V CELL DIVISION 767 



apex of a dicotyledon is shown in Fig. 3A, and the changes which precede its 

 conversion to the reproductive phase are seen in Fig. 3B. 



In the root, the extreme apex is made up of the root cap, which is continually 

 being added to by division of cells of the meristem, which in this case is just behind 

 the apex. The root-cap cells do not divide, nor do they enlarge much; they are 

 slovighed off at the extreme tip as fast as they are added. The apical meristem, 

 commonly about 1/2- 1 mm long, comprises very small cells which elongate exten- 

 sively and rapidly. There is no folding out at the flanks to produce anything cor- 

 responding to the leaf primordia. Lateral roots (see section Vllb, p. 801) appear 

 at some distance from the apex and quite disconnected from it. 



(b) Secondary meris terns 



Cells of the vascular cambium are quite different from those of the apical mer- 

 istem; they are very long (up to 4 mm) and narrow. They have a good-sized 

 nucleus, usually in the center, and an elaborate system of vacuoles in fine thread- 

 like channels, distinct globules or relatively large units (see Fig. i ) ; these vacuoles 

 change with the season in many plants and the patterns are often characteristic 

 of the species, especially in trees (Bailey, 1930). Unlike apical meristematic cells, 

 the walls of vascular cambium may be quite thick. 



In general, division of these cells is longitudinal or oblique, and enlargement 

 takes place transversely (giving rise to increased girth of the stem). Occasionally 

 a long cell will divide up into short ones by short cross-walls; this is the unusual 

 phenomenon of division without enlargement. It sometimes follows wounding, or 

 the abnormal access of oxygen. 



The cork cambium, which is active in stem cortex to cut off long and narrow 

 sub-epidermal cells, resembles the vascular cambium in some ways, but the cells 

 are not exceptionally long. They often contain fully differentiated chloroplasts. 

 After transverse enlargement the cell-walls become heavily "suberized" i.e. coated 

 internally with cork, and the contents die. It was the microscopic appearance 

 ■of cork cells which historically gave rise to the cell theory. 



(c) Morphology of cell division 



Plant cell divisions differ from those in animal cells in two major respects: (a) 

 no aster is formed, (b) the residues of the spindle fibers do not disappear after 

 mitosis, but become incorporated into the cell plate or primary cell-wall initial. 



The process of cell division is not easily studied in the meristems themselves 

 since the cells are small. They are excellent for studying the nuclear phenomena, 

 but for the role of the other parts of the cell, relatively large vacuolate cells are 

 more informative. Such large dividing cells are found just below the meristem 

 in the apex of the growing shoots, or can be induced in other more mature inter- 

 nodes by wounding nearby. In the latter case very large cells often divide; the 

 cytoplasm and nucleus both increase in volume, the nucleus becomes spherical 

 and thereafter the general process is like that for the sub-meristematic cells (see 

 the review of Bloch, 1941). 



The first stage in division appears to be the swelling of the nucleus from a flat 

 spheroid into a sphere and its movement away from the cell wall. Cytoplasmic 



Literature f>. Si 6 



