THE CELL AND TISSUE. 



65 



tion. This is called the circulation (Fig. 97) of the proto- 

 plasm. In other cases it moves as a broad stream around the 

 cell-wall. This movement is called the rotation (Fig. 98). 



5. The cell wall often remains quite thin. In some of the 

 bast-cells, in wood-cells and many others, it becomes very- 

 thick ; but its growth in thickness is not uniform. Only a 

 spiral band becomes thickened in very many cases; occasion- 

 ally a thickened ring or annular band is formed. Sometimes 

 the whole wall except small circular areas here and there at- 

 tains a considerable thickness. Simple pits in the wall result 

 from such growth. Bordered pits are formed in the wood- 

 cells of the Pine family. The sides of the pit arch over as 

 growth in thickness proceeds — the upper margin thus making 

 the inner ring, and the bottom of the pit the outer ring, when 



pi 



Fig. 99. 



viewed on the side of the cell. In some cells the thickened 

 portions are at the angles and extend across at short and regu- 

 lar intervals, being thus scalariform, that is, like a stairway or 

 slightly resembling the rounds of a ladder. Occasionally the 



