52 THE NATURE OF LIFE 



salts and may therefore be said to have 

 a protective action. This applies to 

 animals as well as to plants. The rela- 

 tive proportion of lime salts (as com- 

 pared with other salts) in blood and sea 

 water is about the same and this is also 

 true of many solutions used for the nu- 

 trition of plants. A solution in which the 

 toxic properties of one salt are inhibited 

 by other salts is called a balanced salt 

 solution and such solutions play an im- 

 portant part in biology. 



The role of mineral substances is not 

 very well understood. The same may 

 be said of the hormones and vitamins 

 which are important for plants as well 

 as for animals. 



A hundred years ago it was not be- 

 lieved that any of the food substances or 

 compounds related to them could be pro- 

 duced outside the living cell. But to-day 

 such a feat is a commonplace of chemis- 

 try. We are even able to build up such 

 substances as sugar from carbon dioxide 

 and water, in imitation of the plant. 



