CHAP. V CLIMBING PLANTS 265 



ellipses in the rate of movement, and in different 

 species revolving once or several times within the samo 

 length of time. In the same internode, the move- 

 ments cease first in the lower part and then slowly 

 upwards. In both sets of cases the movement may bo 

 modified in a closely analogous manner by geotropism 

 and by heliotropism ; though few climbing plants are 

 aeliotropic. Other points of similarity might be 

 pointed out. 



That the movements of climbing plants consist of 

 ordinary circumnutation, modified by being increased 

 in amplitude, is well exhibited whilst the plants are 

 very young ; for at this early age they move like other 

 seedlings, but as they grow older their movements 

 gradually increase without undergoing any other 

 change. That this power is innate, and is not excited 

 by any external agencies, beyond those necessary for 

 growth and vigour, is obvious. No one doubts that 

 this power has been gained for the sake of enabling 

 climbing plants to ascend to a height, and thus to 

 reach the light. This is effected by two very different 

 methods ; first, by twining spirally round a support 

 but to do so their stems must be long and flexible ; 

 and, secondly, in the case of leaf-climbers and tendril- 

 bearers, by bringing these organs into contact with a 

 support, which is then seized by the aid of their 

 sensitiveness. It may be here remarked that these 

 latter movements have no relation, as far as we can 

 judge, with circumnutation. In other cases the tips 

 of tendrils, after having been brought into contact with 

 a support, become developed into little discs which 

 adhere firmly to it. 



We have said that the circumnutatiou of climbing 

 plants differs from that of ordinary plants chiefly by 

 its greater amplitude. But most leaves circumnutate 



