v Movements of Plants 89 



movements of twiners and climbers, young shoots 

 and roots, etc. 

 (2) Movements in relation to external influences : 



(a) Downward movement of roots and up- 



ward movement of stems. 



(b) Light -seeking and light - avoiding move- 



ments, especially of stems and leaves. 



(c) The movement of roots towards the 



greatest moisture. 

 B. Movements of Adult Parts 



(1) " Spontaneous " movements, e.g. of Desmodium 

 or Hedysarum gyrans. 



(2) Movements in relation to external influences : 



(a) In relation to light sleep movements of 



leaves. 



(b) In relation to chemical and physical 



stimuli other than light, e.g. the move- 

 ments of the Sensitive plant and of Fly- 

 Traps. 



Mr. Francis Darwin's Discussion of Plant Move- 

 ments. At this point it is appropriate that we should 

 turn to what Mr. Francis Darwin, who collaborated with 

 his father in writing The Power of Movement in Plants, 

 has recently said in his Presidential address to the bio- 

 logical section of the British Association, 1891, which con- 

 tains an important discussion of growth - curvatures, a 

 problem towards the solution of which his detailed 

 researches have largely contributed. A brief summary of 

 this address, although including points already touched, 

 may hence be of service to the student. 



He begins by distinguishing the two main questions : 

 i . " How does the plant recognise the vertical line ; how 

 does it know where the centre of the earth is?" a ques- 

 tion of irritability. 



