EXPOSURE AND GROWTH AND DEVELOPMENT 



239 



species of less exposed habitats and this attribute must be of great impor- 

 tance in selection and survival. 



Figure 3 also shows the gain in dry weight per unit of leaf area per unit 

 time. There is very little difference at the four wind speeds indicating that 

 the dry weight gain has been proportional to the leaf area attained. The 

 actual photosynthetic activity of individual cells appears to have been little 

 affected. The effect of the wind therefore appears to be on the processes 



• INTERNODE LENGTH 



O LEAF AREA . 



O DRY WEIGHT GAIN PER UNIT AREA/UNIT TIME 



-300 



260 



•220 — 



IN 



e 



ISO S 



i- 



a 



■I40 



o 

 o 



100 



10 



20 



Wind speed ( m.p.h.) 



Fig. 3. Average leaf areas and internode length of Helianthus annuus plant grown at 

 I mile/h, 9 mile/h, 19 mile/h and 33 mile/h together with the dry weight increment 

 per unit area per unit time. 



controlling differentiation and the development of leaf area rather than on 

 the photosynthetic apparatus itself. 



The increase in xeromorphy of the treated plants would appear to be 

 advantageous in reducing water loss from the treated plants. Fig. 5 shows 

 the results of an experiment designed to test this hypothesis. Plants of 

 Helianthus annuus were grown for thirty days at four different wind speeds, 

 the soil being maintained constantly at field capacity. The soil and roots 



