EXPOSURE AND GROWTH AND DEVELOPMENT 243 



The foregoing experiment suggests that the anatomical and morpho- 

 logical changes result from interference with processes controlling differ- 

 entiation. Since growth substances are often involved in such processes 

 the experiments were repeated, treating one set of plants with various 

 growth substances. With /? indole-acetic acid, over a wide range of 

 concentration, no differences were observed (Whitehead (1963)) 

 between treated and untreated plants. With gibbereUic acid however 

 highly significant differences were observed. Both in wind treated and 

 soil moisture regime experiments gibbereUic acid tended to reverse the 

 effect of these factors so that less anatomical and morphological change 

 was seen in the treated plants. It is interesting to note that the full range of 

 treatment of wind or soU moisture restriction could not be used because 

 the gibbereUic acid treated plants failed to survive above a relatively low 

 limit of severity of conditions. This is a further demonstration that the 

 changes have survival value. 



A further test of the survival value of these changes was made in the 

 following experiment (Whitehead (in the Press)). Plants of Helianthus 

 annum grown under a range of wind conditions were exposed to an ex- 

 cessively high wind speed (60 m.p.h.). The time was noted that this 

 wind took to so desiccate the plants that permanent wilting and 'death' 

 occurred. The results are shown in Fig. 8. It can be seen that the more 

 severe the conditions under wliich the plants developed the longer they 

 were able to survive. The plants treated with gibbereUic acid were 

 relatively soon kiUed, whilst the untreated plants survived much longer. 



These experiments can be summed up as demonstrating that the main 

 effect of wind is to reduce the water balance of the plant even if the roots 

 are in optimum condition for water uptake. The reduction of water 

 balance in the shoot whether due to excessive loss or to limited uptake 

 appears to upset the processes of differentiation and only in extreme cases 

 is the photosynthetic apparatus itself affected. This modification of 

 differentiation is unaffected by /? indole-acetic acid but is counteracted to 

 a large extent by gibbereUic acid. The anatomical and morphological 

 changes which result tend to produce a phenotype better adapted to 

 growth under xerophytic conditions. The plants are dwarfed but the 

 number of internodes is not affected; the leaf area is smaUer and the leaves 

 themselves have lower transpiration rates per unit area/unit time; the 

 smaUer amounts of assimilates are differently employed in that there is a 

 large increase of root relative to shoot; the internal anatomy of the leaves 

 is also affected being more xeromorphic and with increase in the number 

 of pahsade layers and reduction of inter-ceUular spaces ; the xylem and 



