CELL-DIVISION. 



97 



a second example showing how the division of cells already existing proceeds 

 with their growth. In Fig. A are represented a number of epidermis cells of a 

 very young leaf, seen from the outer surface. The chambers (s) are those from 

 which the two guard-cells of a stoma are formed later. These, as well as the 

 surrounding epidermis cells, grow, become enlarged, and change their form; 

 thereupon new cell-divisions appear, as in figure B, where the older cell-walls are 

 indicated by thick lines, those most recently formed with thin ones. It is observed 

 that around each mother-cell of a stoma (j) a number of new cell-walls have 

 appeared, by which pieces have become, as it were, cut out of the neighbouring 

 epidermis cells ; so that each mother-cell of a stoma {s) is surrounded by a group 

 of new cells. Fig. C shows a mother-cell now divided into two guard-cells {s s), 

 with the group of adjoining cells and neighbouring epidermis cells surrounding 



FIG. 96. — Development of stomata on the \eai oi Co7nmelyna ctxlesiis, A and J very young; C nearly mature; ss {in A and S) 

 mother cells of stomata ; ss (in C) guard cells. In .^ and £ is shown the formation of the neighbounng cells. 



them : all the cells already indicated in £ have grown further, i. e. they have en- 

 larged and changed their form, and the cell-walls have become thicker. By 

 means of the further growth, which now proceeds without cell-divisions, they 

 finally become fully (developed. 



It is evident from what has been hitherto said concerning the division of 

 cells, that each newly produced cell is not only a portion of the mother-cell, 

 but that even its original form depends upon what form the mother-cell possessed 

 at the moment of division, and in what direction the division wall was formed. 

 Very generally the latter happens in such a manner that the division wall is 

 set at right angles on the walls akeady existing, and, when necessary to this 

 enfl, it presents a corresponding curvature. The newly arising division walls 

 are therefore commonly not flat plates, but in most cases more or less, though 

 it may be almost imperceptibly, curved, as seen in the figures. The figures also 



[3] 



