240 



F. H. WHITEHEAD 



were then sealed off and all plants were exposed to a 33 m.p.h. wind. The 

 plants were weighed before and after the experiment and the leaf areas 

 measured. The loss per unit area per unit time decreases with the increase 

 of wind speed under which the plant developed. But the leaf area also 

 decreases in a similar way so that the net water loss per plant is greatly 

 reduced with the increase of severity of the conditions under which the 

 plant developed. This demonstrates that the anatomical and morpho- 

 logical changes are advantageous in respect of water relations. 



eo 



70- 



60- 



^ 50- 



s 



40 



30- 



_l 20- 



10- 



FlG. 4. 



WIND SPEED Cm.p.h) 

 Plant heights and leaf areas of Senecio ttebrodcnsis grown at four wind speeds. 



Two Other factors in nature are associated with exposure to wind, 

 namely precipitation and the soil moisture regime. Both these factors will 

 affect the water balance of the plant and the question of how far the effect 

 of wind operates through them requires consideration. The measurements 

 already described indicate relationship of leaf water saturation deficits to 

 the anatomical and morphological changes. If this relationship is causal 

 then similar changes should result from alteration in leaf saturation deficits 

 however produced. Water balance is, of course, the resultant between two 



