HABITS OF CLIMBING PLANTS. 163 



No differences in this regard are observable in the behavior 

 of exogenous or endogenous stems, or even of those of climb- 

 ing Ferns. Lygodium scandens^ according to Darwin, re- 

 volves like other twiners ; it completes its revolutions in six 

 hours, or on a very hot day in five (moving against the sun, 

 which is much the commoner case) ; this is about the average 

 rate of Phaenogamous twiners, like which it comports itself 

 in all respects. Our own X. j^cd^natum, we find, revolves in 

 the same way, in about four hours, the temperature being 75*^ 

 Fahr. 



The power of revolving depends, of course, upon the gen- 

 eral health and vigor of the plant, and upon the age of the 

 shoot, is retarded by lowering the temperature, is interrupted 

 by any considerable disturbance, such as exposure to cold or 

 to much jarring ; carrying the plant from one place to another, 

 or cutting off a shoot and placing it in water, stojDS the move- 

 ment for a time, just as it does the more vivid automatic 

 movement of Desmodium gyrans. But each internode is so 

 independent that cutting off an upper one does not affect the 

 revolutions of the one beneath. Twining stems are far from 

 being insensible to the action of light (as Mold supposed), 

 the half -re volution toward the light being not uncommonly 

 twice faster than that from it ; but as the rate of revolution 

 by day and by night is nearly the same, the one half of the 

 circuit is accelerated just as much as the other is retarded. 

 This influence of the light is quite remarkable when we con- 

 sider the slenderness of most revolving internodes, the small 

 surfaces they expose, and that their leaves are little developed. 



The design, as we must term it, of this revolving of the end 

 of twining stems is obvious, and usually effectual. Such 

 stems, even when no supporting object is within their reach, 

 will reach each other, and by twining together make a mutual 

 support, from which, as they lengthen, they may reach yet 

 farther. The connection of the revolving with twining is 

 obvious, though the latter is not a necessary consequence ; for 

 many stems revolve which do not twine, but climb in some 

 other way. 



^' When at last the [revolving] shoot meets with a support, 



