236 THE CELL 



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. 386. 



388. 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 funda- 

 mental substance of cell -walls is a carbohydrate known as 

 cellulose. The cellulose generally stains blue with hema- 

 toxylin. Often by incrustations or deposits of one kind 

 or another, the cellulose reaction is lost or obscured. Two 



387. Rotation of protoplasm in Elodea Canadensis (often known as 

 Anacharis). Common in ponds. 



of the most common additions are lignin, forming wood, 

 and suberin, forming cork. The walls then are said to be 

 lignified or suberized. 



389. In all the cells studied in the above experiments 

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

 have thin walls are called parenchymatous cells. Some 

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

 very thick walls, and are called sclerenchymatous cells. 

 In many cases the cell -walls are intermediate between 

 these extremes. 



390. Cell -walls often thicken by additions to their 

 inner surface. This increase in thickness seldom takes 

 place uniformly in all parts. Many times the wall re- 

 mains thin at certain places, while the most of the wall 

 becomes very thick. Again the walls may thicken very 

 much in angles or along certain lines, while most of the 

 wall remains thin. As a result of this uneven thickening 



