CLIMBERS AND TWINERS 



49 



of attachment, as shown in Fig. 40. Some tendrils are modified 

 leaves or stipules, as shown in Fig. 98 ; others are modified stems. 

 63. Irritability of tendrils. The coiling of tendrils is due 

 to their irritability, aroused by the stimulus of contact with a 

 solid object. After a latent period, varying with different species 

 from a few seconds to more than an hour, the bending begins. 

 It is caused either by contraction of the side in contact or by 

 expansion of the opposite side; the exact mechanism of the 

 process is not yet fully under- 

 stood. The tendrils of the passion- 

 flower plant will respond to the 



FIG. 41. Coiling of petiole of dwarf 

 nasturtium ( Tropceolum) 



FIG. 42. Twining stem of hop 

 After Decaisne 



pressure of a bit of thread, hung on the tendril and kept in 

 motion, whose weight is only a few millionths of a grain. 



64. Twiners. Only a few of the upper internodes of the stem 

 of a twiner are concerned in- producing the movements of the 

 tip of the stem. This is kept revolving in an elliptical or cir- 

 cular path until it encounters some roughish and not too stout 

 object, about which it then proceeds to coil itself. 



The movements of the younger internodes of the stems of 

 twiners are among the most extensive of all the movements 



