38 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



The truth is probably that the effects of the wind are largely due to 

 the consequent increase in transpiration leading to desiccation, as was 

 suggested by Wiesner in 1887, by Kihlman in 1890, and by Warming 

 in his lectures in 1889.^ Wind has a desiccating action which increases 

 with its force. It dries the soil : places very exposed to the wind acquire 

 a relatively xerophilous vegetation. It dries plants, so that these must 

 adapt themselves to their conditions in order to avoid desiccation. In 

 a calm atmosphere the air adjacent to plants becomes humid, so that 

 transpiration is obstructed. By even weak movements of the atmosphere 

 air is constantly carried away, and fresh, less-humid portions of it come 

 into contact with the plant. Even when the atmosphere is very humid 

 its uninterrupted renewal will lead to strong transpiration. The drier 

 the air and the stronger the wind, the greater will be the drying action. 

 Evaporation is proportional to the square root of the wind-velocity. 

 The shaking of the plant-organs also operates in the same direction. 

 By this transpiration the growth in length of axes and leaves is decreased 

 (nanism), many leaves and whole shoots are killed, so that irregular 

 branching results : thus all the observed phenomena receive an explana- 

 tion that is not forced. The deviation of the shoots in the direction 

 of the prevalent wind is possibly almost without exception caused by 

 a kind of sympodial growth. That the crown on the lee-side acquires 

 a gradually ascending shape, is caused by the circumstances that the 

 shoots, both living and dead, on the windward side screen the parts 

 on the lee-side from too rapid a renewal of the air. In this matter 

 once more we see the fundamental significance of water to plant- 

 life. 



The force of the wind is far less on the ground than at some distance 

 above it ; consequently short plants are much better protected from 

 the wind than are taller ones.^ 



The danger arising from the wind is increased when at the same 

 time the activity of the root is decreased by coldness of the soil, so that 

 the loss of water is covered with difficulty or not at all ; hence in Central 

 Europe when there is too little snow in winter cereals and other plants 

 perish. This circumstance is of particular importance in arctic and alpine 

 situations. The espalier-shape, mentioned in p. 26, as assumed by shrubs 

 growing in these places, may be caused by wind, and we often see the shoots 

 directed straight away from the windward side. 



The cushion-like growth of the herbs (Draba,^ Androsace helvetica,* 

 and others) living under similar unfavourable conditions in windy cold 

 places, may obviously arise in the same way. Even arctic mosses show 

 similar construction. ^ Herbs of this kind acquire, for want of water, 

 short shoots and small leaves, become as a whole very stunted pygmy- 

 forms ; they are richly branched, consequently often of extraordinarily 

 dense growth, and are very like miniature shrubs found in scrub. Often 

 cushion-plants, for instance Silene acaulis, are dried up on the windward 

 side. That dryness can really bring into existence such cushion-like forms 

 is confirmed by plants growing in arid, hot, but tolerably calm, desert 

 places. 



' See Warming, 1884, p. 99. ' Wiesner, 1887. 



^ See illustration in Kjellman, 1884, p. 474. 



* Kihlman, 1890 ; G. Andersson, 1902. "^ Ottli, 1903. 



