364 



LECTURE XV. 



and further, the rate is not uniform for any given plant as the 

 above table shews. The variations in the rate of revolution of 

 any one plant are doubtless to be attributed to some extent 

 to variations in external conditions; but it appears from Dar- 

 win's observations that the rate of revolution is generally 

 slower at the commencement of circumnutation than it is sub- 

 sequently. 



The following diagram will serve to illustrate circumnutation. The 

 small upper circle divided into segments represents an ideal transverse 



S 



FIG. 41. 



section of the region of nutation of a stem exhibiting circumnutation, 

 The large circle below represents the orbit of circumnutation as seen 

 from above, and the small circles upon it represent different positions of 



