21 O LECTURE XIII. 



phenomena. We obtain an idea of the enormous magnitude of the forces here 

 coming into consideration only through knowledge of the fact that during the 

 penetration of water into dry starch-grains ^ a rise of temperature of several degrees 

 centigrade takes place; for this fact can scarcely be otherwise explained than by 

 assuming that the penetrating water is condensed, and a corresponding heating 

 effect follows. Since, however, a condensation of water by which a rise of 

 temperature of only 07ie degree is brought about requires a pressure of several 

 hundred atmospheres, we arrive at the conclusion that the condensation of the 

 water as it forces asunder the particles of the imbibing body must be equivalent 

 to a pressure of many hundred atmospheres. 



But organic bodies capable of swelling are able to absorb not merely pure 

 water, but also aqueous solutions : here however peculiar phenomena make them- 

 selves apparent. It depends entirely upon the nature of the atoms distributed in 

 the water of the solution, on the one hand, and that of the swelling body on 

 the other hand, how much of the former enters simultaneously with the 

 water between the particles of the latter. In many cases a swelling cell-wall 

 absorbs a larger quantity of water, but a smaller quantity of dissolved matter, 

 from a somewhat concentrated salt solution, than corresponds with the concen- 

 tration of the solution. It follows from certain phenomena which we will examine 

 more closely later on, that living protoplasm absorbs only pure water from 

 certain solutions, leaving the dissolved substances behind. In other cases, again, the 

 swelling body absorbs a far larger quantity of the dissolved substances than accords 

 with the percentage composition of the solution. This is particularly conspicuous 

 with many colouring-matters, which may be so strongly absorbed, especially by 

 bodies consisting of proteids, such as dead protoplasm and crystalloids, that these 

 bodies become intensely and darkly coloured, even when the solution itself contains 

 but little colouring matter and is very light-coloured. 



According to circumstances, the various phenomena of swelling shortly 

 indicated here play a part in the vital phenomena of plants. The changes in 

 volume which cell-membranes especially undergo by imbibition and desiccation, 

 may cause various movements of dead masses of tissue or of individual 

 cells. The dehiscence of dry capsular fruits, for the purpose of distributing 

 their seeds, is in general brought about by the fact that, on the drying up 

 of the pericarps, either their outer or inner sides lose relatively more of the 

 water of imbibition, by which curvatures and even ruptures of particular parts 

 of the pericarp are produced. In some cases these curvatures produced by 

 unequal contraction and dilatation are, according to the stmcture of the organ, 

 combined with spiral windings or with the rolling up and extension of band-like 



1 That heat is set free when water enters into organised, and to a smaller extent also when 

 it passes into unorganised bodies, was, according to Pfeffer, first established by Pouillet. Jimgk and 

 I observed the increase of temperature during imbibition by starch in 1865 (cp. 'Lehrbuch der 

 Botanik," 1868, p. 500). Naegeli (' Theorie der Gährung; 1879, p. 133) found that the increase 

 of temperature when perfectly dry starch absorbed water, amounted to 11.6° C, both bodies before 

 the combination being at the temperature of 22° C. Since now, according to Joule, water is heated 

 0.03° C. by a pressure of 34.3 atmospheres, it follows that a rise of temperature of 11° must corre- 

 spond to a pressure of enormous magnitude. The water penetrating first into the starch grains 

 undergoes the most considerable rise of temperature, and therefore the greatest compression. 



