ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 39 



pyridine seems to me, judged in the light of experimental results, 

 to be the one which most nearly reproduces the conditions in 

 question. Upon white flowers pyridine, very generally, exerts a 

 yellowing influence. Sulphurous acid does not. 



(4) The brown precipitate is another of these complex 

 phenomena. In some cases the presence of tannin in the cells 

 may account for it. Bouvarclia buds (and flowers), especially 

 white varieties, are prone to become blotched here and there 

 upon the corolla and calyx with deep brown. Sulphurous acid 

 (whether weak or strong) calls forth nothing comparable to this. 

 A drop of pyridine (and several other bodies of the pyridine 

 series), volatilising under a bell-glass beside a shoot oiBouvardia, 

 is sufficient to produce identical blotches on the buds. Histo- 

 logical examination shows a dark precipitate which begins in the 

 outside epidermis, which, if the action be continued long enough, 

 gradually extends to the subjacent layers, finally involving the 

 whole bud. These characters absolutely agree with those of the 

 blotches produced in the same positions in fog. Micro-chemical 

 examination of uninjured buds show that the tissues in question 

 are copiously provided with tannin. I do not make any definite 

 inference here, as the whole subject is being further inquired into 

 in my laboratory. 



Very striking and remarkable is the action of phenol, applied 

 as a vapour, upon white flowers. One may divide white flowers 

 into those which turn brown with phenol and those which are 

 simply killed, without change of colour. Codocjyne cristata, 

 Lilac, and Camellia may be mentioned as flowers which speedily 

 turn to a rich chocolate-brown; the Chinese Primrose, Allium, 

 Narcissus, and Cyclamen as exhibiting no such coloration. 

 Aniline, nitro-benzene, and certain other coal-tar products 

 behave, in general, similarly. A short discussion upon the 

 action of phenol upon foliage has been introduced already, so I 

 will add nothing here. The matter is still under observation. The 

 only point to which I desire to draw attention is this. In phenol 

 we have one of a number of organic compounds which produce 

 colour-changes in some flowers and not in others.* Amongst 



* It is no question of mechanical resistance of cell-membranes. I have 

 no doubt on that point. Nor is the production of a marked brown colora- 

 tion in any way coincident with the presence or absence of tannin. White 

 flowers which are rich in tannin show all degrees of colour-change, and so 

 do those destitute of this substance. 



