Atkins — Hydrogen ton Concentration of the Soil. 201^ 



greatly diminished amount, at pll 7 to 8. It accordingly seemed probable that 

 the blue colour of the flower stalks and petals, shown" iii habitats of the more 

 acid type, was due to the presence of traces of ferrous salts not required in the 

 general metabolism of the leaves. The ferrous salt in excess may then react 

 with the natural anthocyan pigment, which is pink, giving the' blue colour 

 which is observed. 



This conclusion, as it has since been found by the writer, was pre^'iously 

 arrived at by Molisch (1897), whose work is discussed in a following section. 

 On this view the hydrangea flower may be taken as an indicator of the 

 availability of iron, and indirectly of the soil reaction. Such variations of flower 

 colour with habitat do not appear to be common. 



An interesting example has been examined by Boresch (1920), who found 

 that the Cyanophycean Fhormidium Betzii var. nigro-violacea has normally an 

 olive-green colour; this, however, becomes violet or yellowish-brown in cultures 

 deficient in iron salts, addition of which restores the usual colour. 



Qualitative and Quantitative Tests for Iron in the Hydrangea Flower. 



The facts and considerations of the preceding pages rendered it of interest 

 to see whether it was possible by direct tests to demonstrate the presence of a 

 greater amount of iron in blue hydrangea flowers than in pink. For this 

 purpose the delicate hematoxylin test of MacaUum (1897) was tried. Dried 

 flowers (no fresh were available) were boiled in twice distilled water containing 

 a dilute solution of well-washed hamatoxylin crystals. Such a solution is a pale 

 yellow, and iron, where present, acts as a mordant for the stain. Pink flowers 

 and others, blue to mauve, all from the same plant, were used. Control tests 

 without heematoxylin showed that the blue flowers always appeared slightly 

 darker than the pink long after their natural colour had been extracted. With 

 hffimatoxylin there was, however, a very definite darkening, amounting in parts 

 to the development of the typical blue purple of iron haamatoxjdin, but in the 

 blue or bluish flowers only. No trace of the blue purple was ever seen in the 

 pink flowers, though some possibly showed an almost imperceptible darkening. 

 Since this test is only given by iron in inorganic, or in at least ionisable condition, 

 the absence of such iron in the pink hydrangea and its presence in the blue may 

 be taken as demonstrated. It seemed that quantitative results were desirable, 

 so flowers, including the blue or pink portions of the stalks, were dried and 

 incinerated. The almost white ash was then dissolved in hydrochloric acid, and 

 after oxidation with nitric acid was treated as usual for the precipitation of 

 ferric hydroxide. None coulcl be seen, however, so the white precipitates were 

 redissolved, made up to the same concentration with respect to ash, and the 

 traces of iron estimated colorimetrically with ammonium thiocyanate. Standard 

 N/10,000 ferric iron was diluted, and it was seen then that the solution from 

 the blue hydrangea ash lay near a 0-5 dilution, viz., 28 milligrams of iron per 

 litre, whereas that from the pink corresponded to 0-3 dilution, or 1-7 milligrams. 

 Since the volume of the blue hydrangea solution was approximately one-eighth 

 of a litre, the iron in the ash was only aboat 0o5 mgrm., an amount too small 

 for gravimetric analysis. The ash of the blue hydrangea flowers was found to 

 be 5-9 per cent, of the material taken after drying at 95° and over sulphuric 

 acid. It contained 024 per cent, of iron. The pink hydrangea gave 4o per 

 cent, of ash, of which iron amounted only to three-fifths as much as in the blue, 

 namely, 014 per cent. It seems probable that both types of flower have a 

 certain amount of iron in the form of complex organic substances, but in the 

 blue only is there an excess available *to combine with the natural sap-soluble 

 pigment. 



