346 Tranmctions. 



The wind is, indeed, a factor of paramount importance. For several days 

 at a time the north-west gale will rage, accompanied by heavy rain in the 

 west, hut in the east quite dry. Apart from its powerful effect on transpira- 

 tion, the wind is strongly antagonistic to tall growths, especially where 

 its presence is most felt, as in narrow river-valleys, and the low growth 

 of the majority of the steppe-plants, as also the divaricating-shrub form, 

 are " adaptations " against both the mechanical and transpiration -promot- 

 ing action of the wind, whatever their origin in the first instance. The 

 south-west wind is of quite minor importance in the Rakaia Valley, but on 

 the heights it may at first bring more or less snow at any season. In the 

 east its effect is more marked. It is a cold wind, and is frequently 

 accompanied at first by rain on the lower and snow on the higher 

 ground. 



The winter snowfall is most felt at above 1,200 m. altitude, and plant- 

 distribution according to altitude, as Cockayne pointed out some years ago 

 (1900, p. 128), depends, in part at any rate, upon the average length of time 

 the snow lies upon the ground. In gullies and hollows where the snow 

 accumulates the higher alpine plants may occur at below their usual altitude. 

 Besides acting as a warm covering to the plants and checking transpiration, 

 snow acts mechanically, especially on the evergreen subalpine shrubs, press- 

 ing their branches to the ground. The fell-field herbs and subshrubs are also 

 in places submitted to a great pressure, and look, after the snow has melted, 

 as if a heavy roller had passed over them ; but this flattening has no effect 

 on their aftergrowth, the shoots rapidly assuming their usual position when 

 the weight is removed. Very effective is the work of snow in keeping the 

 ground cold during the vegetative season, and thus more than in any 

 other way does it regulate the belts of vegetation and local distribution. 

 Further, its gradual melting leads at the higher levels to a constant supply 

 of soil-water for a considerable period, and this is especially noticeable in 

 hollows which may have a distinctive vegetation dependent partly thereon 

 and partly upon the longer time the snow lies. 



Frost occurs at all seasons of the year, and in positions where there is no 

 snow-covering young shoots are exposed to its effect. One and the same 

 species may grow, however, in spots protected or unprotected by snow, and 

 in each be equally undamaged. Certain species of Cehnisia in winter may be 

 coated with ice, and this will function in checking transpiration. Probably 

 even at the highest altitude the thermometer never sinks below 18° C.,* 

 and we should not be surprised if the average minimum is higher. 



With regard to xeromorphy, too much stress cannot be laid on the effect 

 of quite short rainless periods in the wettest mountain climate. Plants 

 accustomed to a wet soil, a moist atmosphere, and a frequent downpour are 

 exposed all on a sudden to quite opposite conditions, the porous soil also 

 rapidly losing its available moisture. In such a case xerophytic structure 

 or form sufficient to ward off the danger of excessive loss of water even for 

 a few days is imperative. 



* In 1908-9, when on one occasion the thermometer at Kew sank to 10° Fahr., the 

 previous weather having been mild, Gaya Lyallii was destroyed to the ground-level, 

 Sophora tetraptera was killed, Gri.telinia liftoralis, though escaping the winter frost, was 

 much damaged by the Minds of March, and ten species of Veronica were killed or severely 

 injured (Kew Bull., 237, 1909). Many other examples could be cited, but this sufficently 

 testifies to the compai'atively mild chmate of the New Zealand lower subalpine belt at 

 •any rate. 



