202 Scientific Proceedings^ Rotjal Dublin Society. 



the first four inches, which is insufficient to get near the absorbent portions of 

 the roots. 



Sap-soluble Pigments as Possible Indicators. 



Since the sap-soluble colouring matters of many i^lants act as indicators, as 

 showii by Haas (1916), it seemed possible that the diverse colours in hydrangeas 

 were due to varying degrees of acidity, the more so as it is known that certain 

 colour changes from red to blue, according as the flower fades, are explained 

 correctly by this hypothesis. As against this must be set the remarkable con- . 

 stancy in pH value given by the leaves, stems, and roots respectively of members 

 of the same species. 



To test the matter directly, pink and blue flowers were treated with dilute 

 acetic acid, but the pink did not change to blue. In fact, no very marked 

 change was noticed in either case. Furthermore, flowers were obtained from a 

 single plant bearing both pink and light mauve or blue. Petals were crushed 

 with an agate pestle, and two drops of water, and one of indicator, added to each. 

 Using brom phenol blue as indicator, both appeared to be close to pH 40, and 

 were indistinguishable. This indicator is, however, dichroic, so an exact com- 

 parison in a turbid drop is not easy to make. Methyl orange covers a somewhat 

 similar range, and with it both the pink and blue petals were ascertained to be 

 at pll 42, and were, as before, indisting-uishable. It is accordingly clear that 

 the colours of hydrangea flowers are not due to the natural pigment acting as an 

 indicator. 



Floiver Colour and Availability of Iron Salts. 



Among gardeners the practice of potting -with iron nails is well known as a 

 means of producing blue hydrangeas. It therefore seemed probable that the 

 solubility of iron salts might be the direct cause of the production of the blue. 

 Attempts were made to induce cut flowers to change from pink to blue by 

 placing the stalks in dilute solutions of ferrous and ferric salts. A deep dark 

 green appeared in the stems, and spread slowly into the petioles and veins of 

 the petals. The flowers then withered. Possibly with iron salts in much 

 smaller amount a blue might ha,ve resulted. 



Culture experiments by Duggar (1920) have shown that certain salts 

 ordinarily considered as insoluble are quite effective as plant nutrients, since 

 the solids yield a continual supply of a minute amount in solution. There is, 

 however, a limit to the availability of "insoluble" compounds, as shown by 

 McCall and Haag (1921) to be the case with ferric salts. These workers found 

 that culture solutions containing ferric salts were adequate for nutrition of 

 wheat when the reaction was pH 40 or more acid, but solutions from pH 4-0_ to 

 pH 70 gave rise to chlorosis in the plants grown in them. Patten and Mains 

 (1920) liave shown that ferric hydroxide is precipitated in quantity between 

 pH 3-5 and pH 6-0, at which the process is complete. In nature, however, few, 

 if any, soils are as acid as pH 3-5; yet plants grow in natural solutions, 

 according to species, up to p>H 8 or 9. As previously suggested by the writer 

 (1922, 1)", this is probably due to the presence of iron in the ferrous condition. 

 Further M'ork (1922, 2) has shown that ferrous hydroxide does not begin to be 

 precipitated until pH 5-1; and though it comes down in quantity at pH 55 to 

 2}H 6-5, yet even beyond pH 71 a small amount exists in the solution, and is 

 slowly precipitated as it becomes oxidised to the ferric state. It thus appears 

 that under the reducing conditions met with in soils, especially perhaps in badly 

 aerated acid soils, in which the colloids are not aggregated as in the presence of 

 calcium bicarbonate, iron may be readily available at pH 6, and even, though in 



