TENDRIL-CLIMBERS 49 



The periodic inhibition of the contact irritability at the apex of Cuscuta affords 

 a good instance of the influence of a realized activity upon subsequent development, 

 and it has the importance of allowing the plant to spread from one host to a 

 neighbouring one, and of enabling more rapid extension over a single host. The 

 persistence of the irritability in the absence of a support gives a better chance of 

 one being immediately utilized when reached by the circumnutation of the elongating 

 apex. 



Since Cuscuta usually gains a support by the aid of its circumnutation, the 

 coiling follows the direction of circumnutation 1 , but it is uncertain whether the 

 contact irritability suffices by itself to produce definite coiling. Since coiling takes 

 place around a rod of moist gelatine which exercises no contact stimulation, it is 

 evident that circumnutation alone produces fairly good coiling. Cuscuta, like other 

 twiners, usually produces no further coils around a support laid horizontally. This 

 is owing to the fact that during the phase when the apex is non-sensitive to contact, 

 the terminal internodes free themselves from the support and strive to become 

 erect 2 . 



A rod of any material suffices to produce the coiling of Cuscuta and the forma- 

 tion of haustoria, which however only attain their full development when they penetrate 

 an appropriate host-plant. Since the production of haustoria is dependent upon the 

 stimulus of contact, they are only formed on the side pressed against the support, 

 although all sides of the stem are capable of producing them. 



Heliotropism. The negative heliotropism of certain tendrils aids them in acquiring 

 a support. This applies to the tendrils of Viiis vinifera, Ampelopsis hederacea*, 

 Bignonia capreolata, Eccremocarpus sealer *, as well as to the root-tendrils of Vanilla 

 planifolia 6 . The tendril of Smilax aspera 6 possesses very weak negative helio.tropism, 

 which causes it to circumnutate somewhat more rapidly away from the light than 

 towards it. The reverse is the case in the feebly positively heliotropic tendrils of 

 Passiflora 7 , whereas Darwin could detect no heliotropism at all in the tendrils 

 of Pisutn*. Tendrils, like the stems of twiners, are therefore only feebly helio- 

 tropic. This also applies to the stems of Cuscuta, although when they have been 

 rotated horizontally for some time on a klinostat they become distinctly positively 



1 Peirce (1. c.) observed no coiling in the opposite direction, but Koch (1. c., 1874, P- I2 4) 

 this to occur occasionally. 



2 Peirce, I.e., p. 115. According to Koch (1. c., p. 124), Cuscuta is also able to twine around 

 a horizontal support. 



3 Knight, Phil. Trans., 1812, p. 314; Mohl, Ranken- und Schlingpflanzen, 1827, p. 76; 

 Darwin, Climbing Plants, 1875, p. 144; Wiesner, Die heliotropischen Erscheinungen, 1880, Th. n, 

 p. 38. 



* Darwin, I.e., pp. 86, 103. Beccari (Bot. Jahrb., 1884, I, p. 27) observed that the tendrils of 

 Cissus do not apply themselves to strips of mica, possibly because of the negatively heliotropic 

 action of the reflected light. 



5 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. xv, p. 237. 



6 Darwin, I.e., pp. 118, 184. 7 Id., I.e., p. 153. 



8 Id., I.e., p. 112. Wiesner (I.e., p. 38) finds that the tendrils of Pisum are positively 

 heliotropic in weak light, negatively heliotropic in strong light. Derschau (Einfluss von Contact 

 und Zug auf rankende Blattstiele, 1893, p. 12) finds the petioles of Lophospermum scandens to be 

 fairly strongly positively heliotropic. 



PFEFFER. HI E 



