459 



dew at night are the chief conditions." After the blossoms open, dry 

 warm weather must prevail Sandy soil is especially suitable. In soils 

 rich in humus the plant runs to foliage. Clay soil is absolutely unsuitable, 

 since it does not let the water percolate through. 



However, examples of adaptation to the climate are known. Thus, 

 V\'ebber and Bessey^ report that -cotton, when carried from the Bahamas to 

 Georgia, did not thrive at first, but gradually adjusted itself to the temper- 

 ate climate. 



However, fogs, even of the English variety, may become disastrous, 

 especially near cities with many factories. P. W. Oliver-, upon the re- 

 quest of the Royal Horticultural Society, has published the most extensive 

 studies on London fog. The most troublesome admixture is the smoke, 

 the elements of which coat not only the plants but window panes, etc., with 

 a sooty covering. An analysis of this sooty covering shows : 



carbon 39-00 per cent. 



hydrocarbons 12.30 



organic bases 2.00 



sulfuric acid 4.33 



hydrochloric acid 1.43 



ammonia 1.37 



Metallic iron and magnetic oxid 2.63 



Silicate, iron oxide and other mineral substances. 31.24 



The injuries to plants are usually only phenomena of discoloration. 

 However, different plants are more susceptible ; hence the fog may cause 

 the dropping of the leaves. In injuries of the first kind, leaf tips and edges 

 become brown, but the remaining leaf surface is still capable of functioning 

 (Pteris, Odontoglossus, etc.). The dropping of leaves with yellowing and 

 browning, or even without any external signs of injury, is the most frequent 

 result. Sulfuric acid is considered as the cause of the leaf destruction; in 

 addition, Oliver ascribes as an injurious influence also metallic iron. In 

 deciduous plants which remove all the starch from the leaves before they 

 fall, the most important agent exciting abnormal leaf fall is sulfuric acid. 

 Experiments determining a rapidly reduced transpiration show reactions 

 similar to these from fog, if at the same time the light was decreased. I 

 also ascribe the emptying of the cells to the lack of light, for with the action 

 of the acid alone, I found in my experiments that the whole cell contents 

 died quickly and were deposited on the wall. 



Of the tar compounds, pyridine was found in fog in especially large 

 amounts. When exposed to vapors of this substance, the leaves became 

 Hmp and a darker green. The cells were plasmolyzed; the cyptoplasm in 

 the epidermis had turned brown, but the chlorophyll did not change. As a 



1 Yearbook of the Dept. of Agriculture, 1899, p. 463. 



2 Oliver, F. W., On the effects of urban fog upon cultivated plants. Journ. 

 Hortic. Soc. Vol. 16, 1893; cit. Zeitschr. f. Pflanzenkrankh. 1893, p. 224, und Gard. 

 Chron. 12, 1892, p. 21, 594, 648, etc. 



