164 COLLEGE BOTANY 



are not necessaa-y but may not be injurious; (c) it sometimes 

 prevents the entrance of injurious compounds; {d) it prevents 

 the outward diffusion and loss of food-making materials. 



When the cell dies, the cytoplasmic layer or membrane loses 

 its osmotic power, becomes permeable and permits all substances 

 to pass through it in either direction. Therefore, coloring mate- 

 rials, sugars and other substances will not pass out of the living 

 cell into the surrounding liquid but will very readily pass out 

 of the dead cell. This is very evident in beets, which retain 

 their coloring when alive, but give it out very readily after the 

 cells are killed. 



Chemical changes are continually taking place within the 

 living cells and these changes are not necessarily the same in all 

 parts of the plant. In fact, they are quite different in the dif- 

 ferent parts of the plant, especially in the different parts of the 

 higher plants, in which the differentiation of cells into tissues 

 is most complex. These changes are very evident when we take 

 into consideration that only certain parts of the plant contain 

 chlorophyll, and that only certain parts of the plant are used for 

 food. These differences in the chemical character of the contents 

 of the cells results in a loss of osmotic equilibrium and very 

 naturally resiult in an osmotic interchange between the cells. 

 Therefore, there must be a constant movement of liquids and an 

 interchange of cell contents through the greater part of the plant 

 during the growing season. The character of these membranes 

 and the behavior of the cell contents present may complex phe- 

 nomena which has not as yet been fully explained by scientists. 



Txirgor. — The movement of the water into the cell results in 

 forcing the protoplasm against the cell wall, which, being more 

 or less elastic, becomes distended. This is very noticeable on 



