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 ne- glected, 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 1 7 72, 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. The^ could not in fact find their place in the general system until N�geli 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