182 



BOTANY OF THE LIVINGj>PLANT 



Climbing by prehensile methods has gained more attention than 

 straggling, because it is so well represented in the Native Flora, 

 and because the advantages which it brings are so obvious. 

 The attachment to the support may be by a twining stem, as in 

 the Hop, Scarlet Runner, or Convolvulus ; or by tendrils of various 



sorts, and by prehensile leaves. The 

 twining stem originates from an upright 

 seedling, the tip of which soon takes 

 an oblique or horizontal position, its 

 apex revolving in a circle. This apical 

 region receives the stimulus of gravity. 

 The reaction is an inequality of growth 

 successively on the various sides of it 

 (lateral geotropism), and this causes 

 the revolving movement. If the stem 

 comes in contact with an upright 

 support of suitable thickness, it laps 

 round it with a continuous spiral course 

 upwards. There is here little morpho- 

 logical change beyond an elongation of 

 the internodes, and frequently a delay 

 in the development of the leaves till 

 their support is assured. Such climbers 

 may be dextrorse, following the hands 

 of a watch (Hop), or sinistrorse, show- 

 ing the reverse, which is the more 

 common, as in Convolvolus or Phaseolus. 

 There is here no contact stimulus. The 

 twining is a geotropic phenomenon 

 (Fig. 135). 



But tendrils grasp their support as a 

 consequence of contact-stimulus, which 

 reacts by disturbing the growth while 

 young. The tendril 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. 136.) 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 



FIG. 135. 



Twining stems. A, Sinistrorse shoot of 

 Pharbitis. B, Dextrorse shoot of Myrsi- 

 phyllum. (After Strasburger.) 



