248 Charles Darwin. 



all plant life. Such modificatioh>-ai;e always in the 

 direction of the plant's advantage, anH^may be so 

 great as to become difficult of recognition as forms 

 of circumnutation. 

 / I need not labour to convince you that any modi- 

 / fication which is an advantage to the plant will be 

 / secured by the process of natural selection. It is the 

 f glory of the great genius whose labours we are here 

 \ to commemorate to have demonstrated this truth to 

 \ the entire satisfaction of the united scientific world. 

 Darwin has actually solved the great problem of 

 phytology, so long supposed to be incapable of solu- 

 tion, viz.: Why does the root grow downward and 

 the stem upward ? Briefly and roughly stated, the 

 answer to this question is that, as the bursting seed 

 pushes out its two germinal points these circumnu- 

 tate from the first, and thus explore their surround- 

 ings for the means of benefiting the plant. To 

 employ Darwin's own word, they "perceive" the 

 advantage that would result from the penetration of 

 the soil on the one hand, and from the ascent into 

 the free air and sunlight on the other, and through 

 the pre-Darwinian law of the " physiological division 

 of labour," the one becomes geotropic and the other 

 heliotropic — the one develops into a radicle and 

 then into a root, while the other develops into an 

 epicotyl and then into a stem. 



I will only add to the thoughts already presented, 

 that Darwin's discovery of the existence in all plants 

 of an innate and spontaneous mobility belonging to 

 them as forms of organic life possesses an important 

 ulterior significance. 



