PROTOPLASM AND PLANT-CELLS. 



in some cases, e.g. diatoms and the superficial cells of 

 joint-rushes and grasses, silica or other mineral matters 

 are deposited. On the other hand the cellulose may 

 degenerate into mucilage, e.g. gum arable, cherry gum, 

 flaxseed, many water-plants, etc. 



9. The cell-wall may be thickened uniformly, or, as 

 more frequently happens, some portions may be much 

 more thickened than others. When it is uniform the 

 wall shows no markings of any kind, but when otherwise it 

 shows dots, pits, rings, spirals, reticulations, etc. etc. (Pig. 

 5). This thickening gives strength to the cell- wall, and 



FiQ. 5.— -Longitudinal section of a portion of the stem of Garden Balsam, 

 c, ringed vessel; d', a vessel with thickenings which are partly spiral 

 and partly ringed ; i)", u'", «"", several varieties of spiral vessels ; «'"", a 

 reticulated vessel. 



serves either to protect the protoplasm, as in many spores 

 and pollen-grains, or to help in building up the frame- 

 work of the plant. 



10. Careful examination of the cell-walls, even when 

 much thickened, shows that the protoplasm of contiguous 

 cells is not completely separated. Delicate fibrils of pro- 

 toplasm extend through minute openings in the walls, con- 

 necting the greater part of the cells throughout the plant. 



11. Cells in plants are of various sizes and shapes. The 

 largest (with a few exceptions) are scarcely visible to the 

 naked eye, while the smallest tax the highest powers of the 



