558 History of the Doctrine of [BOOK in. 



the clear account given of the tension which is produced 

 between the vascular bundle and the turgescent layer of 

 parenchyma, and the reference of the periodic movements 

 and of those of irritation to the movements of water in the 

 antagonistic masses of parenchyma. The details were still 

 imperfect, but one great advantage was secured, namely, the 

 doing away with the mysticism associated with the idea of 

 irritability, from which even von Mohl was not entirely free. 



A full enquiry into the downward curvature of roots, pub- 

 lished by Wigand in 1854, deserves mention, because it threw 

 some light on the theory of the strictly mechanical questions 

 connected with a subject which had been for some time neg- 

 lected, and because, while containing other instructive matter, 

 it refuted the theory, founded on endosmose and on the struc- 

 ture of tissue, which had been suggested by Dutrochet and 

 adopted by von Mohl, since it showed that one-celled organs 

 also exhibit geotropic curvatures. The great theoretical im- 

 portance of the fact that all the various phytodynamical phe- 

 nomena, with the exception of movements of irritability, are 

 manifested in one-celled organs, was not fully understood till 

 after 1860. 



It has been already observed, that no theoretical result was 

 obtained from the discovery of circulation in cells made by Corti 

 in 1772, and repeated by Treviranus in 1811. The same may 

 also be really said of the later observations of Amici, Meyen, 

 and Schleiden, which went to show that such movements occur 

 very generally in vegetable cells. In like manner the move- 

 ments of swarm-spores, of which a considerable number of 

 instances had been observed before 1840, were rather the 

 subject of astonishment than of scientific consideration. They 

 could not in fact find their place in the general system until 

 Nageli and von Mohl discovered in 1846, that it is in the pro- 

 toplasm that the so-called movement of the cell-sap takes place, 

 and Alexander Braun made it known in 1848 that the swarm- 

 spores are naked masses of protoplasm, and indeed true 



