470 PHYSIOLOGY 



tendrils of climbing plants exhibit the most remarkable sensitiveness to 

 mechanical stimuli, and it is by this means that their attachment to 

 supports is secured. Tendrils are slender, even threadlike, lateral or- 

 gans, branched or not, sometimes occupying the usual place of a branch, 

 sometimes that of a leaf or of one or more leaflets of a compound leaf. 

 They are therefore formed successively with the development of the 

 main axis and its chief branches, so that the plant is constantly laying 

 hold of a support by younger and younger tendrils. It may thus climb 

 to great heights, while the main axes remain very slender and wholly 

 unable to support their own weight, much less that of foliage, flowers, 

 and fruit. The most important feature of the tendril is its irritability 

 to contact, and the curvatures which follow as end reactions. 



Behavior. — When a tendril is young and only about one fourth grown, 

 it may be either straight or curled up into a loose spiral, of which the 

 convex surface corresponds to the under side. If coiled, it unrolls as 

 its period of rapid growth begins, at which time also begin nutating 

 movements that are almost as regular as the revolving movements just de- 

 scribed in the twiners. These tendril movements, however, are not due to 

 any known external stimulus, but must be called at present autonomic. 

 The tip is thereby swung in all directions and is thus likely to come into 

 contact with some suitable support. When it does so, it quickly wraps 

 around it. After a time, through continued and unequal growth in 

 length, spiral coils appear in the region between the axis and the attached 

 part, increase in number and closeness, and become more and more firm, 

 until this part has become a veritable spiral spring by which the plant 

 is slung to its support. These results are attained in the following way : 



Stimulus : friction. — The tendril is sensitive to contact, usually 

 throughout its whole length, and on all sides, but most so towards the 

 tip. Yet it is not sensitive to contact in the narrow sense; it is because 

 things come into contact with the tendril in more than one place when 

 they touch, so that it is only by multiple and successive contacts, and 

 usually by shifting contact or friction, that the tendril is excited. Liquids 

 (even the heaviest, mercury), if entirely free from solid particles, and 

 perfectly smooth solids, like gelatin, do not produce excitation. Rain, 

 therefore, does not cause useless movements of tendrils. But very slight 

 rubbing movements of excessively light objects suffice to start them. 

 It has been found, for example, that a bit of thread, weighing by estimate 

 only 0.00025 mg., if moved by the wind over a very sensitive tendril, will 

 induce curvature. 



