THE STRUCTURE OF THE CELL WALL 



65 



associated with lignin, hemicelluloses, or other membrane substances. In ex- 

 treme cases secondary thickening may continue until the wall occupies most 

 of the interior of the cell. The formation of a secondary wall prevents any 

 further enlargement of the cell. All cells have a middle lamella and a pri- 

 mary wall, but secondary' walls are present only in certain types of cells. 



Fig. 12. Diagram illustrating formation of a plant cell wall. (A) cell plate 

 between two daughter nuclei, (B) cell plate (middle lamella) completely separating 

 the daughter cells, (C) primary wall has been formed on each side of the middle 

 lamella. One of the newly formed cells has undergone considerable enlargement. 



Phloem fibers, stone cells, tracheids, and wood fibers are typical examples of 

 cells with prominent secondary walls. 



Increase in thickness of the wall usually appears to take place by the addi- 

 tion of definite layers of cellulose or other cell wall constituents to the inner 

 surface of the existing wall. In most walls these la3'ers are too thin to be 

 detected without swelling or otherwise treating the wall. 



The Structure of the Cell Wall. — The basic unit in the structural or- 

 ganization of the cellulosic framework of the plant cell wall is the cellulose 

 "molecule." Cellulose "molecules" are not units of definite molecular weight 

 but long chains of varying length formed by the condensation of at least a 



