vegetable cells. A new substratum for the movements in plants, and one of the simplest kind, was thus obtained; and
Nägeli attempted in 1849 a mechanical explanation of the
movements of swarm-spores, while in 1859 De Bary exhibited
in the Myxomycetes most instructive examples of such movements. If Nägeli failed in his attempt, yet it seemed possible
that the protoplasm had an important share in the production
of all phytodynamic phenomena, and the idea appeared
capable of a very wide application when linger pointed out
in 1855 the resemblance between vegetable and animal protoplasm. It is true that not one of these later observations
led to any conclusive results till after 1860 ; but that the whole
subject of phytodynamics had made considerable advance as
early as 1850 is apparent from the account given of it by von Mohl in his 'Vegetabilische Zelle' of 1851, and by Unger in his 'Lehrbuch der Anatomie und Physiologic der Pflanzen' of 1855. Von Mohl chiefly exposes the unsatisfactory nature of the attempts that had been made to explain the phenomena; Unger, on the other hand, shows how much that is fundamentally important had been already established.
The mechanics of growth had not been included by former writers among the phenomena of phytodynamics, nor was it so included by either Unger or von Mohl. It seemed to be supposed that there was a fundamental difference between growth and other movements in the vegetable kingdom, and this idea was entertained even in the most recent times. From the time of Mariotte and Hales no one had made the mechanical laws of growth the subject of special investigation or theoretical consideration; yet some observations had been made on the formal relations of growth and its dependence on external influences. Ohlert (1837) was the first after I)u Hamel who studied the distribution of growth in the root; Cotta in 1806, Chr. F. Meyer in 1808, Cassini in 1821, Stcinhcil and others made measurements in connection with the same question in the stem, but only with the result of showing that the distribu-
