MOVEMENTS OF CURVATURE 



and cortical cells at the point of contact, and in Sicyos angulatus and 

 a few other Cucurbitaceae the fixation is aided by a viscid secretion \ 



The physiologically radial stems of Cuscuta europaea, C. epilinum, and 

 of Cassytha twine like typical climbers, and in addition coil and produce 

 haustoria as the result of contact 2 . When this has occurred and a few 

 close coils with haustoria have been formed, the acropetal portion of the 



stem loses its contact irritability for a 

 time, and a few much steeper coils are 

 formed by circumnutatory coiling. These 

 coils are often loose and form no haustoria. 

 If, however, no support is found, the new 

 growths of the stem of Cuscuta remain 

 continually sensitive to contact, which 

 shows that it is the satisfaction of the 

 desire for contact which causes the periodic 

 inhibition of the contact irritability. 



In addition the stimulus of gravity is 

 necessary to maintain the irritability of 

 Cuscuta^ for on a horizontally rotated 

 klinostat not only the circumnutation but 

 also the power of responding to contact 

 disappear, while after three days' rotation 

 the irritability only returns after twenty- 

 four hours' rest under normal conditions 3 . 



It is uncertain whether in other cases the 

 stimulus of gravity may be necessary to maintain 

 contact irritability, for typical tendrils as well as 

 the hyphae of Phycomyces appear to remain 

 irritable when rotated on a klinostat. Whether 

 this also applies to the feebly irritable stems of 

 the petiole-climber Lophospermum scandens, 

 which rarely coils in nature, is unknown 4 , 

 although many instances have been found in 

 which the sensitivity and power of reaction 



FIG. 16 Cuscuta epiiinum on i m patitns 



'"' 



are more or less dependent upon geotropic induction. 



1 Miiller, Cohn's Beitrage z. Biol., 1887, Bd. iv, pp. 107, 123, &c. ; Schenck, I.e., p. 200. 



2 First observed by Mohl ^Ranken- u. Schlingpflanzen, 1817, p. 131) ; further studied by Koch 

 (Hanstein's bot. Abhandl., 1874, Bd. n, p. 121; Die Kleeseide, 1880), and fully explained by 

 Peirce (Annals of Botany, 1894, Vol. vin, p. 53). 



3 Darwin's statement (1. c., p. 100) that the tendril of Echinocystis lobata becomes straight and 

 non-sensitive when there is danger of contact with its own shoot requires further proof. The power 

 of discrimination by which Darwin supposed certain tendrils to be able to avoid coiling around one 

 another does not actually exist, the absence of such coiling being due to the slenderness, pliability, 

 and smoothness of the tendrils, combined with their circumnutation movements. (Cf. Ewart, 1. c., 

 pp. 224-7 ; and Pfeffer, Unters. aus dem bot. Inst. zu Tubingen, 1885, Bd. I, p. 495.) 



4 Darwin, Climbing Plants. 



