Page:History of botany (Sachs; Garnsey).djvu/352

332 von Mohl had observed with accuracy a number of important facts, but that Nägeli added largely to them, and, which is the main point, elaborated them into a comprehensive theory embracing all kinds of cell-formation. How important the correct distinction of the protoplasm from the rest of the cell-contents was for the perfecting of the theory of cells is seen from Nägeli's declaration, that he retracts his former view which rested on the authority of Schleiden, because it sprang from a time when he was ignorant of the significance of the mucilage-layer (the protoplasm), though it is true that he indicates at the same time other points and new considerations which definitively set aside Schleiden's theory. After investigating the different modes of free cell-formation and finding the processes there quite different from Schleiden's account of them, he proceeded to search for free cell-formation where Schleiden had affirmed that it invariably occurs, namely in growing vegetative organs in the higher plants. But this investigation led him to the conclusion that all vegetative cell-formation is true cell-division, and that even the reproductive cell-formation in some Algae and Fungi is effected by division; the reproductive cells of most plants are the result of free cell-formation, but it should be observed that the term free cell-formation is here used not exactly in the modern sense, inasmuch as Nägeli included in it the formation of four-fold grains (tetrads) in spores and pollen. If the distinction between cell-division and free cell-formation had often been suggested by former observers, Nägeli was the first who distinctly defined it, though not exactly as it is now defined. ' In cell-division the contents of the mother-cell separate into two or more portions; a perfect membrane forms round each of these portions, which at the moment of its appearance rests partly on the wall of the mother-cell and partly on the adjacent walls of the sister-cells. In free cell- formation a smaller or larger part of the contents of a cell, or even the whole of them becomes isolated. On its surface is formed a perfect membrane, which is everywhere free on its