The Structures and Processes of Roots 173 



of that which is drawn away, and this keeps the amount of 

 water in the cells of the root nearly constant. 



Osmosis. The third physical process that aids in the 

 absorption 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 

 its minute pores. The water continues to move through until 

 its level is the same inside and outside (Fig. 100). 



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 dif- 

 ference from the diagram in Figure 101. The membrane (C) 

 allows water molecules to pass through it freely, but it per- 

 mits scarcely any of the sugar molecules to pass. The outer 

 side of the membrane is completely covered with water mole- 

 cules (B), tending to diffuse through the membrane. The 

 inner side (A) is only partly covered with water molecules, 

 since part of the area is occupied by sugar molecules. Con- 

 sequently there are fewer water particles on the side A tend- 

 ing to diffuse outward than there are on the side B tending to 



