36 



Elementary Botany 



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dissolves up the various food materials, and thus brings them 



into a fit state to be taken. 



The outside surface of the root is entire, not perforated by 



holes, hence the solution of food has to be absorbed into the 



cells of the plant itself This is accomplished by means of the 

 process of osmosis. If two liquids be se- 

 parated by a membrane permeable to both, 

 they will flow through it and intermingle. 

 This can be very easily demonstrated 

 by means of a simple experiment. If a 

 vessel is closed below by a piece of bladder, 

 whilst a tube is tightly fastened in the neck 

 (fig. 62), and if a concentrated solution of 

 cupric sulphate is placed in the vessel so 

 that it stands a short way up the tube, and 

 then the whole apparatus is plunged into 

 a vessel of water as shown in the diagram, 

 the two liquids will interchange, some of 

 the cupric sulphate passing through the 

 bladder into the outer water, whilst some 

 of the latter enters the vessel. The two 



I'l -Ss* liquids do not, however, flow with equal 



, J — ■* ^ rapidity. The law in all such cases is that 



the denser liquid flows more slowly than 

 the less concentrated. As cupric sulphate 

 solution is denser than water, a larger 

 amount of the latter passes through the 

 membrane than of the former, and therefore 

 the liquid will rise in the glass tube. The 

 passage inwards is called endosmose, whilst 

 that outwards is termed exosmose. Sugar 

 syrup may be used in the above experiment 

 instead of cupric sulphate. 

 Let us apply this process to the root. Within the cells 

 there is the sap, without there is the water with mineral salts 

 in solution ; we have here two liquids of different densities 

 separated by a permeable membrane, the cell wall. As a con- 

 sequence osmosis ensues, some of the external water passing 



Fig. 62.— Apparatus for 

 measuring osmose : h, a 

 vessel filled with cupric 

 sulphate closed helow 

 hy a permeable mem- 

 brane, and placed in a 

 vessel of water. As the 

 water passes through 

 the bladder to mingle 

 with the cupric sul- 

 phate, the level of the 

 fluid will rise in the tube 

 r in connection with the 

 vessel ^, but will fall at 

 « in the outer vessel. 



