ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 



5 



The general aspect of the question of the relation of fog to 

 vegetation was very lucidly stated by Mr. W. T. Thiselton-Dyer, 

 Director of the Royal Gardens, Kew, in a letter to me, dated 

 Oct. 16, 1890. From this letter I make the following extract : — 

 " Roughly, I think the matter stands in this way : — 

 " i. Sulphuric acid is the principal cause of injury to trees 

 and shrubs because their parts are permanent and it is cumulative 

 in its action. It perpetually parts with and acquires water. If 

 you write on a piece of wood with dilute H 2 S0 4 there is no 

 immediate apparent effect. Hold the board before a fire, and 

 wherever the H 2 S0 4 has touched the wood it is charred. The 

 reason is that the dilute H 2 S0 4 first parts /with -its water, and 

 when it has reached a certain concentration immediately de- 

 hydrates the wood. The H 2 S0 4 deposited by fog and smoke on 

 trees acts during warm weather as a persistent and gradual 

 caustic ; the same H 2 S0 4 acts again and again ; it eats into the 

 tissues, and as it is perpetually being added to in amount by 

 fresh deposits, it probably gains upon the loss due to the washing 

 action of rain. 



" ii. Sulphurous acid acts in an entirely different way, and is 

 mostly injurious to herbaceous and so-called soft- wooded plants. 

 It is a powerful deoxidiser. It enters into the intercellular 

 passages and probably acts directly on the protoplasm. It may 

 do this by stealing oxygen from it and, so to speak, asphyxiating 

 it, or it may have some direct toxic effect not understood. 

 Anyhow, in the case of flowers I believe it acts by killing the 

 protoplasm and destroying the turgescence of the cells. To this 

 may be ultimately due the falling of the flowers and the dis- 

 articulation of parts of the inflorescence. The phenomenon is so 

 rapid that I can hardly believe there is a normal formation of a 

 zone of 1 abscission.' 



" hi. Fog is no doubt injurious to trees by the actual deposit 

 of a coating of finely divided carbon. 



" iv. The more obscure part of the phenomenon is the part 

 played by the various products of the destructive distillation of 

 fuel which are present in it. Of these there are probably a large 

 series. 



"a. Frankland has shown that in so-called 'dry fog' 

 the watery particles are inclosed in an envelope of hydro < 

 carbons. 



