238 



F. H. WHITEHEAD 



A similar increase in xeromorphy was observed. Examination of the 

 relative proportion of root to shoot dry weights showed that over most of 

 the range of the experiments there was a greater proportion of root to 

 shoot with increase of wind speed despite the smaller total dry weight 

 (Fig. 2). At wind speeds approaching 40 m.p.h. the plants failed to survive 



250- 



200- 



C7> 



E 

 — ' I50H 



01 



IOC- 



SO' 



• TOTAL DRY WEIGHT 



© SHOOT DRY WEIGHT 



O root/ SHOOT RATIO 



X ROOT DRY WEIGHT 



I I 



10 20 



Wind speed (m.p.h.) 



— T— 



30 



o 



4 2 



> 



3 8 



■ S 



Fig. 2. Graph showing alterations of dry weight accumulation over 30 days in 

 Hdianthus amiuus grown at four different wind speeds. 



the full time of the experiment and these plants did not show an increase 

 in root proportionally to shoot, indicating they were unable to produce 

 phenotypic change with potential advantage beyond a certain critical level 

 of conditions. Fig. 3 shows average intemode length and average leaf 

 areas o£ Helianthus annuiis grown at four wind speeds. These two characters 

 show a marked decrease in value with increase of wind speed. Fig. 4 shows 

 data of the plant height and leaf area for the mountain species Senecio 

 nehrodensis grown for the same length of time and at a range of wind 

 speeds. It can be seen that the 'critical level' beyond which compensation 

 does not occur is much higher together with the fact that the degree of 

 response to any one wind treatment is greater. It would appear that 

 the degree of response and tolerance in mountain species is greater than 



