ADJUSTMENT TO GRAVITY, CONTACT, AND SHOCK 141 



16 1. Response to contact. Many climbing plants have 

 developed organs called tendrils, by means of which they cling to 

 their supports. Tendrils are characteristic of climbers, Micram- 

 pelis, Sicyos, Vitis, etc., in contrast to the twiners, such as the 

 bindweed, morning-glory, etc. Tendrils are specialized branches 

 or leaf parts which have become especially sensitive to contact. 

 Similar sensibility in a less degree seems likewise to occur in the 

 stems of twining plants. The growing point of roots is also sensitive 

 to contact, and is consequently enabled to pass a hard substance 

 which lies in its path. 



The sensory area of a tendril is usually restricted, though in 

 a few cases it extends over the major portion of it. As a rule, 

 the basal portion is scarcely or not at all sensitive, and the sensory 

 area is confined to the concave side of the curved or hooked tip. 

 A few tendrils respond to contact upon any side, while others respond 

 only to lateral contact, in addition to that upon the lower side. 

 Generally speaking, they are not sensitive when either very 

 young or very old, but react only while they are growing and 

 showing circumnutation, i.e., the constant movement of the 

 tip in a circular manner. The, value of circumnutation in bringing 

 the tip in contact with a possible support is evident. When 

 the sensory area is brought into contact with a stimulus of the 

 proper kind, the tip begins to curve. The curving may begin in 

 less than a minute after the contact, or several hours may elapse 

 before it becomes evident. Sensitive tendrils respond to a 

 momentary slight touch, but for the majority a stronger stimulus 

 is necessary. The size and surface of the support have much to 

 do with the presence and nature of the response. 



In certain plants the stimulus affects only the point of contact, 

 but in the majority of cases it is also transmitted, and results in 

 the spiral coiling so characteristic of many tendrils. This reaction 

 usually begins near the sensitive tip and travels toward the base. 

 Its advantage is evident in that it lifts the stem and serves to hold 

 it firmly, but not so rigidly that it may be easily torn away. The 

 curvature which produces attachment as well as the spiral coil 

 is the result of unequal growth. The cells of the sensory side 

 either do not elongate at all after contact, or they do so more 

 slowly than the cells of the opposite side. The coil becomes 

 fixed in consequence of the development of the tissues into a 

 more permanent form. 



