MOVEMENTS AND ATTITUDES OF PLANTS 143 



as shoots, execute definite movements, and even in parts 



that are mature, definite movement occurs. As we have 



seen, plant organs execute movements 



in response to such stimuli as light, ^* 



gravity, and moisture. Roots usually 



turn away from the light and shoots 



turn towards it ; underground stems 



ascend and descend in the soil and are 



aided in their descent by contractile 



roots. 



Nutation. Twining plants. We have 

 now to notice the movement character- 

 istic of aerial stems. Plants growing 

 in woods, hedgerows, and other shady 

 places, tend to develop longer and more 

 slender stems and thinner leaves than 

 plants of the same species grown in open, 

 sunny places. If the stems of these 

 plants be observed it will be found that 

 the growing tips move in a more or less 

 circular orbit . ' This movement is called 

 nutation. Plants such as the Convol- 

 vulus (Fig. 89) and Black Bryony 

 (Tamils communis) (Fig. go) develop 

 long, slender internodes and relatively 

 large leaves, and the stems, too weak to 

 hold the shoot erect, lean on other and 

 sturdier plants for support. Their 

 growing tips describe a wide spiral, 

 making a complete revolution in from 

 one to two hours, and, on coming into 

 contact with a shoot of suitable diameter, 

 wind round it. As growth continues, the spiral so formed 

 is drawn tighter, clasps the support firmly, becomes thicker 

 and stronger, especially on the convex side, so that it cannot 



Fig. 89. 

 Twining Stem of 



Convolvulus 

 arvensis (Pfeffer). 



