Processes in Movement of Materials 89 



in turgidity. Water is also held in cells against evaporation by 

 the conditions that give rise to osmosis and imbibition ; that is, 

 water loss from cells is retarded by the same internal conditions 

 that facilitate water absorption. Salts and sugar pass from cell 

 to cell by simple diffusion. We can account for the rate of move- 

 ment of soluble food substances only in part, but diffusion is 

 certainly one of the physical processes involved. 



PROBLEMS 



1. Apply the principles of diffusion discussed in this chapter to : 



a. The opening and closing of stomata ; 



b. The movements of leaves having pulvini ; 



c. The wilting and recovery of plant tissues ; 



d. The interchange of gases between a land plant and the surrounding air ; 



e. The interchange of gases between a water plant and the surrounding water. 



2. Why do red beets retain their red color when placed in cold water, but lose it 



when placed in boiling water? 



