8 MORPHOLOGY OF THE CELL, 



newly constituted protoplasm-mass clothes itself, sooner or later, with a cell-wall. 

 All processes of cell-formation agree in these points ; but a description which goes 

 more into detail requires a distinction to be drawn between different cases in which 

 the process now varies in many different ways. 



It must first be noted that the formation of new cells does not always result in 

 an increase in the actual number of cells. Three types may be distinguished in this 

 respect: — (i) The Renewal or Rejuvenescence of a cell; that is, the formation of a 

 single new cell from the whole of the protoplasm of a cell already in existence ; 

 (2) The Conjugation or Coalescence of two (or more) protoplasm-masses in the 

 formation of a new cell ; (3) The Multiplication of a cell by the formation of two 

 or more protoplasm-masses out of one. Each of these types shows a series of 

 variations and transitions into the others. The greatest diversity is exhibited in 

 the multiplication of cells ; two cases must be distinguished, according as part 

 only of the protoplasm of the mother-cell is applied to the formation of the new 

 cells, or as the whole mass is converted into daughter-cells. The latter, by 

 far the more common case, again exhibits variations, according as the protoplasm- 

 masses, which collect around new centres, expel water, contract and become 

 globular; according as the cell-wall is secreted during division or after its com- 

 pletion; and, finally, according to the way the cell-sap and nuclei make their 

 appearance. 



The different processes of cell-formation are in turn brought into play throughout 

 the life of the plant ; — Rejuvenescence, Conjugation, Free cell-formation, and Cell- 

 division with contraction and rounding off, are the forms connected with repro- 

 duction ; while in the growth or increase in volume of an organ by the formation 

 of new cells, cell-division only comes into play, and almost invariably by division 

 of one cell into two without any considerable contraction and rounding off of the 

 new cells ; the multiplication of cells in growing tissue may therefore be described 

 as a bipartition of those already in existence. In the formation of reproductive 

 cells, the tendency to isolation and rounding off is most strongly displayed ; while 

 in cell-formation accompanying growth, the mother-cells are divided by partition- 

 walls in such a manner that the resulting daughter-cells resemble their mother- 

 cells, or are able to develope into any required form. 



The more important modes of cell-division must now be examined in a few 

 examples. 



A. Cell-formation in relation to Reproduction. 



1. Cell-formation by Rejuvenescence. — A good example is afforded in the 

 formation of the swarm-cells of Stigeoclonium insigne (Fig. 3, p. 4). The protoplasm 

 of a cell of a filament contracts, and expels a portion of the water of the cell-sap ; the 

 arrangement of the differentiated protoplasm-mass is changed, and the chlorophyll-bodies 

 become indistinct ; its form alters as it escapes from its cell-wall ; from almost cylindrical, 

 the protoplasm-mass becomes ovoid, with a broad green and a narrower hyaline end ; 

 after its movement has ceased, the latter becomes the base, and the green end alone 

 grows when the new cell clothes itself with a cell-wall. The observations of Pringsheim 

 on (Edogonium also show that the direction of growth of the renewed cell is at right 

 angles to the original direction of growth before the renewal; for the hyaline or 

 radicular end of the swarm-cell, which afterwards becomes the point of attachment, is 



