234 THE CELL 



nucleolus. Cell -walls are so often absent that it is quite 

 as well to think of a cell as a single nucleus with its attend- 

 ant protoplasm. The nucleus is an essential part of every 

 cell, and is intimately connected with the wonderful process 

 of cell -division. In some very low forms of plants, as in 

 some of the bacteria, no nucleus has yet been clearly 

 made out. 



384. NATURE OF PROTOPLASM. Protoplasm, with its 

 nucleus, forms the essential part of all living, acting 

 cells. It is possible in many cases to find a small mass 

 of living protoplasm with a nucleus but without a cell- 

 wall. Protoplasm is not entirely homogeneous, for when 

 examined with a microscope of very high power it is often 

 found to be of a foamy or honeycomb nature. This mesh 

 or network contains many minute granules, called micro- 

 somes, and lies in a clear "ground mass" composed of cell- 

 sap. On a glass slip mount in a drop of water some com- 

 pressed or brewer's yeast which has been growing in a thin 

 syrup of white sugar for twenty-four hours; place over the 

 drop a thin cover-glass, and examine with the compound 

 microscope, first with the low power and then with the 

 high. The individual cells should be visible. Note the 

 shape and contents of the cells, and make a sketch of a 

 few of them. A similar study may be made of the soft 

 pulp scraped from a celery stem; of hairs 

 scraped from the surface of a begonia leaf; 

 of threads of spirogyra; cells of protococ- 

 cus ; soft white cells of an apple ; the thin 

 sss. Ceils in petiole leaves of various mosses ; the epidermis of 



of begonia leaf. 



at *>. waxy plants. 



385. VACUOLES. Protoplasm often does 

 cium oxaiate. not en ti re ly fill the cell. There may be a 

 number of cavities or vacuoles in a single cell. These 

 vacuoles are filled with cell-sap (v., Fig. 385). In some 

 parts, as in buds and root -tips, where the cells are most 



