OSMOSE. 211 



parts. The pentamerous fruit of Erodium gruinum, for example, separates into 

 five single parts, each enclosing a seed, and each of which is provided with a 

 long awn which, on the drying up of the fruit, becomes twisted into a spiral at 

 its lower end; if this organ is moistened the awn is extended quite straight, 

 and the very energetic movements produced by alternate drying and wetting, 

 together with various additional adaptations, lead finally to the result that the 

 fruit bores into the earth, to germinate there in the following spring. The 

 awns of many grasses behave similarly. The so-called warping of wood on drying 

 is also caused by the unequal changes of volume during the drying — i. e. the loss 

 of the water of imbibition of the wood ; and in the same way many movements 

 of the branches of trees during intense cold are to be referred to the same 

 principle, since the solidification to ice of the imbibed water acts in the same 

 way as desiccation, and, if it occurs unequally on different sides of a branch, 

 must produce curvatures. 



The great force with which water penetrates into cells capable of swelling 

 will, as already mentioned, subsequently enable us to understand how the highest 

 trees are enabled to raise the large quantities of water transpired from their foliage, 

 from the roots through the stem into the leaves. A third fact also comes into 

 consideration here : the fluid moving in the substance of the cell-walls as water 

 of imbibition is not pure water, but a very dilute solution of those materials which 

 the roots absorb from the soil, and on account of which (since they are necessary 

 for assimilation) the entire water-current towards the leaves is set in motion. The 

 conveyance of substances produced in any cell whatever towards neighbouring cells 

 can also only take place by the cell-walls, as well as the protoplasmic linings of 

 the cells being able to absorb not merely pure water but also aqueous solutions. 

 This process of the movement of substances from cell to cell requires, however, the 

 consideration of a far more complicated phenomenon, depending upon the processes 

 of solution and swelling hitherto described, and termed Dtosmosis (Osmose). 



If a wide glass tube is closed at the lower end with an organic mem- 

 brane, and a quantity of salt solution is poured into the tube, which is then im- 

 mersed in pure water, the latter penetrates the closing membrane as water of 

 imbibition, is taken up by the molecules of salt of the solution in the tube, and 

 serves for the dilution of this solution. If this process continues sufficiently 

 long, a remarkable increase in the volume of the Uquid in the interior of the glass 

 tube takes place, and the fluid ascends in it, under certain circumstances, to 

 a very considerable height: the cause of this movement lies in the attraction 

 of the particles of salt for the water which permeates the closing membrane 

 from below. This process, here described so simply, is designated Endosmosis 

 (Endosmose). Under certain circumstances Exosmosis (Exosmose) also may occur. 

 If the membrane closing the tube is capable of absorbing or imbibing the salt 

 solution in the tube, the attraction of the external water causes a portion of the 

 dissolved salt molecules which have penetrated into the membrane to diflfuse out 

 into the water, while at the same time a larger quantity of water molecules ascend 

 through the membrane into the salt solution. It depends however entirely upon the 

 nature of the membrane and the dissolved salt, whether the latter can exude at all 

 by exosmose. In the process of osmose therefore, there are two points of prominent 



p 2 



