SIO JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



INJURIES TO PLANTS BY LONDON FOG, 

 AND BY ENGINE SMOKE. 



By Rev. Prof. G. Henslow, M.A., V.M.H., &c. 



Lecture to the Students at the Society's Gardens, July 4, 1901. 



Injuries to plants in the suburbs of London may result from both 

 London fogs and railway engines. The former prevail with anticyclonic 

 conditions of the atmosphere, when the barometer is high and a north- 

 east or east wind prevails. Hence the effect of the fog is seen in the 

 west and south-westerly directions. The air being comparatively heavy, 

 the fogs are felt near the surface of the earth. With these " dry " fogs, 

 ^bs they are called, it is found that the water-particles are invested with a 

 carbonaceous and, as it proves, a poisonous substance. 



Much injury has been done to gardens in the open, and plants under 

 •»lass, by the smoke of stationary engines when " blowing off steam," as it 

 is called. 



In both cases the general effect, allowing for various degrees of injury, 

 is the same, and these are of two classes : viz., first, injury resulting from the 

 ■arrest of light ; and secondly, from the poisonous nature of the ingredients 

 of smoke. 



I will first consider the effects of the partial arrest of light, due to 

 the adhesive nature of the carbonaceous materials, which cling tenaci- 

 ously to the surface of the glass. Indeed, it cannot be at all easily 

 washed off, but has to be actually scraped with a knife, to be satisfactorily 

 removed. 



On looking at the sun through a London fog, if it be visible at all, it 

 ^vill be seen to be of an orange-red colour. This means that the more 

 refrangible half of the solar spectrum is absorbed to a greater extent than 

 the less refrangible or " red end." 



Similarly, in testing a piece of glass from a greenhouse, thickly coated 

 Avith sooty matter from a railway engine, the blue rays proved to be more 

 'Strongly absorbed than the red ones. 



In both cases there is a great deficiency of light. 



Experiments have proved that when plants are grown under glasses 

 which transmit red, yellow, green, blue, and violet, as the predominant 

 rays — i.e. they appear to the eye to be only the colours named — then the 

 first obvious effect is the elongation of the stems under red, yellow, and 

 •green, with a short stem under blue and violet. Such, at least, was 

 obviously the case in the writer's experience with Lettuces, &c. 



The next effect is the relative degrees of impoverishment. The 

 process of assimilation gave two maxima, one under yellow and another 

 cinder blue glass, while in all cases under glass the deficiency was 

 very marked, i.e. as compared with plants grown normally in the 

 open air. 



Analogous results occurred in glasshouses, on the roof of which engine 



