84 



General Botany 



W. S. Cooper 



Fig. 46. The barrel cactus {Echinocactus cylindraceus) of the Colorado desert, California. 

 These plants are highly resistant to water loss because of the presence of mucilaginous 

 carbohydrates which imbibe and hold water. 



brittle, is now soft and pliable; it is also more transparent than 

 it was. 



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

 particles of water have diffused into the gelatin and have forced 

 the particles apart. Since the gelatin particles have been forced 

 farther apart, the gelatin is more phable and the particles chng to 

 one another less firmly. The cell walls of a plant take up water 

 in the same way. Hence when a piece of dry wood is put in 

 water, it imbibes water and swells. 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. 



When 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 hving plant. The external 



