204 HOW PLANTS CLIMB. 



gradually bend down until at right angles to the stem ; remaining 

 there for a time, they arch downward until the leaf points to the 

 ground with its tip incurled. This is their fashion of looking out 

 for a support, which the revolving motion of the shoot may bring 

 them near to. They then act as I have just indicated, according 

 to whether they find the support they seek, or fail to find it. 



A petiole coming into contact with a support for a short time 

 only, usually continues curved for some time, but can afterwards 

 regain its upright position, and so be ready to act once more ; 

 but if it clasp its support for any length of time, then it cannot 

 straighten again. In some species, markedly so in some of the 

 Clematis family, the petiole having coiled around its support, in 

 two or three days begins to swell, and gradually thicken, either 

 laterally, or through its whole diameter, until it becomes twice as 

 thick as an unclasped petiole. It then becomes much more 

 woody internally, like a stem, and instead of being easily snapped 

 in two, it is so tough and rigid that force is needed to break it. 



This change of structure gives greater durability, firmness, and 

 strength ; it hinders the unwinding of the petiole, and of course 

 enables it to withstand the force of the wind, or of shock from 

 any other cause. The appearance of a cross-section of such a 

 petiole under the microscope, shows a complete riiig of woody 

 tissue, as opposed to the semi-lunar one of an ordinary leaf-stalk. 

 It is a fact worthy of note that this change is effected merely by 

 the act of clasping a support. 



Petioles are usually sensitive only when young. They are 

 sensitive on all sides, although this differs in different species. 



The rate at which they respond to a touch varies. In some 

 species of TropcEolum a slight rub took effect in three minutes ; 

 in others the response occupied six, ten, or even twenty minutes. 

 In other cases — for instance, in some species of Clematis — it took 

 several hours. In others, two or three days or more pass by 

 before the process is complete. 



The degree of sensitiveness varies. In some, a weight of only 

 one-sixteenth of a grain will cause bending to ensue ; in others, 

 the touch of the exceedingly fine flower-stalks of the Quaking- 

 grass ( Briza). 



This sensitiveness extends in some cases to the stems and to 



