IRRITABILITY. 487 



When, now, a tendril is brought into contact with a sup- 

 port, either by its circumnutation or by the wind, in such a 

 way that its irritable surface is touched, it begins to curve 

 round the support. As it does so, new areas of the irritable 

 surface are stimulated, and the curvature increases and extends 

 until the whole of the tendril lying between the original 

 point of contact and the apex is wound in a spiral round the 

 support. In some few cases, which are naturally also those 

 in which there is no transmission of the stimulus, this is all 

 that happens. In the great majority of cases, however, this 

 coiling of the apical portion of the tendril round the support 

 is followed by the spiral coiling of some part of that portion 

 which lies between the point of contact and the insertion of 

 the tendril upon the stem, provided that it is mechanically 

 possible for the spiral coiling, which necessarily involves con- 

 siderable shortening, to take place. It cannot possibly take 

 place, for instance, when the stem bearing the tendril and the 

 support around which its apical portion has twined are both 

 immovably fixed. The turns of the spiral coiling are not, 

 however, all in the same direction : they are grouped into two 

 or more spires, separated by short straight portions, the turns 

 of any two successive spires being in opposite directions. This 

 is a mechanical necessity of the spiral coiling of a filament 

 attached at both ends. 



As a rule, according to Darwin, the spiral coiling usually 

 begins in the attached tendril close to the support and then 

 travels downwards towards the base. If, however, the tendril 

 is very slack, the unattached portion becomes at first flexuous 

 throughout nearly its whole length, and then it gradually 

 becomes spirally coiled. 



The especial physiological interest of this spiral coiling of 

 the free portions of tendrils is that it offers a striking example 

 of the transmission of a stimulus : the free portion is stimu- 

 lated to coil spirally by the stimulus transmitted from the 

 portion which had coiled round the support. Its general 

 biological importance is also great. By the spiral coiling of 

 the tendrils the stem bearing them is raised, and is held firmly 

 in position, but not rigidly, for the spirally coiled tendrils act 



