88 



iastance, strong solutions of sugars and other substances 

 exercising a powerful attraction for water are formed by the 

 protoplasm and are held by it in the cell, so that a 

 current of water is drawn rapidly into the cell by endos- 

 mosis just as happens in the case of the pig's bladder. Any 

 Rubstances also which are in solution in the water surround- 

 ing the cell which are either absent from the cell-sap, or of which 

 a smaller proportion exists in the cell-sap than is present 

 in the surrounding water, will also pass into the cell-sap with 

 the water current, provided that the protoplasm permits them 

 to pass. 



Now, although the protoplasm in this way allows a solution 

 of particular materials to pass through its substance into the 

 cell-sap, it does not permit it to filter back again out of the cell, 

 but holds it fast. As water continues to pass into the cell 

 the protoplasmic lining is pressed outwards against the cell- 

 wall which in its turn becomes distended, but which, by its 

 resistance to the pressure, develops a state of tension in the 

 cell. A cell in this state becomes stiff just like a bladder 

 distended by air and is said to be turgid, or in a state of tur- 

 gescence. 



The young growing leaves and shoots of the higher plants, 

 whioh consist almost entirely of living cells, each one of which 

 closely resembles our algal cell, are enabled to stand erect 

 and to exhibit considerable rigidity, owing to a number of the 

 cells being turgid and stiff. If a young shoot is cut and kept 

 without water we know that it soon droops and becomes 

 flaccid, the cells composing it having lost water and being 

 no longer able to remain turbid. 



Again, if a living cell is brought into contact with a con- 

 centrated solution of a substance which has a stronger attraction 

 for water than have the substances contained in the cell-sap 

 water may be drawn out of the cell to such an extent that it 

 is no longer able to remain turgid and the protoplasm becomes 

 inactive and unable to perform its functions. It is on this 

 account that plants are, as a rule, unable to obtain their supplies 

 of water and hence also of dissolved mineral salts from concen- 

 trated solutions. 



Transpira- 35 . As the mineral salts required 



as raw food materials must thus enter the living turgid cell 

 as very dilute solutions, in order that the cell may obtain a 

 sufficiency of salts a large quantity of water must continually 

 be got rid of, and the protoplasm, while not allowing the salts 

 which it requires to pass out of the cell and be lost, does 



