200 HOW PLANTS CLIMB. 



gonnm does its work only during the middle of summer ; it grows 

 vigorously in autumn, but with no tendency to climb. 



With regard to the direction which the revolving movement 

 takes, there are very many interesting facts. We can only notice 

 at present that some twine from left to right, or against the hands 

 of a watch, while others take the opposite direction. These terms 

 are used in different senses by different writers. The simplest and 

 best method to clearly apprehend the terms is to imagine the pole, 

 or support in front of the observer, and then to note in which direc- 

 tion the first revolution is made. If it be from right to left — i.e., 

 with the watch — it is called sifiistrorse ; if from left to right, it is 

 dextrorse ; the terms being used to specify the hand towards which 

 the shoot twines. 



By far the greater number of twiners revolve from left to right, 

 dextrorsely ; the purple and white Convolvuluses, French bean, 

 and Morning Glory are examples. A few take the opposite 

 direction, as, for example, Hop, Honeysuckle, and Black Bryony 

 [Tanws). Very rarely do plants of the same order twine in 

 different directions. Darwin met with no two species of the same 

 genus that did so, but different individuals of the same species 

 are sometimes found to twine in two ways : the Woody Nightshade 

 [Solannm dulcamara) of our hedges, for example. In some cases, 

 as in the Chili Nettle, some individuals twine in one way, some in 

 the other, others in both, the petioles of its opposite leaves afford- 

 ing a fulcrum for the reversal of the spire. This double move- 

 ment in the same plant is rare. It occurs in Hibbcrtia, where the 

 twining is always dextrorse, while the revolving movement varies ; 

 thus the plant is adapted for twining in order to ascend, and at 

 the same time is able to wind from side to side through the thick 

 Australian scrub. 



Our indigenous twiners can ascend a support as thin as ordi- 

 nary thread, some, such as Woody Nightshade, being able to climb 

 only round very thin and flexible stems ; they can ascend stems of 

 moderate thickness, but Honeysuckle is the only one that ever 

 twines around tree-trunks. In the tropics, on the contrary, 

 twiners can ascend forest-trees, and this is needful for them, or 

 they would be unable to reach the light and air. In England, 

 our annual twiners would be unable in their single season to reach 



