242 PLANT PHYSIOLOGY 



of the tendrils. They are most sensitive when they are 

 half or three quarters grown and until after they have at- 

 tained their full length. A considerable degree of warmth 

 and humidity increases their sensitiveness, warmth being 

 the more important factor. Most tendrils are more sen- 

 sitive on one side than on the others. The part most 

 sensitive to contact is that between the tip and middle, 

 the tip itself and the base not being sensitive to contact. 

 The base is, however, sensitive to gravity and in this re- 

 spect differs from the rest of the tendril. The zones of 

 greatest sensitiveness and of greatest growth, as in roots 

 (see p. 201), are not coincident. 



A tendril of P&ssiflora in the right stage of growth and 

 under favorable conditions will soon bend toward the 

 side touched, if gently stroked with a pencil. The extent 

 and duration of the bending will be proportioned to the 

 degree of irritation. The rougher the surface of the object 

 which the tendril touches, the more pronounced the bend- 

 ing. Because the surface of tendrils is so smooth, they so 

 slightly irritate each other when they chance to touch that 

 they rarely bend about each other. A succession of light 

 touches, each too light to induce a pronounced or per- 

 manent bending, will stimulate a tendril to the same degree 

 as continuous contact with the same object, but unless the 

 contacts are made at very short intervals, so that the ten- 

 dril finally catches and entwines the stimulating object, 

 thereby producing an enduring contact and a prolonged 

 stimulus, the tendril will cease to bend more and will finally 

 straighten out. Presumably loss of sensitiveness precedes 

 loss of ability to bend, but since the reaction is the only 

 evidence we now have of the sensitiveness, it is impossible 

 to say positively. This experiment shows, among other 

 things, that there is an accumulation of stimuli, that the 

 response is not immediate but induced, and that the ten- 

 dril finally ceases to respond to one degree of inconstant 

 stimulation, becoming either accustomed and unsensitive 

 or else fatigued and unresponsive. This condition has its 

 parallel in the familiar state of the fatigued muscle or 

 organism. A fresh horse lightly touched by the whip re- 



