MODIFIED CIRCUMNUTATION. 407 



1059. An}' given twiner generally twines in one direction 

 only ; for instance, the hop moves in the direction of the hands 

 of a watch, or to use another expression, follows the sun ; the 

 Morning Glory moves in an opposite direction. But there are 

 some cases in which the direction of twining is reversed even 

 during a comparatively short distance. In the tropics this 

 reversal is said to be common. 1 



1060. The time required for the revolution of a twiner varies 

 in different plants, and is by no means constant for the same 

 plant at different stages of its development. In the case of the 

 Morning Glory, the average time required for the revolution of 

 a thrifty shoot under favorable conditions is about three hours. 



1061. Twiners are affected somewhat by the amount of light 

 received, but the revolving goes on uninterruptedly night and 

 day. The increase of rate when a revolving shoot is approach- 

 ing a window may be equal to a tenth, or somewhat more, of the 

 whole period of the revolution. Such acceleration is very differ- 

 ent for different plants. 



1062. Modified circnmnntation. The effect of the influence of 

 light in increasing the rate of movement in a twiner is a good 

 example of a large class of modified movements. These move- 

 ments have already been considered in the chapter on "Growth," 

 under the terms Heliotropism, Geotropism, etc., but must be 

 again referred to in connection with the universal movement, 

 circumnutation. When it is desirable to free any circumnu- 

 tating part from the influence of a disturbing factor, for instance 

 light, great care must be taken to avoid subjecting it to abnor- 

 mal conditions such as result when a seedling is kept in the 

 dark in order to free it from the influence of light on its 

 movements. When so kept it undergoes changes of form with 

 its blanching, and therefore little security is felt that all its 

 behavior is normal. In the instance of green plants which 

 demand light for their health}' activity the removal of disturbing 

 factors is a task of considerable difficult} 7 . 



A part of the difficulty is removed by the use of some instru- 

 ment by which the plants can be made to revolve slowly in a 

 given plane, thus exposing the different sides successively to 

 the action of the force. A simple form of this appliance is 



. l Fritz Miiller is quoted by Darwin as saying, that the stem of Davilla 

 twines indifferently from left to right or from right to left ; and that he once 

 saw a shoot which had ascended a tree about five inches in diameter reverse 

 its course. 



