of every minute portion of cell-membrane, and made better use than von Mohl himself had made of the comparison which he had suggested, namely, that the structure of a cell-wall with cross-striation and at the same time with concentric stratification resembles that of a crystal cleaving in three directions. He first gave expression to this conception of the structure of the cell-wall in 1862 in his ‘Botanische Untersuchungen,’ I. p. 187, and further developed it in the second volume of the same work at p. 147. But the true starting-point of Nägeli’s theory of molecular structure is to be found in his searching investigations in 1858, into the constitution of starch-grains. From the way in which they resist the effects of pressure, drying, distention, and withdrawal of a part of their substance, he arrived at the conclusion that the whole substance of a starch-grain is composed of molecules, whose shape must be not spherical but polyhedral, that these are separated from one another in their normal condition by envelopes of water, and that the amount of water in the stratified substance depends on the size of these molecules, the water being less when the molecules are larger; this view could at once be applied to the structure of the cell-wall, the growth of which may be explained as the increase in size of the molecules already present, and the intercalation of new small molecules between the old ones. These molecules of Nägeli are themselves very compound bodies, for the smallest of them would consist of numerous atoms of carbon, hydrogen and oxygen, and ordinarily a molecule would be composed of thousands of those aggregates of atoms, which the chemists call molecules.