208 Scientific Proceedings, Royal Dublin Society. 



2. Ferrous salts remain in solution after ferric salts have been precipitated 

 or rendered completely insoluble as the hydroxide. The precipitation of ferrous 

 hydroxide does not begin until about pll 51, and even at pH 71 an appreciable 

 amount remains unprecipitated. Since plants grow at pll values beyond the 

 limits for the complete precipitation of ferric salts, thej^ must, under these 

 conditions, utilize ferrous salts only. 



3. The difference between the pink and blue flowers of hydrangea is not 

 due to acidity, since both kinds from the same plant were found to be at 

 precisely the same hydrogen ion concentration, pH 4-2. 



4. The use of Macallum's ha^matoxylin test shows that blue hydrangea 

 flowers give the typical purple and brown reactions for "inorganic" iron, 

 whereas pink flowers have only minute traces of ''inorganic" iron. 



5. Colorimetric estimations with ammop-ium thiocyanate show that the ash 

 of the pink flowers contains only 06 as much iron as that of the blue. Calculating 

 on the dry weight of the flowers themselves, the blue contain about 140 parts per 

 million of iron and the pink about 60 p. p.m. 



6. Dilute solutions of alum and of aluminium sulphate give a reaction of 

 about pH 4, ranging up to about pll 36 for more concentrated ones. These 

 substances therefore are convenient reagents for increasing soil acidity without 

 risk of attaining injuriously high hj'drogen ion concentrations by accident. To 

 the increase in aciditjr, and consequent liberation of iron, is due the blue colour 

 of hydrangeas found by Molisch to result from this treatment. It is possible, 

 however, that the aluminium, as well as the iron, may form a blue complex 

 with the anthocyanin, which is pink in absence of excess of these salts. 



7. The precipitation limits for ferrous and ferric salts given in section 2 

 of this summary throw light upon the availability of iron in the soil and on 

 the occurrence of chlorosis in certain plants when grown on alkaline soils. 



8. The formation of iron pan may also be considered from this view-point, 

 and the lessened solubility of iron salts when converted into the ferric condition 

 is of importance in explaining the formation of pan v/here an acid soil solution 

 percolates into a less acid region which is still sufficiently near the surface for 

 oxidation of the precipitated hydroxide to proceed; ferric salts, if present, 

 would be precipitated slightly before the ferrous, as would also those of 

 aluminium. 



The author wishes to record his indebtedness to Mrs. E. W. Sexton for 

 drawing his attention to the problem of the flower colour of the hydrangea; 

 also to those friends who so kindly provided him. with soil samples. The re- 

 agents necessary for the hydrogen ion determinations were supplied out of a 

 grant from the Department of Scientific and Industrial Research; and thanks 

 are due to the Director of the Marine Biological Laboratory, Plymouth, for 

 general laboratory facilities. 



Bibliography. 



Arndt, C H., 1922.— The growth of field corn as affected by iron and aluminium 

 salts. Amer. J. of Bot., 9, 47-71. 



Aeehenius, 0., 1922, 1. — Hydrogen ion concentration, soil properties, and 

 growth of higher plants. Arkiv. for Botanik, 18, 1-54. 



Arrhenius, 0., 1922, 2.— Absorption of nutrients and plant growth in relation 

 to hydrogen ion concentration. J. C4en. Physiol, 5, Sl-SS. 



