VI] MACALLUM'S EXPERIMENTS 287 



to the phenomena of surface energy and of adsorption*. And 

 all this implies that the study of these minute structures, if it 

 teach us nothing else, at least surely and certainly reveals to us 

 the presence of a definite "field of force," and a dynamical polarity 

 within the cell. 



Our next and last illustration of the effects of adsorption, 



which we owe to the investigations of Professor Macallum, is of 



great importance ; for it introduces us to a series of phenomena 



in regard to which we seem now to stand on firmer ground than 



in some of the foregoing cases, though we cannot yet consider that 



the whole story has been told. In our last chapter we were 



restricted mainly, though not entirely, to a consideration of figures 



of equilibrium, such as the sphere, the cylinder or the unduloid ; 



and we began at once to find ourselves in difficulties when we were 



confronted by departures from symmetry, as for instance in the 



simple case of the ellipsoidal yeast-cell and the production of its 



bud. We found the cylindrical cell of Spirogyra, with its plane 



or spherical ends, a comparatively simple matter to understand; 



but when this uniform cylinder puts out a lateral outgrowth, in 



the act of conjugation, we have a new and very different system 



of forces to explain. The analogy of the soap-bubble, or of the 



simple liquid drop, was apt to lead us to suppose that the surface 



tension ivas, on the whole, uniform over the surface of our cell; 



and that its departures from symmetry of form were therefore 



likely to be due to variations in external resistance. But if we 



have been inclined to make such an assumption we must now 



* Traube in particular has maintained that in differences of surface-tension 

 we have the origin of the active force productive of osmotic currents, and that 

 herein we find an explanation, or an approach to an explanation, of many phenomena 

 which were formerly deemed peculiarly "vital" in their character. "Die Differenz 

 der Oberflachenspannungen oder der Oberflachendruck eine Kraft darsteUt, welche 

 als treibende Kraft der Osmose, an die Stelle des nicht mit dem Oberflachendruck 

 identischen osmotischen Druckes, zu setzen ist, etc." (Oberflachendi-uck und 

 •seine Bedeutung im Organismus, Pfliiger's Archiv, cv, p. 559, 1904.) Cf. also 

 Hardy {Pr. Phys. Soc. xxvm, p. 116, 1916), "If the surface film of a colloid 

 membrane separating two masses of fluid were to change in such a way as to lower 

 the potential of the water in it, water would enter the region from both sides at 

 once. But if the change of state were to be propagated as a wave of change, 

 starting at one face and dying out at the other face, water would be carried along" 

 from one side of the membrane to the other. A succession of such waves would 

 maintain a flow of fluid." 



