166 



BURTON EDWARD LIVINGSTON 



toplasm itself, simultaneously a colloidal suspension and a true 

 solution, of many substances. This mass appears to possess con- 

 siderable cohesion, being neither a true solid nor a true liquid, 

 and whateN'er cohesive force is here present must tend to resist 

 stretching and vacuolar enlargement. 



(4) As a true solution, the protoplasmic mass must resemble 

 the vacuolar solution and its solutes should exhibit a diffusion 

 tension, this latter developing an effective osmotic pressure at 

 the two peripheral films. The inwardly directed pressure tends 

 directly to oppose the vacuolar pressure and the outwardly di- 

 rected one should tend to enlarge the outer plasmatic surface, to 

 increase the volume of the protoplasmic mass itself, and to cause 

 the virtual entrance of water into the latter from without. The 

 outward pressure should be opposed by the external surface ten- 

 sion above mentioned, as well as by any osmotic or other inwardly 

 directed pressures developed in the immediate surroundings of 

 the cell. 



(5) As a colloid, the protoplasmic mass possesses considerable 

 power of imbibition and may attract water at both peripheries. 

 This force of imbibition should act in the same general manner as 

 does the osmotic pressure of the plasma, tending to prevent enlarge- 

 ment of the vacuole and to increase the volume and water content 

 of the protoplasmic mass itself. 



The water-' mbibing power of protoplasm has recently been 

 given a prominent place in general physiology through the bril- 

 liant work of Martin H. Fisher, who has formulated and defended 

 the thesis that many phenomena of water absorption and of tur- 

 gidity, which have heretofore been attributed to osmotic pressure, 

 are in reality due to colloid imbibition. Wliile there appears to 

 be no ground for doubt that Fisher's contention must hold, at 

 least in many instances, for non-vacuolated cells (such as compose 

 the majorit}^ of animal tissues), the student of plant physiology 

 will not be readily convinced that osmotic pressure does not play ' 

 the prime role in the maintenance of turgidity in the commonly 

 vacuolated cells of plants, where the protoplasmic mass, itself 

 usually possessed of no very great viscosity, is relatively very 

 small as compared to the vacuole. 



