IN TROD UCTION. 



substances are produced by the alteration of the cellulose or 

 other constituents of the original wall. (4) An excessive 

 deposit of mineral matters in the 

 wall is known as mineralization. 

 Such walls may even retain their 

 form after all organic matter is 

 burned out, as in the skin of the 

 scouring rush or horsetail. 



10. Growth of the cell-wall. 

 As the cells become older the wall 

 may increase in thickness. It must 

 also increase in area as the cells 

 grow in size. The growth in area 

 is usually accomplished by putting 

 new particles between the older 

 ones. Growth in thickness is rarely Fu ? "..-Cells from a liverwort 



J showing thickened walls A^ 



uniform. When the wall grows haifaaeiat ^' art re 



highly magnified; B, a cell from 



thicker except at certain spots, these th . e , ^wer part of the thaiius, 



with reticulate thickenings 



remain as pits or pores in the (shaded); c, A rhizoids with 



isolated branched thickenings. 



thickening layers. When only cer- Highly magnified. - After 



Sachs. 



tain spots or lines grow thicker, 



the wall shows projecting spikes, bands, or threads, which 



give it the appearance in figs. 10, n. 



