84 BIOLOGICAL SOCIETY OF WASHINGTON. 



subject of the most absorbing interest, and of which you have 

 already listened to so able a presentation by Prof. Riley from the 

 point of view of the entomologist. 



But Darwin's great service has been to show that these varieties 

 of activity are simply modes in which inherent arid spontaneous 

 activities manifest themselves under these varying external influences. 



His preliminary investigations into the nature of these innate 

 powers of movement were directed to that large class of plants 

 known as twiners and climbers, whose revolving motions were so 

 thoroughly described in his work on " Climbing Plants." It was 

 here that he laid the foundation for those later studies which 

 eventually resulted in that great work, almost his last, on the 

 "Power of Movement in Plants." In this work he demonstrates 

 by an enormous induction that the ample sweeps of the twining 

 plant are but the most obvious manifestations of a class of 

 phenomena which are common to the entire vegetable kingdom. 



Amid the varied forms of movement which plants present Darwin 

 has succeeded in finding one fundamental and generic one to which 

 every other may be referred. To this universal form of plant 

 activity he gives the name " circumnutation." Not only twining 

 stems and tendrils, but parts of flowers, tips of growing shoots, 

 caps of penetrating roots and rootlets, radicles, epicotyls, cotyledons, 

 and even full-grown leaves, are incessantly describing circles, 

 ellipses, and other more or less regular geometrical figures ; and he 

 conclusively shows that it is out of this primary form of activity 

 that all the more specialized forms already mentioned have been 

 developed. All movements of the parts of plants are thus to be 

 interpreted as modified forms of this innate periodic circumnutation 

 which is common to all plant life. Such modifications are always 

 in the direction of the plant's advantage and may be so great as to 

 become difficult of recognition as forms circumnutation. 



I need not labor to convince you that any modification which is 

 an advantage to the plant will be secured by the process of natural 

 selection. It is the glory of the great genius whose labors we are 



