GROWTH. IRRITABILITY AND MOVEMENT 



161 



tropism, or curvature in response to contact, is best known in tendrils (p. 

 216). Contact of the tendril with a solid object causes growth on the side 



Fig. 97 a. 



Pollen-grains germinated in a nutritive medium under a cover glass, of which the 

 margin is shown. The tubes curve away from the margin, that is, away from 

 the supply of oxygen. (After Molisch.) 



away from the point of contact to be considerably accelerated, with the 

 result that the object, provided it is of suitable dimensions, tends to become 

 entwined by the tendril. 



4. Nastic Movements. 



So far we have studied the effect on plant organs of directional 

 stimuli such as gravity, line of incident light, unequal distribution 

 of water and so on. Many plant organs in addition display a sensi- 

 tiveness towards non-directional and uniformly distributed stimuli 

 such as variations in light intensity and temperature, or mechanical 

 shock, and show change of orientation or movement under the 

 influence of such stimuli. In these cases the new orientation is 

 determined not by the stimulus, as in tropic responses, but by the 

 structure of the plant. Such responses are said to be of the Nastic 

 type. Whereas tropic movements are due to growth changes and 

 are therefore confined to growing organs, nastic movements are 

 mostly, though not always, due to changes of turgor. Such changes 

 can occur in mature parts, provided that mechanically the organ is 

 suited for bending. They are frequently more rapid and striking 

 than tropic movements, though of less importance biologically. 



The opening and closing movements shown by many flowers and by 

 some foliage leaves are examples of nastic responses. They usually 

 occur in the morning and evening, the evening movements being 

 commonly termed " sleep movements." It so happens that the 

 opening and closing of flowers are due to growth changes in the petals 



-B.B . 



