CIRCULATION OF THE SAP. 



143 



ments in the contents of cells and vessels take place when fluids of 

 difierent densities are separated by an animal or vegetable mem- 

 brane. 



If, on opposite sides of an animal or vegetable membrane, we place 

 two fluids of unequal density, having an affinity for the interposed 

 membrane and for each other, the fluid on the one side being thick and 

 gelatinous, whilst the other is thin and watery, two unequal and 

 opposite currents are at once established — the thin fluid setting with 

 a strong and full current through the membrane towards the thicker 

 fluid, which it penetrates ; the thicker fluid, with a more feeble current 

 and in less quantity, reaching the thin fluid with which it mingles. 

 This constitutes Osmose. The inequality in strength and amount of 

 the two currents depends, not so much on the density of the liquids, as 

 on their character, those of a gluey or albuminous nature passing 

 slowly, whilst those of a more liquid' nature transude very rapidly. 

 If the membrane form a sac or bladder, in which the thick gelatinous 

 fluid is contained, then the thin fluid rapidly passing 

 inwards into the sac penetrates the thick fluid, and 

 thus the amount of fluid in the bladder is increased 

 and its walls are distended. To this inward current 

 the term Endosmose is applied, and conversely, Exos- 

 mose refers to the slow and feeble outward current of 

 the thick contained fluid. In this instance the Endos- 

 mose current is the stronger, but a reversal of the 

 relation of the fluids to the membrane renders the 

 Exosmose current the stronger, consequently the con- 

 tents of the sac are diminished in amount and its 

 walls collapse. The relative rapidity of the Exosmose 

 and Endosmose currents depends on the position of the 

 liquids as regards the membrane ; the strongest cur- 

 rent always setting in towards the most colloid body. 

 In flg. 240 is represented the mode of showing en- 

 dosmose by means of a bladder full of syrup,' which is 

 attached to the end of a tube, and immersed in water. 

 In this case the water passes rapidly into the bladder 

 by endosmose, so that the fluid rises iu the tube, while a portion of 

 the thicker fluid passes out by exosmose. The force of this endosmose 

 may be measured by a graduated tube, as in the flgure, or by a tube 

 with a double curvature, as fig. 242, the lower part of which is filled 

 with mercury. In the flatter case the mercury is pushed upwards 

 into a graduated tube, and thus an endosmometer {liir^ov, a measure), 

 or measure of the force of endosmose, is formed. 



Pig. 240. 



Fig. 240. Instrument to show Endosmose and Exosmose, consisting of a Madder con- 

 taining syrup attached to a tube, and plunged in a vessel of water. The inward motion of 

 the water (endosmose) exceeds the outward movement of the syrup (exosmose). 



