Water Deficit or Unsatisfied Hydration Capacity. 109 



gives rise to most of the phenomena known as permeabiUty and osmosis. 

 The external layer, as well as the entire colloidal mass, reacts to solu- 

 tions in a manner determined by its composition and its history, 

 especially with respect to the salts which have come into the cell since 

 its formation, and particularly to the salts which may be dissolved in 

 the water in which the cell is immersed. The absorption of such salts 

 alters the capacity of the colloids in various ways. In the case of the 

 alkaline solutions, it has already been pointed out that nearly all plant 

 tissues show a long-continued swelling which may be reasonably 

 ascribed to the formation of a salt between the sugar and the hydroxide, 

 which salt has a greater capacity for hydration than the carbohy- 

 drates alone. 



Acids decrease the hydration capacity of agar and of the pentosans 

 which enter into the composition of certain types of plant cells, of 

 which the tissues of the potato would be a good example. 



The death of cell-masses by desiccation probably produces less dis- 

 arrangement of the material of the cell than any other method of 

 kilUng, and not only the colloidal mass, but its external layer, probably 

 retains its condition with respect to phases with but little serious 

 disturbance. The only positive movement which might be recog- 

 nized as of importance in connection with the point now being 

 discussed would be that as drying-out proceeds the salts and acids 

 of the mass would pass toward the periphery and might accumulate 

 at that place, although it could by no means be assumed that such 

 action would result in a uniform deposition in the membrane. The 

 hydration of such dried material would take place as in other dead 

 material, being largely by means of imbibition and adsorption, with 

 osmosis by the action of vacuolar material almost eliminated. The 

 records of many measurements of dried plants, of which fresh and 

 Uving portions have been tested, as described in this chapter, are 

 meant to be explanatory and illustrative, but they show conclusively 

 that any serious development of our conceptions of the water-rela- 

 tions of cell-masses must be based upon and take seriously into con- 

 sideration the hydration processes of the protoplastic colloids. 



Colloids are never at rest, yet it is possible to secure combinations 

 of conditions in mixtures in which hydration, for example, is all but 

 complete. Protoplasm, however, is the seat of complex transforma- 

 tions and is the medium of such diverse diffusion movements that 

 ideally it is never in a condition of satisfied hydration. The amounts 

 which it may take up from different solutions and under various con- 

 ditions previously described may, on proper analysis, serve to show 

 the natiire of the protoplastic colloid with respect to its principal 

 components. The determination of the index of unsatisfied hydra- 

 tion, or the water deficit, is an indispensable feature of any serious 

 effort to analyze growth into its physical components. 



