31 a LECTURE XIII. 



interest— in the first place, the question whether a substance dissolved in water is 

 able to pass through a given membrane; and, secondly, the force with which the 

 water on the one side and the dissolved substance on the other side of the 

 membrane attract one another. It depends upon this, whether a given substance 

 can pass from one cell into another in the living plant, and with what force cells 

 are enabled to take up water into themselves. If, for example, there arises by 

 chemical decompositions, in any cell whatever, a combination of atoms which 

 cannot diffuse out through the protoplasm and the cell-wall into the neighbouring 

 cells, this solution must accumulate in the cell in question, and even attain a 

 high degree of concentration, without passing into the neighbouring cells ; on the 

 other hand, chemical compounds which are contained in a cell-tissue may become 

 accumulated to a large extent in any given cell, if they suffer a change of their 

 condition of aggregation in the latter. If, for example, the sugar penetrating into a 

 cell is used for the formation of starch-grains, fresh sugar is enabled to penetrate 

 continually into this cell so long as this change takes place ; and, as we shall 

 see later, the plant makes the most abundant use of this and similar processes. 

 The distribution of the various chemical compounds in the tissues of plants, their 

 travelling over wide distances, and their accumulation in definite organs, depend 

 upon such processes ; a point to which we shall return more in detail subsequently 

 in the theory of nutrition. 



The use made of the force with which water penetrates by endosmose 

 into the cell is no less varied and general. The first phenomenon produced by 

 this is the so-called turgescence of the celP; which however can take place 

 only in living cells still provided with protoplasm. We have to picture such a 

 cell as a double-walled vesicle, closed on all sides : the external wall consists of 

 cellulose, the internal or second wall, closely applied to it, consists of protoplasm, 

 and the enclosed cavity is filled with solutions of salts (the cell-sap). Let us suppose 

 the simplest case, and that such a cell lies in water. The water of imbibition 

 contained in the walls is taken up by the salt-molecules contained in the cell-sap, 

 and an equal quantity penetrates into the two peripheral layers from the exterior. 

 If this process continues for a long time, a large quantity of water gradually 

 penetrates into the interior of the cell ; and this is rendered possible only by the 

 double wall becoming distended to a corresponding extent. When this distension 

 finally ceases, the wall opposing it, no further flow of water inwards can take 

 place. The cell is now in the state of turgescence — i. e. the walls are distended 

 by the water which has forcibly penetrated into the cell, and since they strive 

 to contract elastically, they exert a pressure on the internal fluid. The main 

 point to be noticed here, is that the force by which the wall is pressed outwards 

 arises from the attracting force of the salts dissolved in the cell-sap towards the 

 water surrounding the cell ; and the opposite pressure, which prevents the further 

 penetration of water, is afforded by the elasticity and cohesion of the cell-wall. 

 In this, moreover, another fact is to be especially observed. The cellulose 

 wall, as is known from numerous observations, is, it is true, but slightly extensible 



' Further particulars as to the turgescence of cells and its significance in growth as well as in 

 irritable movements will be given later. 



