CIRCUMNUTATION. 



401 



some one direction and temporarily diminished or arrested in 

 other directions," it has been found convenient to discriminate 

 between circumnutation and modified circumnutation. Darwin 

 divides the latter into two classes of movements : (1) those 

 dependent on innate or constitutional causes, and independent 

 of external conditions, except that the proper ones for growth 

 must be present ; (2) those in which the modification depends 

 to a large extent on external agencies, such as the daily alter- 

 nations of light and darkness, light alone, temperature, or the 

 action of gravity. It is plain that such a division cannot be ab- 

 solute ; in fact, numerous intermediate cases are known to exist. 



1045. Methods of observation of circumnutation. For meas- 

 uring the rate and determining the exact direction of the move- 

 ments of circumnutating parts when the parts are small and 

 the movements slight, the following methods described by Dar- 

 win l can be employed in nearly all cases where it is necessary 

 to magnif}- the amount of displacement. 



1 Power of Movement in Plants, 1880, p. 6. 



FIG. 177. Angular movements of a leaflet of Averrhoa bilimbi during its evening 

 descent, when going to sleep. Temp. 78-81 F. The ordinates represent the angles 

 which the leaflet made with the vertical at successive instants. A fall in the curve 

 represents an actual dropping of the leaf, and the zero line represents a vertically 

 dependent position. Each oscillation consists of a gradual rise followed by a sudden 

 fall. (Darwin.) 



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