THE SWELLING OF ORGANIZED BODIES 71 



a general standpoint it is unimportant whether the discrete particles of 

 which the swelling substance is composed arc molecules, micellae or 

 other units. Hence for the present no special molecular structure need 

 be assumed. 



The swelling is dependent not only upon the nature of the substance 

 itself, but upon the character of the fluid absorbed : thus cell-walls swell 

 in water but not in alchohol or turpentine, while gum arabic, in water 

 alone, swells indefinitely and finally passes into solution, whereas in the 

 presence of a little alcohol the gum arabic absorbs only a certain amount 

 of water, and swells to a limited extent. A variety of intermediate stages 

 are possible between the limited absorption of water which causes simple 

 swelling, and the unlimited absorption which gives rise to a condition of 

 solution. It is indeed often doubtful whether certain colloid substances do 

 actually dissolve in the fluid throughout which they are distributed l . 



Limited solubility in a given fluid is due to the latter having a restricted 

 power of taking up the particles of the dissolved substance. Hence im- 

 bibition, swelling, and the formation of the so-called solid solutions are all 

 closely related phenomena. Thus when carbon is absorbed by a piece of 

 iron, hydrogen by palladium or spongy platinum, or when mercury is 

 taken up by other metals to form amalgams, the penetration of the foreign 

 element causes a certain change of volume 2 . 



The amount of imbibition of which organized bodies are capable 

 varies within wide limits : thus corky membranes hardly swell at all, 

 while the gelatinous envelopes of the Nostocaceae, &c., according to 

 Nageli 3 , contain 200 of water to one part of solid substance. The 

 dry walls of wood tracheides can absorb 48 to 51 per cent, of water 4 , 

 whereas the amount of water present in the protoplasm varies from 

 60 to 90 per cent. It is true the latter is semi-fluid, but as is shown by 

 the gelatinous membranes of the Nostocaceae, a solid condition is possible 

 even when the amount of water present is extremely great. The semi-fluid 

 nature of the plasma is of the highest physiological importance, as it 

 renders internal and external alterations and movements more readily 

 possible. In the cell-wall on the other hand a certain degree of rigidity 

 is essential, and hence its component particles exhibit a high degree of 

 cohesion, even when the imbibition and swelling are so great that they 

 are relatively widely separated from one another. 



1 Cf. Lehmann, Molecularphysik, 1888, Bd. i, p. 526 ; Barns und Schneider, Zeitschr. f. 

 physik. Chemie, 1891, Bd. vin, p. 278. On Glycogen, &c., Brucke, Vorles. iiber Physiol., 1881, 

 3. Aufl., p. 325 ; and Errera, 1'Epiplasme des Ascomycetes, 1882, p. 70. 



2 Cf. Van 't Hoff, Zeitschr. f. physik. Chemie, 1890, Bd. v, p. 325; Kiister, ibid., i> s >>4> 

 Bd. xm, p. 445. 



3 Nageli, Starkekorner, 1858, p. 312 ; other examples by Hofmeister, Pflanzenzelle, 1867, p. 214. 

 * Sachs, Arb. d. Bot. Inst. in \Yurzburg, 1879, Bd. II, p. 312. Cf. Sect. 27. 



