CHAPTER III 



GROWTH AND CELL-DIVISION 



SECTION IT. General. 



THE existence of large non-cellular plants such as Vaucheria, 

 Caulerpa, and Mucor shows that cell-division does not form an essential 

 condition for the continuance of growth, but in cellular plants a stoppage 

 of cell-division is always ultimately followed by a cessation of growth, 

 and similarly when growth is prevented cell-division also soon ceases. 

 Under normal conditions the cell divides when it reaches a certain size. 

 The rapidity of growth and the frequency of division are in fact intimately 

 correlated in each cell, the former regulating the latter. The magnitude 

 at which division occurs varies greatly in different plants, and in the 

 case of the cells of bacteria it is reached while they are excessively 

 minute, and far smaller than the cells of other plants can ever become. 

 Even when growth is inhibited by external pressure, the meristematic 

 cells and nuclei in the cambium and in the young apices retain about 

 the same size that they average during their most active growth and 

 division. The apical cells of Sphacelaria and Callithamnion^ as well as the 

 embryonic cells of a Spirogyra filament, behave similarly, although they 

 are always considerably larger than the cells in the primary meristem of 

 a flowering plant. 



The size of the embryonic cells usually varies within narrow limits, 

 but the segment-cells derived from them undergo secondary differentiation 

 to subserve a variety of purposes, and their properties and size are subject 

 to corresponding modification. Thus in the sporangia of Mucor and Sapro- 

 legnia, just as in the somatic segments of Sphacelaria \ the dividing-cells 

 decrease in size. During tissue-differentiation, however, the reverse is usually 

 the case, for the segment-cells divide less frequently, or not at all, as they 

 increase in size by stretching-growth and attain their adult shape. Indeed 

 in the laticiferous tubes of Euphorbia the cellular segmentation is suppressed, 

 in spite of their steady growth and of the frequent divisions of the nucleus. 



1 For figures see Goebel's Outlines, Clar. Press, 1887, p. 67 ; also Jahrb. f. wiss. Bot., 1865-66, 

 Bd. IV, Taf. 34. 





