TENDRILS AND TWINING STEMS 



113 



and root are scarcely a millimeter apart, they respond 

 oppositely. Since the cells of both have been only re- 

 cently derived from the same single cell there must be 

 a very early differentiation of cells in this respect. 

 Likewise, a lateral branch originates within a minute 

 fraction of an inch from a stem-tip and yet shows a 

 response to light and gravity which is different from that 

 of the main stem, since the former grows horizontally 

 and the latter vertically. The response to light is called 

 phototropism. It serves to bring stems into the most 



Fig. 23. — Roots on the Stem of English Ivy, which originated on 

 the dark side next the chimney on which the plant grew. Note that 

 these roots grow away from the light and toward the moist brick 

 chimney. 



favorable position for exposing the leaves to light 

 and air. 



Tendrils and Twining Stems are guided in part by 

 light and gravity, but their special characteristics are 

 chiefly due to their reaction to contact, i.e., thigmotrop- 

 ism. A twining stem in the course of its growth causes 

 the tip portion to swing around in a circle. Contact with 

 a suitable support stimulates growth in such a manner 

 that the coils of the stem are wound about it. 



