Page:History of botany (Sachs; Garnsey).djvu/341

] first part having been laid before the Paris Academy in 1831-32, Mirbel distinguished three modes of cell-formation; in the germination of the spores of Marchantia new cells are formed from the germ-tube and new cells again from these by a similar process, much in the same way therefore as that which actually occurs in the germination of Yeast-fungi ; he found a second kind of cell-formation in the production of the gemmae of Marchantia, where he distinctly observed the successive appearance of the dividing walls, but formed an erroneous idea of the proceeding on the whole ; in the further development of the gemmae and in other cases of growth he considered that new cells are formed between those that are already present in the manner supposed in his earlier theory.

Von Mohl's dissertation on the multiplication of vegetable cells by division, published in 1835 and reprinted in 'Flora' of 1837, shows how strange these processes even then appeared; in this work he expresses some doubts about Mirbel's statements, but he accepts them on the whole, and only makes incidental mention of his own more numerous and better observations on the development of spores ('Flora,' 1833), though he had there seen several cases of cell-division and free cell-formation with tolerable distinctness. Adolph Brongniart ('Annales des sciences naturelles,' 1827) also had observed, though imperfectly, the formation of pollen-grains in their mother-cells in Cobaea scandens, and Mirbel, in the appendix to the work mentioned above, had given a correct description and good figures of the formation of pollen-cells; and yet von Mohl neglected to compare these important observations of cases of cell-division with his own; even in 1845, when he published the latter in a revised form in his 'Vermischte Schriften,' he overlooked the close relation between the formation of those pollen-grains and spores, and the cell-division in Cladophora. Still this treatise of von Mohl's is of great importance in the history of the theory of cell-formation, because it described a case of cell-division for the first time step by step and brought all the salient points into relief.