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BURTON EDWARD LIVINGSTON 



of the protoplasm, these are an osmotic pressure of dissolved sub- 

 stances and an imbibition force; for the cell wall, too, is to be 

 regarded as both a colloid and a true solution. As a colloid, much 

 of its material is coagulated, it is hydrogel, and like other hydro- 

 gels it exhibits the power of imbibition. By virtue of this power 

 the cell wall tends to absorb water at both its surfaces, from the 

 contained protoplasm on one side and from the cell environment 

 (which, in our present instance, is a weak aqueous solution) on 

 the other. Of course this tendency toward water imbibition (or 

 resistance to water loss) results in a second force which opposes 

 the stretching force from within. But at the same time, absorp- 

 tion of water automatically reduces the force of imbibition, so 

 that as water enters the wall from without, this inwardly directed 

 force soon vanishes, the wall approaching imbibitional saturation. 



Furthermore, as has been remarked, the imbibing force varies 

 not only with the water content of the imbibing colloid, but also 

 with the nature of the colloid, and vegetable cell walls vary greatly 

 in this latter respect, being composed of different sorts of material 

 in different cells and also in the same cell at different times. 

 With alteration of a cell wall, as by impregnation with wax-like 

 substances, the water content corresponding to imbibitional satura- 

 tion is of course lowered. It thus comes about that many cell 

 walls, notably the external ones of peripheral cells, may be imbibi- 

 tionally saturated with water and at the same time may contain 

 only a very small amount of this liquid. Such walls are often 

 said to be imper\dous to water, though of course the}^ are always 

 permeable to some extent. In these waxy walls the gradient of 

 imbibitional attraction for water, between the condition of com- 

 plete dryness and that of saturation, is apparently very steep, 

 so that the force of imbibition, only a little before saturation is 

 attained, is still great. 



What we have thus far considered as imbibed water in the cell 

 wall is of course not pure water but a solution. In so far as the 

 outer protoplasmic film is permeable to the solutes within its 

 mass these should diffuse outward and bring the imbibed solution 

 of the wall into concentration equilibrium with the protoplasmic 

 solution. Indeed, there must always be freedom of interchange 



