THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 55 



in attaching themselves to flat or irregular supports, of as much as 3 or 

 4 cms. diameter if these are in close proximity. 



The contact not only produces the attachment to the support and 

 accelerates the coiling but also induces an increase in the strength of the 

 tendril, and in some cases the formation of special growths such as suckers. 

 It is, however, uncertain whether the increase of strength by lignification, 

 or by secondary growth where this occurs, is directly due to the stimulus of 

 contact or is the result of the mechanical demands made upon the attaching 

 organs. A decision is by no means easy, since an increase of pressure at 

 the point of contact not only increases the contact-stimulus but also the 

 mechanical demands made upon the organ, and, further, the stimulus of 

 contact may be transmitted some distance away from the directly stimulated 

 area. It seems indeed that both factors enter into play, for Ewart observed 

 a slight thickening in hook-tendrils allowed to pull against gelatine-covered 

 rods where little or no stimulus of contact could be exercised, and observed 

 in other cases a thickening caused by contact without any appreciable 

 strain being set up in the organ, and that where a tendril was in contact 

 with two supports the thickening was mainly shown at the points in contact 

 and not in the region between ] ; similarly Derschau found that the petiole 

 of a leaf-climber exhibited a slight secondary thickening after temporary 

 contact with a support too light to exert any appreciable stress upon the 

 petiole 2 . 



Hegler's statement that tension in general increases the strength of 

 ordinary stems is incorrect, for Ball 3 , under similar conditions, and in part 

 with the same plants as those used by Hegler, observed in no case any 

 perceptible increase in the tensile strength. It is possible that positive 

 results may be gained with other plants, but further experiment is necessary 

 to determine whether the increase in the tensile strength of attached tendrils 

 without any secondary growth is due to the stimulus of tension, of contact, 

 or to other causes. 



The acceleration of the ultimate coiling of the tendril due to contact 

 is sometimes very pronounced. Thus Darwin found that an attached 

 tendril of Passifiora quadrangular is coiled as much in two days as an 

 unattached one in twelve. The tendrils of Vitis vmifera, Ampelopsis 

 hederacea (guinquefolid) , and of various species of Cissus, only coil when in 

 contact with a support 4 . The same applies to the hook tendrils of Strychnos 

 and to the branches of the tendril of A mphilobium mutisii which are thrown 

 off in the absence of a contact-stimulus 5 . 



The coiling of a free tendril usually begins when growth is reduced to 



1 Ewart, 1. c., pp. 193, 215, 222, 227. 2 Derschau, 1. c., p. 30. 



3 Jahrb. f. wiss. Bot, 1903, Bd. xxxix, p. 305. 



4 Darwin, I.e., p. 125 ; v. Lengerken, Bot. Ztg., 1885, p. 360; Schenck, 1. c., p. 145. 



5 Ewart, I.e., pp. 208, 218. 



