62 WATER-CULTURE. [CH. Ill 



long, shoots of plants with the cut end in the solution 

 may be used ; shoots of Alisma plantago and Scrophularia 

 aquatica are good for this purpose, and when it is 

 convenient to have a woody stem, branches of Acer 

 pseudoplatanns or Tilia europcea answer well. 



(75) Potassium salts necessary. 



Take three plants, A, B, C, as nearly as possible of 

 equal weight and equally developed. Dry A at 100^ C. 

 and determine its dry weight. Grow B in normal culture 

 solution and C in a fluid containing the same salts as the 

 normal solution but with an equivalent weight of sodium 

 — instead of the potassium — salt. Continue the cultures 

 for about three weeks, then take out the plants B and C, 

 dry them at 100° and determine their dry weights. B 

 should be considerably heavier than C. 



To confirm the fact that the greater increase in 

 weight shown by B is associated with the actual ab- 

 sorption of the potassium, B and C should be incinerated 

 after weighing and the absolute amounts of KoO in the 

 whole ash of each determined. 



Instructions for obtaining the ash and making an 

 accurate estimation of the K2O are given in Part li. 



(76) Phosphoric acid necessary. 



The same method is used as in the last experiment 

 but a somewhat longer time will be required for satis- 

 factory results. The solution which contains no salt of 

 phosphoric acid may have the usual calcium phosphate 

 replaced by an equivalent quantity of calcium nitrate. 



