92 Plant Tissue Culture 



sented by ten-thousandths of a per cent, the purity 

 of the water used is of prime importance and can- 

 not be overemphasized (White, 1932, 63, 1933, 

 141). ''Tap water" is, of course, worse than use- 

 less in any critical studies on nutrition. Many 

 laboratory supplies of "distilled water" are like- 

 wise worthless, because of significant amounts of 

 impurities of many sorts. These may range from 

 traces of lead from improperly soldered joints in 

 faucets to distillable petroleum-oil fractions com- 

 ing from the pumps used in wells or pumping 

 plants from which the ' * tap water ' ' is derived. In 

 order to insure a truly pure supply, water should 

 be first run through a softener and filter (see 

 Chapter III), then distilled at least twice, and 

 finally stored in vessels of hard glass or Pyrex. 

 Carboys can be covered with inverted beakers to 

 keep out dust or can be plugged with wads of clean 

 cotton enclosed in well-boiled and rinsed cheese- 

 cloth. Rubber stoppers should not be used, and 

 care must be taken that water does not come in 

 contact with cotton plugs. 



Salts. Plant tissues require for satisfactory 

 growth at least eleven elements besides oxygen, 

 hydrogen, and carbon. These are calcium, potas- 

 sium, magnesium, nitrogen, sulfur, phosphorus, 

 iron, manganese, zinc, boron, and iodine. While 

 not essential, sodium and chlorine are to be con- 



