204 MODIFIED CIRCUMNUTATION. CHAP. V. 



We shall now consider the many cases in which 

 eircumnutation has been modified for various special 

 purposes ; that is, a movement already in progress is 

 temporarily increased in some one direction, and tem- 

 porarily diminished or quite arrested in other direc- 

 tions. These cases may be divided in two sub-classf s ; 

 in one of which the modification depends on innate or 

 constitutional causes, and is independent of external 

 conditions, excepting in so far that the proper ones for 

 growth must be present. In the second sub-class the 

 modification depends to a large extent on external 

 agencies, such as the daily alternations of light and 

 darkness, or light alone, temperature, or the attraction 

 of gravity. The first small sub-class will be considered 

 in the present chapter, and the second sub-class in the 

 remainder of this volume. 



THE CIRCUMNUTATION OF CLIMBING PLANTS. 



The simplest case of modified eircumnutation is that 

 offered by climbing plants, with the exception of 

 those which climb by the aid of motionless hooks or 

 of rootlets : for the modification consists chiefly in the 

 greatly increased amplitude of the movement. This 

 would follow either from greatly increased growth over 

 a small length, or more probably from moderately in- 

 creased growth spread over a considerable length of the 

 moving organ, preceded by turgescence, and acting suc- 

 cessively on all sides. The eircumnutation of climbers 

 is more regular than that of ordinary plants ; but in 

 almost every other respect there is a close similarity 

 between their movements, namely, in their tendency 

 to describe ellipses directed successively to all points 

 of the compass in their courses being often inter- 

 rupted oy zigzag lines, triangles, loops, or smal] 



