THE MOVEMENTS AND SENSITIVITY OF PLANTS 335 



irritation is increased. Finally, the tendril may coil several times 

 around the support, and thus become firmly attached to it. Then 

 it rapidly begins to grow in diameter, at the same time forming a 

 considerable amount of mechanical tissue which is never pro- 

 duced in a tendril that has not come in contact with a support. 

 Thus, an irritation caused by contact not only produces a curva- 

 ture, but the formation of new tissues as well. As soon as the 

 clasping of a support has been completed, the rest of the tendril 

 begins coiling into the shape of a corkscrew, becomes thicker, and 

 draws the stem to the support. 1 



The capacity to perceive a stimulus is localized almost exclu- 

 sively on the concave or the so-called ventral side of the tendril. 

 A touch on the dorsal side will produce no curvature. Apparently, 

 there are present special cells, provided with sensitive pores, which 

 serve the purpose of perception. Such a pore consists of a small 

 canal in the external membrane of the cell. This canal is filled 

 with protoplasm. It is supposed that an external pressure is most 

 readily perceived, where the membrane is thinnest. Similar cells, 

 however, have not been found in all plants, consequently, they do 

 not seem to be indispensable structures for thigmotropic sensitivity. 



Twining plants do not have sensitive tendrils. They attach 

 themselves to supports by means of quite a different mechanism. 

 Not having any special organs of grasping, they twine around the 

 support with their whole stem. The latter is characterized by 

 very long internodes and a retarded development of leaves. The 

 tip of the stem is therefore stretched into the form of a com- 

 paratively long lash. The upper part of the stem acquires a hori- 

 zontal position and slowly rotates on the lower upright part, thus 

 describing wide circles. These circular movements are of the 

 nature of autonomic nutations, similar to the seeking movements 

 of tendrils, but the grasping of the support by twining plants 

 depends on other causes. On meeting with a vertical or inclined 

 support, the stems of twining plants simply continue their rotating 

 movement and thus finally find themselves twined around it. 

 No thigmotropic irritation, with a subsequent unilateral growth 

 and thickening, has been observed in twining plants. 



The horizontal position of the tip of twining plants is due to 



1 The coils of the spiral are reversed at the center, which obviates the 

 necessity of rotating either plant or support to make the spiral. 



