Page:History of botany (Sachs; Garnsey).djvu/350

330 statement that the nuclei are formed after the division. It is surprising that after these considerations von Mohl thought that his own observations necessarily confirmed Schleiden's theory of cell-formation, although he noticed beside that the nucleus never forms a part of the cell-wall, an essential feature in that theory ; but in fact von Mohl took the membrane which according to Schleiden separates from the nucleus for the primordial utricle. But these mistakes are soon followed by the right conjecture, that the substance of the primordial utricle may be identical with the mucilaginous mass, which commonly encloses the nucleus, and so with that which von Mohl two years later named protoplasm. In this later treatise ('Botanische Zeitung,' 1846), in which he proves that the well-known movements in the interior of cells are made not by the watery cell-sap but by the protoplasm, he states (p. 75) that it is the protoplasm which produces the nucleus, that the organisation of the nucleus ushers in the formation of the new cell, and that contrary to Schleiden's theory the protoplasm completely envelopes the nucleus, which always occupies the centre of very young cells, as is the case especially in the endosperm-cells observed by Schleiden. He then shows how the protoplasm of young cells at first solid afterwards forms sap-cavities and stretches between them in walls, bands or threads, the substance of which exhibits the streaming movement. Von Mohl strangely neglected on this occasion to compare carefully his former observations on the origin of spores and the division of Alga-cells with his new results, and to seek for the essential resemblances between them ; on the contrary he said emphatically that the cell-division in Cladophora is probably a quite different process from the multiplication of tissue-cells in higher plants.

The discoveries of Unger and von Mohl up to the year 1846 were quite sufficient to refute Schleiden's theory, but not to give a clear and general view of the processes in the formation of cells; the different kinds of cell-formation were neither carefully distinguished from one another, nor could they be