766 



PLANT GROWTH 



lO 



V. CELL DIVISION 



Any consideration of growth at the celkilar level must consider the two aspects of 

 cell division and cell enlargement or "meresis" and "auxesis." In bacteria and many 

 unicellular and filamentous algae, the cell divides only when it has reached a 

 certain size. Thus division is preceded by 

 enlargement. The pattern of growth in 

 higher plants is generally the reverse, that 

 is, division \% followed by enlargement. The 

 most active division in higher plants takes 

 place in meristems, which it will be conve- 

 nient to consider here. They were defined 

 in section II; the only real criterion of a 

 meristematic tissue is active cell division. 

 However, in some cases the division is only 

 potential and the meristem said to be 

 resting or dormant. The location of the 

 meristem, the cell types in it and the be- 

 havior of the cell after division may all vary 

 widely. 



(a) Apical meristems 



Apical meristems are composed of cells 

 dividing rapidly and enlarging slowly, thus 

 forming a nest of small cells. Commonly 

 they are rich in cytoplasm, and the vacuole 

 is present only as a network of threads or 

 globules which does not coalesce to a single 

 sap-vacuole until the cells have enlarged 

 (Fig. i). But sometimes, as in the apical 

 cells of ferns or the cells near the top center 

 of many other apical meristems, quite large 

 vacuolated cells are present. Occasionally 

 even cells with a high oil or tannin content 

 may occur, though it is not certain that 

 these are rapidly dividing. The typical 

 vegetative meristem is most active on its 

 flanks, where it gives rise to a series of 

 leaf primordia ; for a time these are con- 

 tinuous with the dividincT meristem of the 



Fig. 3. Longitudinal sections of the apex 

 of Xanthium pennsylvanicum (cocklebur). 

 A: vegetative apex, growing on long 

 days. B: apex becoming reproductive, 

 after six 8-h. days. The vegetative apex 

 has vacuolated non-dividing cells in the 

 center and actively dividing cells forming 

 leaf primordia on its flanks; the repro- 

 ductive apex has changed shape, due to 

 centrally located cell division and en- 

 largement, and the non-dividing cells in 

 the center have become active (Courtesy 

 of Prof R. H. Wetmore). 



apex, but then they get pushed away from 



it by the elongation of some of the cells that have divided to form the usually 

 long and narrow leaf initial (see section Vila, p. 797). Indeed the enlargement 

 that goes on at the base of the meristem is also primarily elongation, so that the 

 apex becomes pushed upwards at the top of an elongating cylindrical stem. Why 

 the cells do not grow in all three dimensions is not known. A typical vegetative 



