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TEXTBOOK OF PLANT PHYSIOLOGY 



In climbing plants, these movements are especially marked. The 

 clasping of a support is a tropism, or the acceleration of growth on a 

 definite side of the tendril, in this case of the external, more convex 

 side, in response to an irritation. The stimulus is of the nature of 

 contact or scraping, and is in many ways similar to irritation by 

 touch in animals. The response to it on the part of the plant goes 

 by the name of thigmotropism. 



The thigmotropic sensitivity of tendrils is extremely high. 

 A transient touch with the thinnest hair, weighing no more than 

 a thousandth part of a milligram, is sufficient to produce an appre- 



Fig. 137. Tip of the stem of Bryonia with tendrils: (1) a tendril coiled into a 



spiral; (2) a tendril in a condition of highest sensitivity; (3) a tendril clasping the 

 support and pulling the stem; (4) a tendril not attached to a support and there- 

 fore dying (after Pfeffer) . 



ciable bending of the tendril. In this instance the presentation 

 time is also relatively short and the tendril continues curving after 

 the removal of the object which caused the irritation. It is of 

 interest to note, however, that in order to produce irritation a con- 

 tact with a solid body is absolutely necessary. Drops or streams 

 of water and even of quicksilver do not produce any thigmotropic 

 curvatures. 



If the stimulus is weak and of short duration, the curvature 

 formed is again straightened out by the force of autotropism. 

 If the support remains in place, new parts of the tendril come into 

 contact with it in the process of clasping round the support and the 



