CHAPTER IX 

 IMBIBITION 



If a handful of the seeds of any species which do not possess seed coats 

 impermeable to water be dropped into a vessel of water, within a few hours 

 they will have swollen visibly. Many other materials will swell in a similar 

 way when immersed in water. Among these are starch, cellulose, agar, gelatin, 

 and kelp stipe. Some substances will swell similarly when immersed in other 

 liquids. All of these phenomena are examples of the process usually called 

 imbibition. The amount of water which may enter substances in imbibition is 

 often very great in proportion to the dry weight of the substance which swells. 

 A piece of dried kelp stipe, for example, can absorb as much as fifteen times 

 its own weight of water. 



Water may be imbibed as a vapor as well as in the liquid state. The 

 swelling of doors and woodwork generally during damp weather is a familiar 

 example of this phenomenon. Plant structures, if sufficiently low in water 

 content, also imbibe water-vapor. The water content of "air-dry" seeds, for 

 example, generally fluctuates with the vapor pressure of the atmosphere, rising 

 with an increase in vapor pressure, and vice versa. 



The Dynamics of Imbibition. — Imbibition is usually considered to be 

 basically a diffusion process, but capillary phenomena are probably also in- 

 volved. Imbibing substances are often permeated with minute submicroscopic 

 capillaries, and it is impossible to determine how much of the liquid enters 

 by diffusion and how much by capillary movement through invisible pores. 

 Fundamentally, however, the cause of imbibition may be regarded as a dif- 

 ference in the diffusion pressure between the liquid in the external medium 

 and the liquid in the "imbibant." As long as the latter is less than the former, 

 movement of water into the imbibing substance will continue. An equilibrium 

 will be reached, as in diffusion or osmotic phenomena, only when the diffusion 

 pressure of the water in the two parts of the system has attained the same 

 value. 



A substance which imbibes water does not necessarily imbibe other liquids. 

 Dry kelp stipe, for example, swells enormously when immersed in water, 

 but does not swell when immersed in ether or other organic liquids. Con- 



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