CHAPTER XIX. 



INJURIOUS EFFECTS OF CULTURAL METHODS. 



A. Coating Substances. 



1. Tar. The inside of the framework of conservatories is often 

 coated with tar in order to increase its resistance to great dampness. We 

 are confronted with a long list of complaints that, after setting out the 

 plants in the tarred greenhouses, a blackening and falling of the leaves 

 takes place. I noticed the same phenomena near freshly tarred fences. The 

 conditions found agree in all essentials with those described for asphalt 

 fumes and are explained by the exhalations from the fresh tar coating. The 

 injurious results do not appear if the tarring has taken place a few months 

 before the plants are brought into the greenhouses. I found a method used 

 in the vicinity of Berlin which acted as well. The boards and framework 

 v/ere treated with hard coal tar and after this had dried were coated with 

 cement. 



An attempt has been made recently to keep the paths in gardens and 

 public parks free from dust by means of a thin layer of tar. The process 

 is much recommended^ and the experiments made in France and Italy have 

 shown that even paved streets can be treated advantageously in this way. 

 This process necessitates, however, the edging of the path with a strip of 

 galvanized tin 8 to lo cm. high, since the injurious elements of the tar would 

 otherwise attack the vegetation. This process, which despite its necessary 

 annual renewal is said to be still cheaper than asphalting and less trouble- 

 some than oiling, or the treatment of the streets with "W'estrumit," must 

 still be tested by further experiments. 



2. Refuse from Gas Works. According to a report from Mr. 

 Klitzing, at Ludwigslust, where roads on sandy soil have been hardened by 

 the use of such refuse, a dying back of the street trees was caused. 



3. White Lead. In a case of which I have heard, it was necessary to 

 put potted plants in greenhouses a short time after these had been coated 

 with white lead, and then the unpleasant discovery was made that the plants 

 dropped their leaves. 



1 Das Teeren von Fuss- und Fahrwegen in Garten und Parks. Der Handels- 

 gartner, herausgegr. von Thalacker, Leipzig--Gohlis 1906. No. 50. 



