INTROD 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 minera/iza/ion. 

 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 FlG 

 uniform. When the wall grows 

 thicker except at certain spots, these 

 remain as pits or pores in the 

 thickening layers. When only cer- 

 tain spots or lines grow thicker, 



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

 give it the appearance in figs. 10, 11. 



Cells from a liverwort 

 showing thickened walls A, 

 half an elater; A', a part more 

 highly magnified; B, a cell from 

 the lower part of the thallus, 

 with reticulate thickenings 

 (shaded); C, D, rhizoids with 

 isolated branched thickenings. 

 Highly magnified. — After 

 Sachs. 



