Processes in Movement of Materials 85 



cells of the root are in contact with the water of the soil. Inside 

 the root the water is being used and removed by being drawn up 

 through the stem to the leaves. More water then passes into the 

 cell walls and protoplasm to take the place of that which is drawn 

 away, and this tends to keep the amount of water in the plant 

 nearly constant. Imbibition becomes very powerful in plants 

 that have mucilage, gums, and pectic compounds in their tissues, 

 both in absorbing water and in holding it against evaporation. 



Osmosis. A third form of diffusion that aids in the absorp- 

 tion of water is osmosis. If an animal membrane, as a piece of 

 bladder, is tied over the broad end of a thistle tube and the bulb 

 of the tube is immersed in water, the water will gradually pass 

 through the membrane. The membrane is permeable to water ; 

 that is, it allows water to pass through it. The water continues 

 to move through until its level is the same inside and outside 

 (Fig. 47)- 



When the water level is the same inside and outside the tube, 

 one might think that the water particles were at rest. This is 

 not the case. Water particles are still passing both into the 

 thistle tube and out of it through the membrane. The rate 

 is the same in both directions, however, and so the water level 

 within the tube remains unchanged. 



If we put a little sugar into the thistle tube, something dif- 

 ferent happens, as is shown by the fact that the liquid in the tube 

 begins to rise. Evidently, more water is passing through the 

 membrane into the tube than is passing out, and this change has 

 been brought about by the presence of the sugar. Perhaps we 

 can get a mental picture of what causes this difference from the 

 diagram in Figure 48. The membrane (C) allows water mole- 

 cules to pass through it freely, but it permits scarcely any of the 

 sugar molecules to pass. The outer side of the membrane is 

 completely covered with water molecules, tending to diffuse 

 through the membrane. The inner side is only partly covered 

 with water molecules, since part of the area is occupied by sugar 



