90 



SOCIAL CONJUNCTIVE SYMBIOSIS 



the cambium being periodically renewed outward in the cortex, with 

 the result that concentric zones of wood separated by rings of funda- 

 mental tissue are produced. In many lianas the internal structure 

 shows an eccentric or a flattened arrangement due to the constant 

 lateral pressure against the host. 



The internodes of lianas are usually long, and the longest and 

 largest known tracheae occur in certain climbing stems. Such vessels 

 appear to be correlated with the necessity of transporting water to 

 great heights through a stem that is relatively small. 



Fig. 36.— Wild grape climbing on Crataegus. Social conjunctive symbiosis 



56. Twiners.— Twining plants are perhaps the most specialized 

 of lianas. In these the tip of the shoot executes a rotating movement 

 known as circumnutation. This movement is induced and regulated 

 by the response of the stem tip to the force of gravity and causes the 

 stem to twine about any suitable support. The movement is due to a 

 more rapid growth on one side of the stem. Since the twining of the 

 stem necessarily results in a twisting on its axis the place of rapid 

 growth is constantly changing and migrating around the stem. Each 

 complete revolution of the tip is accompanied by a complete twist 

 on the axis of the stem and the most rapid growth of any given area 

 on the stem is induced when that area is lowermost. 



The direction of twining is specific, that is, with a very few excep- 

 tions, all individuals of a given species twine in the same direction. 



