Iron in the Nutrient Solutions. The source of iron is -to be 

 added (as ferric phosphate) to each nutrient solution after it has 

 been placed in the culture jar, in every case. The bottle contain- 

 ing this phosphate is first to be thoroughly shaken, so as to fur- 

 nish a uniform suspension, and then a sufficient amount of the 

 suspension is to be transferred to the culture jar (with a 1-cc. 

 pipette) , so as to add approximately thr-ee mg. of the precipitate 

 to each quart jar. After this addition the jars are ready for the 

 plants. 



The preparation of the precipitated FePO* should proceed as 

 follows : In a bottle with capacity of 2 liters or more place 500 

 cc. of a 0.04 vol.-mol. solution of KH=PO^ and add thereto, with 

 agitation, a weak solution of ferric nitrate [Fe(N08)''], continu- 

 ing the addition until no more precipitate is produced. There 

 should thus be formed about 0.02 gram-molecule of FePOs which 

 should remain in suspension for considerable time, eventually 

 settling to the bottom of the bottle. 



Now fill the bottle nearly full of distilled water, shake thor- 

 oughly and stand aside till nearly all the liquid is clear. Carefully 

 siphon off the clear liquid without loss of the precipitate, and 

 repeat the filling, shaking, settling and decanting process several 

 times, to wash the FePO^ free from KNO-' and any excess of 

 Fe(NO0' that was originally added. The washing should be 

 thorough, so that 1 cc. of the wash-water, evaporated at from 

 50° to 60° C, in a porcelain dish, shows no residue. Now trans- 

 fer all the precipitate, with its water, to a graduated liter-flask 

 and add distilled water to make a liter. Preserve in a tiglitly 

 stoppered bottle. When shaken thoroughly the suspension thus 

 formed should contain approximately 0.02 g.-mol. of FePO*, 

 about 3 g. per liter, or 3 mg. per cubic centimeter. It will be well 

 to prepare enough FePO* to last for a year or two of this work. 



Enough of the uniform suspension is to be added to each quart 

 jar at each filling to give 3 mg. of FePO*. Of course it will not 

 dissolve appreciably, but it will furnish a constant and adequate 

 supply of iron. This material is not to be added to the germina- 

 tion solution, however, as has been said. 



Repetitions of the Experiments. It is planned that all of the 

 sets of salt proportions (excepting the three that are impossible 

 with that concentration) will be tested at least once with an 

 osmostic value of 1.00 atm. The tests are to be carried out by 

 solution types, so that different cooperators may be working 

 with different types at the same time. The prehminary experi- 

 mentation for any cooperator may thus consist in the testing of 



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