552 History of the Doctrine of [BOOK in. 



point was, that his full investigation of the subject showed how 

 such phenomena must be studied, if we are to arrive at a 

 strictly mechanical explanation of them. 



If von Mohl had attempted to give a mechanical explanation 

 of the processes in the tissue of twining organs he must neces- 

 sarily have failed from ignorance of the agency of diffusion, 

 which must certainly be taken into consideration. This agency 

 was not discovered by Dutrochet till the year (1826) in which 

 von Mohl undertook his investigation, and some time elapsed 

 before it was sufficiently understood to be successfully applied 

 to the explanation of phenomena in vegetation. Dutrochet 

 did indeed attempt so to apply his theory in 1828, and showed 

 that changes in the turgidity of tissue are produced by endos- 

 mose and exosmose, and consequently that a new mechanical 

 method of explanation had been discovered for processes 

 which had been usually referred to a supposed vital principle ; 

 but in his later and more detailed researches into geotropism, 

 heliotropism, periodical movements and movements of irrita- 

 bility, which he collected together in his ' Memoires ' of 1837, 

 he fell into two different mistakes : he assumed conditions of 

 size and stratification in cells which do not actually exist, for 

 the purpose of explaining very various kinds of curvature by 

 endosmose, and he was not satisfied with endosmose in the 

 parenchyma ; he postulated changes in the vascular bundles 

 also, which were supposed to be produced by the influence of 

 the oxygen in a way which he did not explain. Thus there 

 were blots in his explanation of separate processes, and his 

 mechanical theories remained unsatisfactory ; but it is worthy 

 of recognition and was most important for the development of 

 phytodynamics, that he was thoroughly in earnest in his pur- 

 pose of explaining every movement in plants by mechanical 

 laws. Even the opponents of such explanations were obliged 

 to go deeply into mechanical relations in order to refute him, 

 and no one could any longer be imposed upon by the simple 

 assertion that all depends on the vital force ; so devoted 



