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, published 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 neglected, and because, while containing other instructive matter, it refuted the theory, founded on endosmose and on the structure 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 importance of the fact that all the various phytodynamical phenomena, 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 movements 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 Nägeli and von Mohl discovered in 1846, that it is in the protoplasm 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
