14 



THE PLANT CELL 



Fig. 6. — Various stages of cell multiplication 

 by budding of Saccharomyces cerevisiae. (After 

 Reess.) 



The kind of nuclear division above described is called indirect, 

 mitotic, or karyokinetic division. Another method, called direct 

 or amitotic division, is where the nucleus simply constricts itself 



into two (Fig. 7). This 

 is of rare occurrence, 

 being found chiefly in 

 old cells that have about 

 run their course. 



Cell Differentiation. 

 — The new cells formed 

 by the dividing cells of 

 the growing apices and 

 cambium do not long remain in size, form, and other character- 

 istics just as they are when first produced. In cross sections of 

 the growing point of Aristolochia, for example (Fig. 8), the cells 

 are essentially all alike; but a little farther down the stem we 

 find zones and groups of tissues which are 

 readily distinguished from each other because 

 the cells composing them differ in size, form, 

 thickness of walls, etc. These cells and tissues 

 have all arisen by changes in cells that were 

 produced by cell division at the growing point. 

 It appears that these cells, influenced by 

 stimuli and impelled by impulses mysterious 

 to us, set to doing different things: some 

 doing little more than enlarging uniformly; 

 some enlarging and thickening and chemically 

 changing their walls; others greatly elongating 

 and becoming fibers and tubes. It is this 

 power of the living cells to become different 

 things that has made possible division of labor in the plant 

 body, with the consequent successful occupation of all sorts of 

 habitats and the commanding size of our trees and shrubs. 



Sizes of Cells. — After its formation by cell division the cell 

 grows to its adult size which varies within not very wide limits 

 in each tissue. An average" size of cells, not much elongated. 



Fig. 7. — Nucleus 

 dividing by simple 

 constriction. From 

 the lining of the em- 

 bryo-sac of Vicia faba. 

 (After Zimmermann.) 



