74 Chapters in Modern Botany CHAP. 



Darwin's Theory of Modified Circumnutation. 

 Darwin's observations led him to conclude that every 

 growing part of every plant is continually moving, though 

 often on a small scale. And as the most prevalent form 

 of movement is like that of a climbing plant, which bends 

 successively to all points of the compass, so that the tip 

 revolves, Darwin applied to it the term circumnutation. 

 This term he explains as follows : "If we observe a cir- 

 cumnutating stem, which happens at the time to be bent, 

 we will say towards the north, it will be found gradually to 

 bend more and more easterly until it faces the east ; and 

 so onwards to the south, then to the west, and back again 

 to the north. If the movement had been quite regular the 

 apex would have described a circle, or rather, as the stem 

 is always growing upwards, a circular spiral. But it gener- 

 ally describes irregular elliptical or oval figures ; for the 

 apex, after pointing in any one direction, commonly moves 

 back to the opposite side, not, however, returning along 

 the same line." 



In this "universally present movement" Darwin found 

 " the basis of groundwork for the acquirement, according 

 to the requirements of the plant, of the most diversified 

 movements." His particular thesis was that the great 

 sweeps made by the twiners and by tendrils, the move- 

 ments of leaves when they go to sleep at night, the move- 

 ments of various organs to the light or away from it, and 

 even the movement of stems towards the zenith and of 

 roots towards the centre of the earth, are all modified 

 forms of circumnutation, which is omnipresent while growth 

 lasts. In regard to the two last sets of movement he 

 believed, of course, in the influence of light and gravitation, 

 but he regarded these as simply operating upon spon- 

 taneous processes of circumnutation, hastening, diminish- 

 ing, or otherwise modifying them. While maintaining this 



