GENERAL CONSIDERATION OF PLANT DISEASES 293 



in the country along our trunk-line railroads are subjected to conditions 

 which cause unhealthy growth and disease has been proven abundantly. 

 Large factories, power plants and railroad locomotives are pouring out 

 volumes of smoke, which alone is highly injurious, but in addition the 

 acid which is formed in the combustion of coal, when dissolved in rain 

 water, has injurious efifect upon foliage and other plant parts. Its 

 action is seen in the corrosion of tin roofs, rain pipes and ornamental 

 iron work about city houses. 



The following note is of interest to the plant pathologist and plant 

 physiologist.' During the night of Sept. 19, 1913, a light rain fell, 

 followed by a fine drizzle in the early morning of Sept. 20. The wide- 

 open campanulate flowers of the common morning glory {Ipomoea 

 purpurea Roth), growing on a lot in West Philadelphia, four or five 

 blocks from the Pennsylvania Railroad, had their usual quota of rain- 

 drops studded over the upper, inner surface of the purple corollas. 

 Wherever the drops touched the surface of the corolla, the purple 

 color was changed to a pinkish red, and in the process of evaporation 

 of the raindrops the acid of the drops was concentrated, so that after 

 • the complete disappearance of the drops a brown spot was left in the 

 center of the pinkish red circles of discoloration. The explanation of 

 the alteration of color is found in the change of the sap of the corolla 

 cells, where touched by the acid raindrops, from an alkaline to an acid 

 reaction. A similar change can be induced in blue violet petals by 

 bruising them slightly and placing them in an acid liquid. The petals 

 change, like blue alkaline litmus paper, from blue to red, and this re- 

 action with violet petals has proved useful in the physiologic laboratory 

 in the absence of litmus paper. In nature a reverse change, which 

 illustrates the same chemic principle, takes place in many flowers of 

 plants belonging to the family Boraginaceae. For example, in 

 Symphytum and Mertensia, the red flower buds, the cells of which have 

 an acid cell sap, gradually change to blue as the flowers open. That 

 this is a chemic change is proved by treating the red buds with an 

 alkaline fluid and the blue flowers with an acid one. 



Similar spotting, but less clearly discernible and demonstrable, as 

 the delicate reaction with morning-glory flowers, undoubtedly occurs 

 on leaves and fruits, and the suggestion is made here, that such spots 



1 Harshbergee, John W.: The Acid Spotting of Morning Glories by City Rain. 

 Science, new scr., xx.xviii: 548, Oct. 17, 1913. 



