xviii The Botanical Work of Darwin. 
In my opinion the most striking feature of the book is its 
rehabilitation of Dutrochet’s 1 theory, that vegetable move- 
ments are ‘spontanes, executes a l’occasion de l’influence 
d’un agent exterieur et non des mouvements imprimes par 
cet agent/ This remains a signal service to plant physiology, 
whether or no we accept Darwin’s views as to the role of 
circumnutation. 
The localization of geotropic and heliotropic sensitiveness 
in roots and in certain seedlings is perhaps the most striking 
of the discoveries published in the Power of Movement in 
Plants. By some persons these statements and conclusions 
were received with incredulity or contempt. The proof of 
their substantial accuracy is due to Pfeffer 2 and his pupils, 
Rothert, and Czapek. 
Towards the end of his life my father more than once 
spoke of his physiological researches as being undertaken in 
place of the more trying evolutionary work, for which he felt 
himself too old. This has always struck me as remarkable ; 
I should have believed that an old man in bad health would 
have more easily returned to the work on which he had spent 
his best years ; I should have supposed it more difficult to 
attack a comparatively new subject with new methods and 
new lines of thought. When I remember the amount of labour 
necessitated by the Power of Movement, I am astonished at 
his courage and unflagging energy. His manner of attack 
has been so truly described by Sir W. Thiselton-Dyer, that 
I cannot resist the pleasure of quoting it 3 . £ He turned his 
attention to plants doubtless because they were convenient 
objects for studying organic phenomena in their least com- 
plicated forms ; and this point of view, which, if one may use 
the expression without disrespect, had something of the 
1 Pfeffer was the first to call attention to Dutrochet’s remarkable utterance. In 
my address to Section D of the British Association, 1891, I have expressed my 
appreciation of the importance of Pfeffer’s contributions to the study of the 
irritability of plants. 
2 The most brilliant demonstration of the ‘ brain-function ’ of the root-tip was 
published by Pfeffer in the Annals of Botany, 1894. 
3 Charles Darwin, Nature Series, p. 43. 
