HAPTOTROPISM 



495 



contact but perhaps by pressure against the support. Longitudinal growth in the 

 basal region comes rapidly to a standstill, although the tendril may by no means 

 have reached the length attainable if a support be not grasped. Further, 

 a spiral twisting manifests itself in the basal region, which is of great service to 

 the plant inasmuch as it is the means of drawing the branch closer to the support. 

 For purely mechanical reasons this intermediate spiral changes its direction at 

 least once, but several such reversals of the spiral may frequently be noticed 

 (Fig. 153 W). That this reversal is mechanically necessary, may be readily 

 demonstrated by attempting to produce a spiral coiling in a cord or piece of 

 rubber-tubing fixed at both ends. This spiral coiling recalls the autonomous 

 curvatures already noted as occurring in older tendrils that have found no 

 supports, since it always arises by excessive growth on the upper side. There 

 are good reasons for believing, however, that the two phenomena are not iden- 

 tical, since intermediate spiral coils occur, after the clasping of the support, 

 in tendrils which exhibit no free senile coiling {Vitis, Darwin, 1876 a). [Fitting 

 (1903 b) has shown that the incoiling of the base of the tendril after attachment 

 to a support is a stimulus phenomenon which takes place in consequence of a 

 growth acceleration in the median region.] 



The following additional changes taking place in tendrils which have suc- 

 ceeded in finding a support, may also be noted. Not 

 infrequently secondary thickening accompanied by 

 a large development of sclerenchyma appears not only 

 in the part immediately in contact with the support 

 but also in the basal region of the tendril as well, which 

 renders the tendril more capable of fulfilling its function 

 (Treub, 1882-3 ; EwART, 1898). Some tendrils also 

 give off a secretion which helps them to adhere (O. 

 MiJLLER, 1887) ; as to these and other morphogenic 

 results of contact stimulus compare p. 315. Finally 

 we must note that functional tendrils remain alive for 

 a much longer time than those which have been un- 

 successful in finding a support ; the latter soon die, 

 wither, or are thrown off, 



Haptotropism manifests itself not merely in tendrils 

 serving as specific climbing organs and which have 

 entirely relinquished their previous functions and 

 become adapted specially for that purpose, but it is 

 manifested also in other organs which have remained 

 functional so far as their main duties are concerned, 

 but which add as a subsidiary function that of acting as 

 climbing organs. Thus, for instance, ordinary roots may become sensitive to 

 contact in their growing regions and may respond with haptotropic curvatures 

 (Sachs, 1873, 436 ; [Newcombe, 1902 ; Nemec, 1904]) ; and this capacity is so 

 prominent in many aerial roots that we may even speak of them as 'root tendrils' 

 (for literature see Ewart, 1898). Contact sensitivity is also very prominently de- 

 veloped in leaves and especially in petioles (Darwin, 1876 a). Plants belonging 

 to many different families, e.g. Clematis, Maurandia, Lophospermum, Tro- 

 paeolum, Solanum jasminoides, Sec, grasp supports by means of their petioles, 

 while the leaf blades continue to act purely as assimilatory organs. Fumaria 

 officinalis also climbs by means of unaltered laminae. Nepenthes may also be 

 referred to in this connexion, where a certain part of the leaf functions as a tendril, 

 while of the remainder, one part assimilates carbon-dioxide and one part acts as 

 an insect-trap, Lophospermum may be specially mentioned as an example of a 

 climber whose principal axis is sensitive to contact and twines round a support 

 and in addition possesses both sensitive petioles and internodes. None of the 

 examples cited have as yet been fully investigated from a physiological stand- 



F'K- '55- •'i tendril whose 

 tip has just begun to coil round 

 a support. Ink marks have 

 been drawn, in a vertical direc- 

 tion, on the support and on the 

 tendril, on the former at 3, on 

 the latter at i and 2. Another 

 line has been made at 4 on the 

 tendril, at a place as yet at 

 some distancefrom the support. 

 II, the same tendril some hours 

 later, showing the position of 

 the original coil in /; the point 

 marked 4 is now in contact 

 with the support. (After Fitt- 

 ing). 



