298 THE BIOLOGY OF FLOWERING PLANTS 



The sensitiveness of the tendril is very great. In favour- 

 able circumstances the contact of a cotton thread, moved by 

 light air currents, induces a reaction. The contact of water 

 drops, however violent, is ineffective, and this has an obvious 

 biological advantage, in that rain does not cause a movement 

 which could be of no use to the plant. It is probable that 

 an uneven deformation of the protoplasm of the epidermal 

 cells is the immediate effect of the contact. Haberlandt 

 (1906) has described special thin regions in the walls of 

 the epidermal cells of tendrils, sensitive pits, and thin- walled 

 sensitive papillae, as suited to the perception of contact 

 stimulus. 



The reaction is carried out by a considerable increase in 

 the growth rate of the side opposite that stimulated, com- 

 bined with a decrease on the stimulated side. About half 

 an hour after stimulation this condition is reversed and the 

 tendril straightens out. 



The response to contact stimulus is termed haptotropism. 

 It is most prominent in the tendrils, but is also seen in 

 the tentacle of sundews, and in the stems of the dodders. 

 A recent survey by Stark (1915, 19^7) has shown, however, 

 that haptotropism is very common in all sorts of plants 

 and plant organs which do not encircle supports, though 

 in such cases it must be looked for. Of sixty-three species 

 of non-climbers investigated one-third were found to be 

 sensitive to contact stimulus in the petiole, the shoot axis, 

 or the peduncle. Of twining plants, over 50 per cent, show 

 sensitiveness to contact stimulus, and in the case of tendril 

 climbers the sensitiveness is not usually confined to the 

 tendril, but is present, to a lesser extent, in other organs, 

 a condition already recognised by Darwin. Haptotropism 

 is also shown by root tips in the soil. It is thus a very 

 general property of growing parts ; in tendrils the sensitive- 

 ness and the extent of the reaction have become intensified 

 and specialised in relation to a definite and important 

 function. 



The chance of contact with a suitable support in nature 

 is increased by the circumnutation of the apical portion of 



