Page:History of botany (Sachs; Garnsey).djvu/353

] outer face. There are two processes in the formation of a cell; the first is the isolation or individualising of a part of the contents of the mother-cell, the second the formation of a membrane round the individualised portion.' He then proceeds to show that the cell-wall is formed by the separation of non-nitrogenous molecules from the nitrogenous mucilage (proto-plasm). These sentences contain all that is general and essential in vegetative cell-formation. Further on he notices the peculiarities in the various processes in cell-formation; he says that the individualising of the cell-contents assumes four forms; first, single small portions of the contents separate themselves inside the rest, as occurs in the formation of free germ-cells in Algae, Fungi, and Lichens, and of endosperm-cells in Phanerogams; secondly, the whole contents of one cell, or of two by conjugation of associated cells, collect into a free spherical or ellipsoidal mass, as in the formation of germ-cells in the Conjugatae; thirdly, the whole contents of a cell separate into two or more portions, which is now called cell-division; from this Nägeli distinguishes as his fourth form, the process known as abscision (Abschniirung), which occurs in the formation of germ-cells in many Algae and Fungi.

Schleiden had declared it to be a general law in plants, that cells are only formed inside mother-cells. Meyen however, Endlicher, and Unger, had recently assumed the formation of new cells between the older ones; Nägeli maintained that all normal cell-formation, vegetative and reproductive, takes place only within mother-cells.

In opposition to the long-cherished notion that there must be one general and fundamental form of cell, Nägeli pointed to the fact that cells have very different forms at the moment of their production. Those which arise by free cell-formation are, he says, at first always spherical or ellipsoidal; those produced by cell-division have a shape necessarily conditioned by the form of the mother-cell and the manner of division. He showed further that changes in the