EXPOSURE AND GROWTH AND DEVELOPMENT 237 



described fully in Whitehead (1959). The experiment was set up to test, at 

 the same time, the alternative hypothesis that the observed effects were 

 due to shading. This alternate hypothesis was shown to be untenable. 



The growth form of a plant is of great importance in relation to wind 

 effects. A rough classification of such form is considered in Whitehead 

 (1957). In this paper only the exposure sensitive class will be considered. 

 These are plants which show a marked dwarfmg when grown in condi- 

 tions of continuous exposure to wind. In addition to the reduction of 

 height which is not accompanied by a reduction of the number of inter- 

 nodes, there are also extensive anatomical changes. All these changes are 

 such that it can be said that the plant shows increased xeromorphic 

 characters the higher the wind speed under which it is grown. Observa- 

 tions made on plants grown in wind tunnels showed considerable simi- 

 larity to plants of the same species [Senecio nebrodensis) from areas of 

 increasing exposure in M. Maiella. 



In earlier experiments (Whitehead, 1957) it was found that leaf saturation 

 deficits of plants exposed to wind for a time increased despite the fact that 

 the roots were in soil maintained at field capacity. Recovery took place 

 during a subsequent period without wind. Other species of Senecio not 

 found in nature in such exposed conditions, e.g. Senecio elegans appeared 

 to accumulate so large a leaf saturation deficit that under the conditions of 

 the experiment permanent wilting could be expected after a few days. 



Resulting from the experiments, however, it became clear that any 

 genotype has some power of reacting to the environment in such a way 

 that the changes in development can be considered advantageous to 

 survival under those conditions. Accordingly experiments were carried 

 out to investigate the extent of change which could be produced by species 

 not adapted to the severe conditions of the mountains. The species chosen 

 were Zea mais and Helianthus annuus. The plants of Zea were grown in a 

 wind tunnel and their anatomy and morphology compared to control 

 plants not subjected to wind. The results are given in full in Whitehead and 

 Luti, 1962. It was found that the plants exposed to wind were much more 

 xeromorphic in all characters examined. The degree of difference between 

 the treated and control plants was large being of the order of three to six 

 times greater in the treated plants than the controls. 



In further experiments using Helianthus annuus a growth analysis of 

 both treated and control plants was made (Whitehead, 1962). These 

 showed that the plants exposed to wind were shorter and had smaller 

 leaves than the controls. This dwarfmg effect increased with the wind 

 speed under which the plants developed. 



