TWINERS AND OTHER CLIMBING PLANTS 



313 



Young, actively growing tendrils nutate, so as to become hook-shaped. 

 When the tendril comes into contact with a solid object coiling begins near the 

 tip, so that it wraps itself about the support, if this is of suitable size and shape. 

 This coiling movement results from unequal growth on the two opposite sides, 

 brought about as a response to the stimulus of contact. The portion of the 

 tendril that lies between the point of attachment and the base does not remain 

 permanently straight but subsequently also becomes 

 coiled, in the form of a spiral spring, so that the 

 plant is drawn nearer to the support. The stem 

 is thus not supported by a straight and inelastic 

 suspension, which might be easily broken, but it 

 hangs upon a coiled spring, which stretches a little 

 when the plant is moved by the wind, thus largely 

 avoiding the possibility of breaking. The tendril 

 can become attached to a support only while still 

 growing, and those that have not come into con- 

 tact with a suitable support during the growing 

 period generally wither and fall away. Fig. 155 

 represents a portion of the stem of Bryonia dioica, 

 with a tendril attached to a twig of another plant. 

 The middle portion of the tendril forms the spiral 

 spring mentioned above. 



The tendrils of Ampelopsis behave in a peculiar 

 manner when they happen to come into contact 

 with an object about which they cannot twine, as in 

 growing along a wall, for example. Some of the 

 tendrils, which are appressed against the wall by a 

 negative phototropic response and which continually 

 nutate, happen to reach into crevices in the support. 

 Such a tendril becomes thickened at the end within 

 the crevice, so that it cannot be readily withdrawn, 

 thus supporting the plant. [These tendrils also 

 form adhering disks at their tips, by which they 

 become attached to nearly smooth surfaces, 

 (Fig. 156.)] 



Investigations of the anatomy of tendrils show that these possess special 

 arrangements that facilitate the reception of stimuli. The otherwise thick 

 external walls of the epidermis of pumpkin (Cucurbita) tendrils, for example, are 

 characterized by minute pits that extend the cell cavity outward, nearly to the 

 outer surface of the wall, which is very thin at these points (Fig. 157, A). 

 These pits are filled with protoplasm which is continuous with the protoplasm 

 of the cell cavity itself, and these protoplasmic projections frequently contain 

 small crystals of calcium oxalate, which have been thought to play a part in 

 the propagation of the contact disturbance. These structures have been called 

 contact papilla. Such papillae, of the external walls of the epidermis of a pump- 



Fig. 155.- 



Tendril of Bryonia 

 dioica. 



