§2] 



THIGMOTROPISM 



379 



long, suspended above a tendril of Echinocystis, caused such a 

 reaction that the tepdril coiled around and fastened to it. 



Concerning the irritable region, it appears that in most ten- 

 drils there is an especially sensitive side and an especially- 

 sensitive zone. The presence of a more irritable side is not a 

 constant character of all tendrils, however. Thus, Cobsea 

 scandens, Cissus discolor, and others, are irritable on all sides. 

 When a differentiation in this respect occurs, however, a bend- 

 ing takes place towards the more irritable side. The sensitive 

 zone lies either at the tip or immediately below.* 



The period of development at which the tendril is most irri- 

 table must also be considered. Thus, Darwin ('82, p. 174) 

 says : " Tendrils which are only three-fourths grown, and, per- 

 haps, even at an earlier age, but not whilst extremely young, 

 have the power of revolving and grasping any object which 

 they may touch. These two capacities . . . both fail when 

 the tendril is full grown." Pfefper ('85, p. 485) and MiJL- 

 LER ('86, p. 104) likewise find tendrils irritable only in the 

 latter part of their growth-period. f 



There are four periods in the response to a momentary 

 stimulus : (1) a latent period elapsing before the curvature 

 begins to take place ; (2) a period of bending ; (3) a period of 

 quiescence in the bent condition ; and, finally, (4) a period of 



Fig. 102. — Curve of contraction of a tendril. Tlie distance of the curve from the 

 base represents the amount of displacement of the tip ; one unit on the base-line 

 represents five minutes of time ; 1 to 2, latent period and period of contraction ; 

 2 to 3, period of maintenance ; 3 to 4, period of relaxation. (From MacDohgai,, 

 '95.) 



* In Cuscuta, according to Peirce ('94, p. 64), the tip is non-irritable ; the 

 most sensitive zone is .3 cm. below the tip. 



t In Cuscuta, Peirce ('94, pp. 63, 64) found that neither the stalk of seed- 

 lings nor of older plants, during the period of rapid growth which follows the 

 formation of haustoria, are sensitive. Irritability shows itself only when 

 growth becomes less rapid, as it does some time after forming haustoria. 



