HOW PLANTS CLIMB. 13 



when it is cool, and more rapid when it is warmer. Sometimes it stops when 

 everything seems favorable, and starts again after a while. The Hop, Bean, and 

 Morning Glory are as quick as any. In a sultry day, and when in full vigor, they 

 commonly sweep round the circle in less than two hours. They move by night 

 as well as by day. When the free summit of a twining stem is outstretched to 

 two feet or more in length, so as to magnify the motion, this is sometimes rapid 

 enough to be actually seen in some part of the circuit. 



13. Because twining stems are often twisted more or less, some have supposed 

 that the twisting was the cause of the revolving sweep of the free end. If so, the 

 stem below would in a day or two be likely to twist itself off. And twiners sel- 

 dom twist much when climbing a smooth and even support. To learn how the 

 sweeps are made, one has only to mark a line of dots along the upper side of the 

 outstretched revolving end of such a stem (say that of tlie Morning Glory, Fig. 3), 

 and to note that when it has moved round a quarter of a circle, these dots will be 

 on one side ; when half round, the dots occupy the lower side ; and when the revo- 

 lution is completed, they are again on the upper side. That is, the stem revolves 

 by bowing itself over to one side, — is either pulled over or pushed over, or both, 

 by some internal force, which acts in turn all round the stem in the direction in 

 which it sweeps ; and so the stem makes its circuits without twisting. 



14. So the sweeping round of the stem is a movement like that wonderful one 

 of the leaflets of Desmodium gyrans, just described, only slower. And here we 

 see what it is for. The sweeping movement of the stem is the cause of the twin- 

 ing. The stem sweeps round that it may reach some neighboring support ; as it 

 grows it sweeps a wider and wider space, that is, reaches farther and farther out. 

 When it strikes against any solid body, like the stalk of a neighboring plant, it is 

 stopped : but the portion beyond the contact is free to move as before ; and, con- 

 tinuing to lengthen and to move on, it necessarily winds itself round the support, 

 that is, twines. This is the explanation of twining climbers. 



15. leaf-Cllmbers. Some plants climb by their leaves, either the blade, or more 

 commonly the petiole, hooking or coiling round something within reach. Clema- 

 tis or Virgin's-Bower is a familiar instance. In all the common species of Clema- 

 tis the leaves are compound, and the divisions of the petiole, or at first the young 

 leaflets themselves, bend round the stalks or branches of neighboring plants, or 

 any supporting object not too large to be grasped, and so ascend. Lophospermum 

 and Mawandia (handsome flowering herbs of the gardens), and one or two other 



