244 



F. H. WHITEHEAD 



TIME TO "death" AT 40mph WIND 



< 



o 



LLl 



% Treated Gibbcrclllc Acid 

 O Untreated 



Tl ME 



IN 



HOURS 



Fig. 8. Time to 'death' of plants grown at four levels of soil moisture. One set treated 

 with giberellic acid and one set untreated. 



phloem are more strongly developed together with the amount of fibres in 

 the bundles. 



The genetic basis of this phenotypic plasticity, adaptative in nature, 

 must be of importance in species whose habitat involves exposure to wind 

 or drought. It is suggested that there has been positive selection during 

 evolution for genotypes having large potentialities for phenotypic plasti- 

 city of this kind. This is not the only solution that evolution has produced, 

 c.f the many genotypes of Cerastium (Whitehead, i959), but that it is 

 successful may be assumed from the occurrence of many highly plastic 

 species in montane regions. 



My thanks are due to the Royal Society for a grant for the construction 

 of one of the wind tunnels used in this experiment. 



REFERENCES 



Whitehead, F. H. (1957) Wind as a factor in plant growth. In Control of the Plant En- 

 vironment. Ed. J. P. Hudson. London. 



Whitehead, F. H. (1959) Vegetational changes in response to alterations of surface 

 roughness on M. Maiella, Italy.J. Ecol. 47, 603-606. 



