16 EXPERIMENTAL PLANT PHYSIOLOGY 



sugar put into the cavity stands in an empty dish 

 (Fig. 5). 



Observations. — In about an hour's time the pits 

 which contained the sugar are found 

 to be half full of liquid. The 

 empty pit still remains empty. Later 

 the pits containing sugar become 

 full of liquid and begin to overflow 

 Fig 5 while the empty pit gets gradually 



drier. 

 No difference is at first discernible between the 



Sotato surrounded by water and that in the empty 

 ish, but, after a day or so, the one not surrounded by 

 water begins to look shrivelled, especially at the base, 

 whilst the potato in the water remains as turgid as it 

 was at the beginning of the experiment. 



Inferences. — Osmosis has taken place between the 

 sugar solution in the pit and the sap in the cells of the 

 potato : the sugar has attracted the weaker sap from 

 the cells of the potato adjacent to the pit, at the same 

 time a little of the sugar has passed from the pit into 

 these adjacent cells, thus strengthening their cell-sap; 

 as a result of this increase in density, these cells have 

 been able, in their turn, to draw sap from the next 

 layer; this process has been repeated through each 

 succeeding layer until the outermost cells of the potato 

 have been reached. Thus a flow of liquid towards the 

 pit has been set up. 



In the case of the potato surrounded by water the 

 outermost cells of the potato draw the water from the 

 dish and thus a continuous flow is set up from the dish 

 to the pit. 



In Experiment 3 it was proved that absorption 

 decreased as the strength of the solution absorbed in- 

 creased, and that it ceased altogether after a certain 

 strength of solution was reached. The reason for this 

 is now clear. The solutions outside must be weaker 

 than the cell-sap if these solutions are to be drawn 

 into the root, and the greater the difference in the 



