8 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



series of events in some plants with almost equal celerity ; whilst 

 in other plants their development is much more gradual. The 

 discoloration of the leaf, which is the first sign in many cases 

 which catches the attention, is really only the last of a series of 

 changes, the first of which even an expert histologist, aided by a 

 high power of the microscope, would have difficulty in detecting. 

 These changes, due to profound molecular disturbance of the 

 living substance of the leaves, lead up to two very noticeable 

 events. On the one hand, there is the destruction of the chloro- 

 phyll pigment ; and on the other, the production of a definite 

 layer of separation at the point of insertion of the petiole. In 

 what may be called a normal case, both these things happen. 

 But often enough we meet with cases in which the leaf falls 

 green and apparently uninjured. Thus we have Mr. Watson's 

 statement (relating to some recent fogs) in the Gardeners' 

 Chronicle,* that " in the Palm-house bushels of healthy-looking 

 leaves were gathered up almost every morning." On the other 

 hand, instances occur in which no layer of separation is produced, 

 though the other changes referred to are exhibited, and the 

 leaves hang dead upon the stem. 



These two extreme cases are quoted at the outset to show 

 that it is upon a very complex question we enter when we try 

 to understand the meaning of the destructive changes pro- 

 duced in plants by urban fog. 



I will now proceed to give in a summarised form a short 

 account of the general nature of the injuries that are associated 

 with fogs. But, first of all, it will be well to definitely establish 

 the fact that no injury of note results from mist uncontaminated 

 by smoke. Correspondents from various parts of the country 

 assure me that no harm accrues from these mists. Thus towards 

 the close of the winter of 1890-91 the Rev. F. D. Horner, of 

 Kirkby Lonsdale, not only communicated his own experiences, but 

 was so kind as to obtain reports from several other cultivators in 

 his locality. Their united testimony was that the mists which 

 from time to time prevailed were quite harmless. Nor did the keen 

 frost of that winter do any damage among stove and greenhouse 

 plants, notwithstanding the increased fire-heat needed to main- 

 tain a suitable temperature. From Kent, Sussex, and Essex I 

 have many assurances of the non-injurious character of the sea- 

 * November 28, 1891, p. G50. 



