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PLANT PHYSIOLOGY 



surface of the petals, which then curve externally, and the 

 flower opens. When the plant is left for a considerable period 

 in a warm temperature, however, then the growth of the lower 

 side also will be accelerated, and the flower will again close. 



Fig. 135. — Thermonastic opening of saffron flowers {after Kerner). 



Fig. 136.— Photonastic opening of a dandelion flower head (from Bonner Lehrbuch). 



The movements of petals of the water lily and of flowers 

 of the dandelion are good examples of photonasty. Changes in 

 the intensity of light make these plants close, not only in the 

 evenings, but in dull weather as well (Fig. 136). This effect may 

 be produced also by artificial shading, as for instance if the plant 

 is carried into a weakly illuminated room. Some plants, like 

 Nicotiana and the four-o'clock, on the contrary,, close their 

 flowers when the light is increased and open under weaker 



