DISCUSSION QUESTIONS 115 



ing the seeds will have set. The resulting solid cone is removed from the 

 funnel and its base immersed in a dish of water. Water moves upward 

 through the porous gypsum cone by capillarity and permits a continued im- 

 bibition by the pea seeds. Within a few hours the pressure developed by 

 the swelling pea seeds is sufficiently great to rupture the gypsum block. 



The magnitude of the pressure developed during imbibition may be meas- 

 ured quantitatively in two different ways. It may be counterbalanced by a 

 mechanical pressure, the magnitude of which becomes a measure of the imbibi- 

 tion pressure developed. Such a method was used by Reinke as long ago as 

 1879. He stacked disks of dried fronds of Laminaria (a sea weed) in a hol- 

 low metal cylinder, and inserted above the disks a metal piston bearing a plat- 

 form at its upper end. Water was then brought into contact with the dry 

 disks. By placing upon the platform weights of sufficient mass to just prevent 

 swelling of the kelp, the magnitude of the imbibition pressure was determined. 

 A second method of measuring this quantity relies upon counterbalancing the 

 imbibition pressure of a substance by means of a solution of high osmotic pres- 

 sure. The pressure developed by imbibition may be regarded as due to the 

 diffusion pressure of the entering water. If the imbibing substance be im- 

 mersed in a solution in which the diffusion pressure deficit (osmotic pressure) 

 is sufficiently great to prevent any movement of water into that substance 

 the osmotic pressure of that solution is a measure of the imbibition pressure 

 which would be developed in that imbibant. In Table 17 the data show that 

 a solution with an osmotic pressure of nearly a thousand atmospheres was 

 required to prevent any imbibition of water by cocklebur seeds. As deter- 

 mined by either of the two methods just described the imbibition pressure of 

 an imbibant is analogous to the osmotic pressure of a solution, i.e. it is a 

 measure of the maximum imbibition pressure which the imbibant can develop. 



Discussion Questions 



1. How could you decide whether the swelling of dry prunes in water is the 



result of osmosis or of imbibition? 



2. When equal volumes of starch and water are used why is less heat liberated 



when air dry corn starch is mixed with water than when corn starch which 

 has been dried in a desiccator over sulfuric acid is used? 



3. Would you expect gelatin and plant cell walls to show similar swelling 



behavior when immersed in a series of solutions of different pH ? 



4. In a system composed of kelp stipe or wheat seeds plus water would you ex- 



pect shrinkage in volume of the system to be greatest at 10° C. or at 40° C. ? 

 Explain. 



5. How would you expect cations to affect the swelling of an imbibant in which 



the micelles are positively charged? anions? 



6. If pieces of kelp stipe are fastened to the base of a hydrometer which is then 



immersed in a vessel of water the hydrometer will sink as the kelp swells. 

 Explain. 



