74 The Science of Life. 



of position in flowers and leaves. Soon afterwards the 

 number of known cases of plant-movement was con- 

 siderably increased. 



The degeneracy of vegetable histology towards the 

 end of the eighteenth century, and the dominance of 

 the vital-force theory, combined to hinder further pro- 

 gress in regard to the movements of plants. A return 

 to scientific method, however, was well marked in the 

 experiments of Andrew Knight (1758-1838), an English 

 horticulturist and worthy successor of Hales. He 

 showed that the upward growth of stems and the down- 

 ward growth of roots were opposite reactions to the 

 same stimulus "the force of gravitation"; that when 

 germinating plants were grown on a revolving wheel 

 the radicles were directed outwards, in the direction of 

 the "centrifugal force", and the young stems inwards; 

 that the stimulus supplied by moist earth may affect 

 roots more strongly than that of gravity; that the ten- 

 drils of the vine and Virginian creeper grow away from 

 the light (negative heliotropism); and so on. 



In 1827, while still a young student, Von Mohl pub- 

 lished a remarkable essay on tendrils and climbing 

 plants, "the best that appeared on the subject before 

 Darwin wrote upon it in 1865 " (Sachs); Dutrochet 

 extended Knight's experiments with the rotating wheel, 

 and attempted to apply his theory of diffusion to the 

 phenomena of movement; and Briicke in 1848 made a 

 classic research on the sensitive plant, distinguishing 

 the periodic movements from the responses to casual 

 stimulus, and attempting an analysis of both in terms of 

 tension and turgidity. These and other investigations 

 were of much interest, yet Sachs ends his historical 

 survey by remarking that " scarcely any point of funda- 

 mental importance in phytodynamics was cleared up 

 before 1860". 



There is no greater name in the history of modern 

 botany than that of Julius von Sachs (1832-97), and 



Sachs. ke k as probably had a wider influence than 



any other. Not only have many of the most 



prominent living botanists sat at his feet, but his books 



have brought us all into touch with him. He was 



