomes a down-valley wind on 
the other side. 
Such a down-valley wind from the cold plateau is always a rain¬ 
bearing wind, and consequently these valleys present more or 
less the same characteristics as valleys facing directly north; we 
may instance the valley, facing north-west, between A-tun-tsi, above 
