CH. IV] PITTED WALLS. 61 



research and of no little disputing among botanists. It 

 is now generally believed that the lines of stratification 

 represent successive films of cellulose added by the 

 protoplasm to the cell wall. A single sheet of paper, 

 which is thickened by pasting on to it other sheets, may 

 serve as a model of the thickening cell wall, and will in 

 transverse section give the same stratified appearance as 

 an actual cell wall. Fig. 25 represents a model of a more 

 complex case. The original sheet of paper (a) on which 

 the successive sheets are pasted is unperforated, while 

 each of the sheets (6) to be fastened over it is pierced 

 by a circular hole. The resulting thick sheet of paper 

 will when seen in transverse section have the appearance 

 shown in the figure. 



If the perforated sheets had been pasted on to both 

 sides of the whole sheet the model would have represented 

 a dotted or pitted cell wall, in which a similar appearance 

 is produced in roughly speaking the same way. Fig. 26 



FIG. 26. 



CELLS OF A DATE-STONE IN SECTION, 



showing thick cell- walls with numerous simple pits. 



m.l, the middle lamella thickened on both sides, except where the pits occur. 



