MOVEMENT IN PLANTS. 245 



a fresh stimulus which again induces incurving and brings 

 about a permanent spiral coiling round the support. 



(b) When the spiral on the support becomes slack, cur- 

 vature takes place in the part of the tendril just below the 

 support, so that this part comes into contact with and 

 surrounds the support, pushing in front of it the previously- 

 formed but now loose coils. To demonstrate this, when a 

 tendril has made a single coil round the support, make ink- 

 marks on (1) the tip of the tendril, (2) the part of the 

 tendril vertically below this, (3) the support at the point 

 corresponding to (2), and (4) a point on the tendril at a 

 distance of 1 or 2 cm. from the support. After a few hours 

 the point marked at (4) will be found to be in contact 

 with the support. 



338. Changes in Tendril after Attachment. After 

 the completion of the permanent coiling, growth in length 

 stops, and there appears not only in the coils but also 

 in the rest of the tendril a number of changes. 



(a) A spiral twisting occurs in the basal region, whereby 

 the stem is drawn closer to the support. This spiral 

 changes its direction at least once, and that this reversal is 

 due to purely mechanical causes may be demonstrated by 

 fixing a strip of Dandelion stalk at both ends and placing 

 it in water, or by trying to produce a spiral coiling in 

 a piece of rubber tubing fixed at both ends. 



(6) As a rule, marked secondary thickening, accompanied 

 by the development of sclerenchyma, appears not only 

 in the part clasping the support, but also in the basal 

 portion of the tendril. Compare transverse sections of 

 (1) a tendril which has not yet clasped a support, (2) a 

 tendril of the same plant after having made several coils 

 round a support. 



339. Tendrils with sticky Pads. Observations 'should be 

 made on the Self-clinging- Virginian Creeper, which differs 

 from most species of Ampelopsis (usually merged in Vitis, to which 

 the Vine belongs) in that its branched tendrils become attached by 

 means of sticky pads at the tips of the branches. 



