12 THE STORY OF PLANT LIFE 



stimulus of gravity. Hence responses to stimuli are 

 localised or special adaptations to meet special forces. 

 The stimulus may be communicated to surrounding 

 cells, but only for a short distance. 



Another stimulus which causes plants to exhibit 

 movements is light. The response to this factor is 

 called Phototropism, or Heliotropism. In this kind 

 of movement plants place themselves or their stem 

 structures parallel with the direction of the light, 

 but the leaves of plants in a window, or the flowers, 

 in the case of sun plants, during the day which turn 

 their flowers to the sun, e.g. Rockrose, are at right 

 angles. The movement may be towards the light 

 when it is positive, or away from it when it is nega- 

 tive as in roots. 



The leaves take up a position at right angles to 

 the rays of light, and are then diaheliotropic. The 

 stem of the Ivy which clings to a wall or tree is 

 also more or less diaheliotropic. It is the violet- 

 blue rays that influence the movements due to photo- 

 tropism. 



The response to contact with a supporting struct- 

 ure, exhibited by trailing and climbing plants, is 

 again a type of movement, seen in the tendrils of 

 the Clematis, or the rootlike structures on the stem 

 of the Ivy. The roots of plants also exhibit the 

 same kind of sensibility to contact. 



Root-hairs further exhibit a certain affinity for 

 nutritive substances or solutions in the soil, and 

 this is a sensibility of a chemical nature. 



