INDIVIDUAL VEGETABLE CELLS. 29 



retreating, now stopping, or beginning again, in a 

 way differing from all non-living matter. (Fig, 4.) 



Fig. 4. 



6. Sometimes the cell-wall has a deposit of flinty 

 material, (silica]) or of other mineral matter, which 

 is often beautifully 



marked with lines and 

 dots. Different species of 

 plants produce different pat- 

 terns of these deposits, as 

 represented in the figures of 

 Diatoms in Chap. VII. 



7. The cell-wall is often ir- 

 regularly thickened by a de- 

 posit inside, so as to present 

 different appearances in dif- 

 erent cells. In the Pine and 

 Fir tribe the pores in the wall 

 of the wood-cells are sur- 

 rounded by concave spaces 



or depressions. (Fig. 5.) Fig. 5. 



