20 



General Botany- 



Cell differentiation. If we could trace the development of one 

 of the more complex plants, we should find that it begins as a 



Fig. 8. Types of plant cells and tissues, resulting from the difJerentiation of cells: A, a 

 supporting and water-conducting cell (tracheid) from fern stem ; B, water-conducting vessels 

 with spiral thickenings on the walls and soft, thin-walled cells (parenchyma) between them, 

 from sunflower stem ; C, water-conducting vessel with ring-like thickenings ; D, giant stone 

 cell in parenchyma of camellia leaf; E, wood cells from sunflower stem; F, stone cells, 

 with greatly thickened walls, from stem of club moss ; G, stone cells from shell of pecan ; 

 H, wood cells from pine. D, E, F, and G are cross-sections. {After Sachs.) 



single cell. This cell divides, forming two cells, each of which 

 divides again, forming four. Cell division continues until an 

 embryo composed of hundreds of cells, all very much alike, is 

 formed. 



But as the embryo grows farther, some rows or groups of cells 



