548 



SCIENCE 



[N. S. YoL. XXXVIII. No. 981 



tain whether the exposure of the plate to the 

 transmitted rays and to the secondary rays 

 must be simultaneous, but has been unable to 

 produce the anomalous effect by successive ex- 

 posures, that is, by an exposure first with the 

 upper coin in place followed by another ex- 

 posure with this coin removed and the lower 

 coin in place. No vestige of cancellation 

 could be found. T. E. Gorton 



THE ACID SPOTTING OF MORNING GLORIES BY 

 CITY RAIN 



That the trees, shrubs and flowering plants 

 in our large cities and in the country along 

 our trunk-line railroads are subjected to con- 

 ditions which cause unhealthy grovrth and 

 disease has been proven abundantly. Large 

 factories, power plants and railroad loco- 

 motives are pouring out volumes of smoke, 

 which alone is highly injurious, but in addi- 

 tion the acid which is formed in the combus- 

 tion of coal, when dissolved in rain water, 

 has injurious effect upon foliage and other 

 plant parts. Its action is seen in the corrosion 

 of tin roofs, rain pipes and ornamental iron 

 wo,k about city houses. 



1 he following note is of interest to the 

 plant pathologist and plant physiologist. 

 During the night of September 19, 1913, a 

 light rain fell, followed by a fine drizzle in 

 the early morning of September 20. The wide- 

 open campanulate flowers of the common morn- 

 ing glory (Ipomcea purpurea Eoth), growing on 

 a lot in West Philadelphia, four or five blocks 

 from the Pennsylvania Eailroad, had their 

 usual quota of raindrops 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 dis- 

 appearance of the drops a brown spot was left 

 in the center of the pinkish red circles of dis- 

 coloration. 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 reac- 

 tion. 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 reaction 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 

 Borraginacese. 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 caused by the acidity of 

 raindrops serve repeatedly as the points of 

 entry of parasitic fungi, for there are many 

 leaf spots and fruit spots that show concen- 

 tric rings of diseased tissue in the earliest 

 lesions produced. A fungus, which is stimu- 

 lated to growth by an acid condition of the 

 cell sap, would find ideal conditions for the 

 commencement of growth by entering areas 

 influenced by acid raindrops. 



John W. Haeshberger 



University op Pennsylvania 



SCIENTIFIC BOOKS 

 The Genus Iris. By William Eikatson Dykes. 

 With forty-seven colored drawings by F. H. 

 Round, one colored plate of seeds by Miss 

 E. M. Cardew and thirty line drawings by 

 C. W. Johnson. Cambridge, at the Univer- 

 sity Press. The University of Chicago 

 Press, Chicago, HI. 1913. Demy Folio. 

 Pp. viii H- 246. Price £6, 6s. net. 

 Thirty-six years ago J. G. Baker published 

 his " Systema Iridacearum " in the Journal 

 of the Linnean Society, including a revision 

 of all the genera of the family. In this paper 

 the genus 7ns was made to include 81 species, 

 distributed among six " sub-genera," namely, 

 Apogon (33 sp.), Onocyclus (5 sp.), Evansia 



