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referred to a common principle. Both observers had endeavoured to conjecture the course of events from certain data, supplying by inference what they had not directly observed.

Nägeli about the same time took up a different position as an opponent of Schleiden's theory. In an exhaustive treatise on the cell-nucleus, cell-formation, and cell-growth in plants, the first part of which appeared in 1844 in the periodical founded by himself and Schleiden, he collected together all that had hitherto been observed by himself and others from various points of view. All sections of the vegetable kingdom were once more systematically examined with reference to the occurrence of the cell-nucleus and the different kinds of cell-formation; all cases of the latter were carefully compared together in their resemblances and differences, in order to deduce from the observed phenomena that which was essential and universal. The first result was, that Schleiden found himself obliged, in the second edition of his 'Grundzüge' in 1845, to accept the cell-division established by Nägeli in Algae and the mother-cells of pollen as a second kind of cell-formation; thus began the movement in retreat which was destined to end in the following year with the overthrow of Schleiden's theory. This was effected by the continuation of Nägeli's treatise in the third volume of the periodical for 1846. In the first part of his work Nägeli had set out by assuming the correctness of Schleiden's assertions, though he was even then compelled to modify them considerably. In the second part, however, in consequence of further observations Schleiden's theory was declared in plain terms to be utterly incorrect, and was refuted point by point. But Nägeli was not obliged to confine himself to this negative result; his comprehensive investigations supplied material at the same time for constructing a new theory of cell-formation, which not only took in all the various cases, but declared the principle which lay at the root of all. If we compare this second part of Nägeli's treatise with von Mohl's publications from 1833 to 1846, we shall see that