406 MOVEMENTS. 



only two are involved. The iuternodes below the seat of move- 

 ment are rigid. The revolving is associated with growth, but 

 the growth alone is probably not the sole cause of the move- 

 ment. 



1056. It is only the young internodes which are capable of 

 spontaneous movement ; but growth itself, unassociated with 

 changes in the turgescence of the tissues upon the different sides, 

 would not be sufficient to account for the movement. It must 

 be remembered that the young stem possesses remarkable ten- 

 sions, which are easily disturbed by slight internal as well as 

 external causes. The increased turgescence of its cells upon one 

 side, or their diminished turgescence on the other, or the action 

 of both conjointly, followed as this is by an increased growth of 

 the turgescent part, would produce sufficient change in the cur- 

 vature of the stem to bring about the twining movement. 



1057. When a twining stem comes in contact with a smooth 

 support, it generally slides up the support, but fails to grasp it. 

 The check which is given by a smooth support sometimes brings 

 about a change of position in the revolving stern, which is thus 

 described b}^ Darwin: "When a tall stick was so placed as to 

 arrest the lower and rigid internodes of Ceropegia, at the dis- 

 tance at first of fifteen and then of twent3 T -one inches from the 

 centre of revolution, the straight shoot slowly and gradually slid 

 up the stick, so as to become more and more highly inclined, 

 but did not pass over the summit. Then after an interval suffi- 

 cient to have allowed of a semi-revolution, the shoot suddenly 

 bounded from the stick, and fell over to the opposite side or 

 point of the compass, and reassumed its previous slight inclina- 

 tion. It now recommenced revolving in its usual course, so that 

 after a semi-revolution it again came in contact with the stick, 

 again slid up it, and again bounded from it and fell over to the 

 opposite side. This movement of the shoot had a veiy odd ap- 

 pearance, as if it were disgusted with its failure, but was resolved 

 to try again." 1 



1058. Many of the common twiners of temperate climates are 

 able to twine round very slender supports, for instance a small 

 cord, but are unable to twine round a post or trunk of a tree. 

 This does not, however, appear to be wholly dependent upon the 

 amplitude of the revolution. In tropical regions some of the 

 twiners ascend trunks of immense size, but they are generally 

 assisted by adventitious roots, etc. 



1 Climbing Plants, 1875, p. 21. 



