216 



BOTANY OF THE LIVING PLANT 



upwards. There is evidence that the twining is to some 

 tent a geotropic response of a special type. There is here little 

 morphological change beyond an elongation of the internodes, and 

 frequently a delay in the development of the leaves till their support is 

 mred. Such climbers may be dextrorse, following the hands of a watch 

 (Hop), or sinistrorse, showing the reverse, which is more common, as in 

 ( onvolvulus or Phaseohis (Fig. 144). There is here little or no contact 

 stimulus, the twining being a nutatory and geotropic phenomenon. 



1 hit tendrils grasp their support as a consequence of contact-stimulus, 

 which reacts by disturbing the growth while young. The tendril 



Fig. 145. 



Portion of stern of Sicyos, a Cucurbit, with tendril attached to support, x— point 

 of reversal of the coiling of the tendril. See Text. (After Strasburger.) 



is a cylindrical whip-like organ, usually with a hooked tip. Its 

 sensitiveness is sometimes localised along a definite line. During 

 growth it shows movements of circumnutation : if it then comes in 

 contact with a support, inequality of growth causes the tendril to 

 lap round it. (Fig. 145.) Its morphological origin may be various. 

 In the Garden Pea, Vetch, and Cobaea it obviously represents the 

 distal region of the leaf, including several pinnae ; or it may be 

 the excurrent tip of the lamina, as in Gloriosa ; or extended parts 

 of the lower region of the leaf may be prehensile, while the lamina 

 or pinnae develop normally after the lower region has grasped the 

 support, as in Corydalis, Clematis, and Solanum jasminoides ; or 

 lateral ' stipular " structures may be represented by tendrils, as in 

 Smilax ; or again, the tendril may be referable to a whole shoot, as 



