MOVEMENTS DUE TO SWELLING, ETC. 407 



water be warmed, more will be absorbed, and we discover that the power of ab- 

 sorption possessed by gelatine depends directly on temperature. When the 

 temperature reaches a certain degree the absorption becomes unlimited, or, as 

 we are accustomed to put it, the gelatine dissolves in the water. The same thing 

 applies at ordinary temperatures to gum arable — the absorption of water by 

 that substance suggesting at first a case of swelling, but becoming indefinite. 

 Hence we see that swelling may gradually merge into solution. It would be 

 quite incorrect, however, to assume that all bodies capable of swelling are also 

 capable at some definite temperature of dissolving in the medium employed. 

 Cell-walls especially, at the present moment of primary interest to us, remain 

 stationary when they reach a state of maximum imbibition. The changes there- 

 fore which gelatine undergoes in solution we need not discuss. 



In order to obtain a closer insight into the phenomenon of water imbibition 

 we will compare substances capable of swelling, such as a cell-wall, or a piece 

 of gelatine with a finely porous body, such as a plate of plaster of Paris 

 saturated with water and afterwards air dried. If a plate of plaster of Paris be 

 placed in water, it absorbs a certain quantity of it and retains it firmly when with- 

 drawn from the water. The water is, however, retained in pre-existing spaces, 

 as may be seen from the fact that the air escapes in bubbles when the plaster 

 is placed under water ; in other words, the water forces its way into the plaster 

 by capillarity and replaces the air previously present in these spaces. In a cell- 

 wall or a piece of gelatine, on the other hand, capillary spaces containing air 

 cannot be seen even with the best lenses, and, further, impermeability of the 

 substance to air proves that such spaces do not exist. Even were such spaces 

 present there is yet another fundamental difference between finely porous 

 bodies and bodies capable of swelling. The plaster of Paris shows no increase 

 in volume after the absorption of water, such as substances do which are capable 

 of swelling. Further, when water forces its way into previously existing spaces 

 in the latter, these spaces must be enlarged by the water and the minute particles 

 separated from each other, a phenomenon which obviously does not take place 

 in a solid body which does not increase in volume. On the contrary, Askenasy 

 (1900) has observed that in consequence of a capillary entrance of water a 

 diminution in the volume of a non-swelling body may take place, e. g. in deposits 

 on cover-glasses. Bodies capable of swelling must possess a special molecular 

 structure which may not be directly observable but which can only be deduced 

 from observation of their behaviour. 



Among hypotheses of molecular structure, that advanced by Nageli 

 has undoubtedly had the most lasting influence in Botany, more especially in 

 relation to the phenomena of imbibition, and even now it claims a certain 

 recognition. Since, however, some essential parts of the hypothesis have 

 been refuted, it will serve our purpose best if we deal here merely with 

 such parts as are vital to our discussion. Nageli (1858) held that bodies 

 capable of swelling were composed of extremely minute particles, larger than 

 molecules, to which he gave the name of micellae. Since there is no longer any 

 ground for believing in the existence of micellae we need not lay any emphasis 

 on this conception. When the body was in the dry condition the micellae 

 were supposed to be close to each other without any air-spaces between them ; 

 the micellae were regarded as polyhedral in form, and were considered to be 

 held together by mutual attraction. Since the micellae had also an attraction 

 for water, each attempted to surround itself with a film of water. This, how- 

 ever, could not take place unless the force of attraction between water and 

 micella, was greater than that between the micellae themselves. The 

 addition of water thus induced a separation of the micellae, and explained at 

 once the increased volume of the swollen body. Should this be limited, the 

 resistance offered by the separation of the micellae to any further entry of 



