14 THE NATURE OF LIFE 



skin of wine in the cistern of the abbey. 

 When the monk developed an unusual 

 taste for water he investigated and found 

 to his horror that the skin had burst. 

 The wine had taken up water through 

 the skin because it contained substances 

 which attract water (the word "attract" 

 is here used in a somewhat figurative 

 sense). In the living cell there is a pro- 

 toplasmic membrane which corresponds 

 to the skin, and inside this a solution 

 which attracts water. As water is 

 taken up the protoplasmic membrane is 

 stretched, and if there is a cellulose wall 

 outside the living membrane it shares 

 the same fate. The living membrane 

 can be stretched almost indefinitely be- 

 cause the cell can furnish it with new 

 material so that it can continue to ex- 

 pand without rupture. At the same time 

 the cell can produce substances which 

 attract water. It is therefore possible 

 for growth to continue indefinitely. 



The growth of the dead seed is due to 

 imbibition while that of the living seed is 



