172 Science of Plant Life 



dioxid, when once dissolved in the water of cells, move 

 about partly by diffusion. 



Imbibition. The process of imbibition may be illustrated by 

 placing a sheet of gelatin in water. Dry gelatin is a hard, 

 brittle, partly transparent solid. After it has been in water 

 for a few minutes, it will be found to have increased in weight 

 and in length, breadth, and thickness. The gelatin, instead 

 of being brittle, is now soft and pliable ; it is also more trans- 

 parent than it was. 



The increase in size and weight is explained by the fact that 

 particles of water have forced their way between the particles 

 of the gelatin, spreading them apart. Since the gelatin par- 

 ticles have been forced farther apart, the gelatin is more pli- 

 able and the particles cling to one another less firmly. Hence 

 when a piece of dry wood is put into water, it imbibes water 

 and swells. The cell walls of a root, like wood, are largely 

 composed of cellulose and they take up water in the same way. 

 When dry seeds are placed in water, they imbibe water and 

 increase in size. Indeed, most organic substances have 

 the property of imbibing water and swelling. Imbibition is 

 a form of diffusion that results in swelling. Compare the 

 size of a sponge when dry with its size after it has been soaked 

 in water and squeezed as dry as possible. 



WTien a piece of wood becomes saturated, it stops taking 

 up water. If, however, the water were being removed from 

 the inside, more would continue to pass into the wood. This 

 is exactly what happens in the root of a living plant. The 

 external 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 



