56 MOVEMENTS OF CURVATURE 



a minimum, so that the accelerating of coiling produced by contact may 

 be connected with the retardation of growth which usually ensues 1 . This 

 is presumably the result of the correlative stimuli awakened by contact and 

 not of the mechanical tension exercised on the attached tendril. Tension 

 appears usually to slightly retard growth in length, but subsequently to 

 accelerate it. That a free tendril should coil all one way, but that the free 

 portion of an attached one should form two or more reversed spirals is the 

 natural result of the same attempt at coiling combined in the second case 

 with the fixation of the ends of the tendrils 2 . Similar results may be 

 obtained when longitudinal strips of the peduncle of Taraxacum which tend 

 to coil spirally are held at both ends, or when a cord attached at both ends 

 is twisted in opposite directions at two points equidistant from its ends ?. 



The production of the suckers of Ampelopsis and Amphilobium, of the 

 haustoria of Cuscuta^ as well as the thickening of certain tendrils and 

 attaching hooks and of the petioles of leaf-climbers, are undoubtedly due in 

 the first instance to the stimulus of contact. The thickening only attains its 

 full development when permanent contact is assured and when the attaching 

 organ is subjected to increasing tension. The increased pressure at first 

 increases the contact-stimulation but finally retards or inhibits the growth 

 on the applied surface, which usually becomes more or less flattened when 

 the pressure is considerable 4 . The hooks of tropical climbers may attain 

 a considerable increase of strength, in this way their breaking strain often 

 increasing four- or ten-fold, so that they are. able to bear weights of 

 10 to 15 kilogrammes 5 . The same takes place in the tendrils of Amphilo- 

 bium and Bauhinia which undergo secondary thickening, while according 

 to Worgitzky 6 the attached lignified tendrils of Cucurbita and Passiflora 

 become from two to twelve times stronger than unattached ones. 



It is uncertain whether it is the absence of a contact-stimulus or of tension 

 which is responsible for the smallness, shrivelling, death, or abscission of 

 the unattached tendrils of certain plants. This was observed by Darwin on 

 the tendril of Ampelopsis hederacea (quinquefolid) and Bignonia Tweediana> 

 by Muller on that of Cyclanthera pedata, by Leclerc du Sablon on leaf-tips 

 of Flagellaria indica, and by Ewart on the tendrils of Amphilobium mutisii' 1 . 



1 Fitting, Jahrb. f. wiss. Bot., 1903, Bd. xxxviil, pp. 550, 608. This coiling is associated with 

 a single slight acceleration of growth. 



2 Correctly interpreted by Mohl, 1. c., p. 79, and Darwin, 1. c., p. 127. 



3 Noll, Flora, 1899, p. 388. 



* Derschau, I.e., p. 33 ; Ewart, 1. c., pp. 140, 189. 6 Ewart, 1. c., pp. 194, 208. 



6 Worgitzky, Flora, 1887, p. 40. On the tensions to which tendrils are exposed cf. Macdougal, 

 Ber. d. hot. Ges., 1896, p. 153. 



7 Darwin, 1. c., pp. 69, 113, 355 ; v. Lengerken, 1. c., p. 360; Muller, Cohn's Beitr. z. Biologic, 

 1887, Bd. IV, p. 108; Ewart, I.e., p. 219; Leclerc du Sablon, Ann. sci. nat., 1887, 7" ser., T. v, 

 p. 28. The attachment of the coiling portion of the leaf of Nepenthes favours the development 

 of the pitcher according to Goebel, Pflanzenbiol. Schilderungen, 1891, n, p. 98. 



