266 THE CELL 



seen in the cells of many water plants, as elodea (Fig. 439), 

 chara, and nitella. 



443. Besides these and other movements of protoplasm 

 within the cell-wall, there are also movements of naked 

 protoplasm, of two main types: (1) Amoeboid or creeping 

 movements, such as may be seen in a plasmodium of myxomy- 

 cetes, or in an amoeba. (2) Swimming by means of cilia or 

 flagella, illustrated in the swarm-spores of water fungi, and 

 of some algse. By the latter type of movement the unicellular 

 bodies (swarm-spores) are often moved very rapidly. To 

 see movement in protoplasm, carefully mount in water a 

 few hairs from the stamens of tradescantia (spider-wort). 

 The water should not be too cold. Examine with a power 

 high enough to see the granules of protoplasm. Make a 

 sketch of several cells and their contents. It may be neces- 

 sary to make several trials before success is attained in 

 this experiment. If the microscope is cold, heat the stage 

 gently with an alcohol lamp, or by other means; or warm 

 the room. See Fig. 438. 



444. Nature of Cell-wall. — The cell-wall of very young 

 cells is a delicate film or membrane. As a cell grows in 

 size the wall remains thin and does not begin to thicken 

 until the cell has ceased to enlarge. The fundamental sub- 

 stance of cell-walls is a carbohydrate known as cellulose. 



The cellulose usually stains blue with 

 I hematoxylin. Often by incrustations or de- 

 posits of one kind or another, the cellulose 

 reaction is lost or obscured. Two of the 

 most common additions are lignin, forming 

 440. Bordered pits in wood, and suberin, forming cork. The walls 

 pine wood. then are gaid tQ be lignified or su berized. 



445. In all the cells studied in the above experiments, 

 the walls are thin and soft. In general, those cells that 

 have thin walls are called parenchymatous cells. Some 

 cells, as those of nuts and the grit of pear fruit, have very 



