62 



MORPHOLOGY OF THE CELL. 



209. Thin-walled parenchyma cells play an important part in 

 assimilating and storing, and special names are given to cells 

 which have these offices, such as chlorophyll parenchyma, starch 

 parenchyma, etc. In the tissues of most succulents, and in the 

 leaves of a few plants, some of the parenchyma cells are filled 

 with clear sap and more or less mucilaginous matter, and con- 

 stitute the so-called water tissue. 



210. The walls of typical parenchyma cells consist of ordinary 



cellulose ; but even slight deviations from the type furnish good 

 illustrations of lignified and of cutinized membranes. 



211. Lignification may increase the thickness of the cell-wall, 

 greatly reducing the cell-cavity, or it may merely harden the 

 membrane without much thickening. The parenchyma cells 

 found associated with other elements in woody tissues have 

 walls of the latter character ; the grit-cells in pears and many 

 other fruits show good examples of the former. Such hardened 

 cells are called sclerotic parenchyma cells. 



FIG. 40. Sclerotic parenchyma cells from fruit of the pear. (WtfKt 



