evidently taking the places where three wall-surfaces meet for vessels; such vessels he also saw in the milk-cells of Asclepias, described in 1779 by the elder Moldenhawer, who seems himself to have regarded even the intercellular spaces in the pith of the rose as equivalent to these milk-cells. The word vessel even in the 18th century was used in such an indefinite manner, that the broad air-tubes of the wood and the finest fibres were called vessels. Hedwig’s idea of the construction of spiral vessels was strange enough; he took the spiral band itself for the vessel, and supposed it to be hollow because it is coloured by absorption of coloured fluids; in those spiral vessels in which the turns of the spiral band are distant he saw, it is true, the delicate original membrane which lies between the turns, but he supposed that it lay inside the spiral band, which was wound round it on the outside. On the second plate of the first part of the ‘Historia Muscorum’ he even figures the network of ridges which the adjoining cells have left on the wall of the spiral vessel, but explains it as wrinkles caused by desiccation. Hedwig was without doubt a very practised microscopist, and he constantly recommended the extremest care in the interpretation of all that the instrument reveals; but if an observer so careful and practised, who moreover was provided with a glass of tolerably strong magnifying power, fell into such gross mistakes, it cannot surprise us if others, as P. Schrank, Medicus, Brunn, and Senebier, accomplished still less. These highly unimportant achievements are all that mark the close of the 18th century. CHAPTER III. EXAMINATION OF THE MATURED FRAMEWORK OF CELL-MEMBRANE IN PLANTS. 1800-1840. There is no sharp line of division between the 18th and the 19th centuries; the phytotomists who appear on the scene during the first years of the new century are scarcely more successful than Hedwig and Wolff; careful and judicious interpretation of their own and others’ observations is still rare, and they are often misled by preconceived opinions.