424 GROWTH OF PLANTS 



branch showing a pink corymb and a blue corymb. Both were pink at 

 the beginning but the one at the right was changed to blue by spraying it 

 with 0.5 per cent solution of aluminum ammonium citrate. The flower at 

 upper right was produced when the nutrient solution for the plant bore 

 1.34 ppm or less of aluminum. The middle left flower (mauve) was pro- 

 duced on a plant receiving 13.4 ppm of aluminum in the nutrient, and the 

 middle right on a plant furnished a nutrient bearing 134 ppm of aluminum. 

 The plant at the bottom of the plate was grown in a pot with the root sys- 

 tem and lower part of the stem divided with a plate of glass, so the right- 

 hand side has a lower supply of available aluminum than the left. In this 

 way both pink and blue flowers are produced on the same plant. 



It is common knowledge that when this plant is gro\vn in nearly neutral 

 or alkaline soil the flowers are generally pink, while those gro\\Ti in acid 

 soils are generally blue. Neutral or alkaline soils bear little iron or alumi- 

 num in solution, while acid soils bear much more. 



The following statements by the author ^' p -240-241 ^^^^ further evidence 

 for his conclusions and bring out other interesting facts about the problem : 



"Blue flowers from plants grown in sand cultures had an aluminum con- 

 tent of more than 250 ppm; pink flowers contained less than 150 ppm, 

 while mauve flowers varied from approximately 150 to 250 ppm. Bright 

 blue flowers from plants growing in the field usually contained 800 to 

 900 ppm of aluminum, although the amount varied widely depending upon 

 the conditions under which the plants were growing. Aluminum com- 

 pounds added to the soil increased the aluminum content of the flowers. 



"The iron content showed less relationship to the flower color than did 

 the aluminum. Blue flowers from plants growing in soil had a higher iron 

 content than pink ones, but in sand cultures where the amount of iron in 

 the nutrient solution was controlled, some of the blue flowers contained 

 less iron than the pink. 



"Flowers produced on plants grown from cuttings in soil at pH 5.5 or 

 below were blue. Between pH 5.5 and 6.25 the flowers were intermediate 

 between pink and blue. Above pH 6.7 the flowers normally showed no 

 trace of blue. Blue flowers were produced on plants growing in alkaline 

 soil at pH 7.5 or above. There was evidence that aluminum could be accu- 

 mulated in the tissue and bring about mauve or blue color when the plants 

 were transferred to neutral or alkaline soil. 



"Single plants were caused to produce flowers varying from clear blue 

 to clear pink by dividing the root system and growing the two halves in 

 different types of soil. Wherever there was a vascular connection between 

 the roots growing in acid or aluminum sulphate treated soil, the flowers 

 were blue. 



"Some varieties failed to produce clear blue flowers in soil made acid 

 by the addition of aluminum sulphate. Flowers of the same varieties 

 turned blue when aluminum compounds were sprayed on mature sepals 

 or absorbed through a slit in the stem." 



