INTRODUCTION xvn 



tions, scalariform (ladder-shaped), or, less commonly, reticular 

 openings. 



Like the pits, end-wall perforations involve highly selective 

 lytic processes whose regulation and chemical nature is unknown. 

 These sculpturings of the cell wall together with other wall modi- 

 fications, give every appearance of being highly functional changes. 



In phloem, as in xylem, parenchyma fiber and sclerid cells 

 are present together with specialized conducting cells. The funda- 

 mental structural and functional unit is the sieve element. This 

 cell is again distinguished by its wall sculpture— the presence of 

 sieve areas and sieve plates. The former are clusters of fine pores 

 through which cytoplasmic processes extend from one cell to its 

 neighbors. The latter are perforated communicating regions gener- 

 ally confined to end walls. The pores are 0.5-3.5^ in diameter. Just 

 as vessel elements in xylem are joined end— to-end to form vessels 

 centimeters or meters in length, the sieve tube elements are joined 

 to form a continuous chain. Unlike the vessel elements, living 

 cytoplasm is present in the cells although nuclei are lacking. 

 A modified parenchyma cell, the companion cell, is associated 

 specifically with the sieve cell elements. The nature of the relation 

 between enucleate sieve elements and the nucleated companion 

 cells is not known, but it is evident that they are closely associated 

 in function as they are in position and origin from common mother 

 cells. 



These variations in cell type have been reviewed briefly to 

 illustrate a part of the pattern of differentiation. Repeatedly, the 

 condition of the cell wall characterizes these tissues and their 

 component cell types which arise during differentiation. 



The developmental plan which is encountered under mesic 

 condition serves as a convenient reference point for the considera- 

 tion and cell and wall specialization. When the physical environ- 

 ment deviates from these mild, "ordinary'' conditions, however, 

 the plant body must undergo suitable modification if it is to survive. 

 In xerophytes, the place of the cell wall in adaptation is well illus- 

 trated. 



Xeric environments, whether in desert, tundra, or saline regions, 

 select for water-conserving modifications in their plant communi- 



