ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 49 



colour is slow, and the action may be continued over many days, 

 but the result more nearly approximates to a fog-injury. But it 

 diners from this in at least two important respects. Firstly, the 

 time taken is too long ; and, secondly, the leaves by no means 

 always disarticulate. One characteristic fog-injury is never met 

 with when healthy plants are exposed to the action of dilute 

 sulphurous acid — that is the disarticulation of green and 

 apparently uninjured leaves. This effect I could not produce 

 until I more rigidly imposed the conditions prevailing in fog. It 

 was only when I employed plants which had been kept in feeble 

 light for a few days that I was able to cause anything like a 

 rapid defoliation with sulphurous acid ; or the same result was 

 attained by conducting the experiments in darkness or under 

 light -absorbing screens. The best imitations of normal fog- 

 effects can only be obtained in this way. The reduction of light 

 alone is insufficient to account for the observed result. Nor will 

 sulphurous acid in full daylight bring about this result. To the 

 acid must be superadded diminished illumination. The latter 

 produces a loss of tone in the plant, to which I have already 

 alluded. The power of the protoplasm to resist any toxic sub- 

 stance present in the air is lowered. The death of the leaves 

 is hastened, and they disarticulate. As to the reason for their dis- 

 articulation under these conditions, and their non- disarticulation 

 with the acid alone, I shall venture no explanation. 



The Behaviour of Different Classes of Plants. 



And now we are in a position to consider why the leaves of 

 Ferns, Monocotyledons, and Dicotyledons differ in their behaviour 

 towards fog. The facts are these. Ferns, with their tender and 

 delicate foliage, hardly suffer perceptibly. At Kew I have 

 examined the various Fern-houses after spells of severe fog, 

 when the collections of stove plants in adjacent houses were com- 

 pletely disfigured from this cause, without remarking any damage 

 to speak of. At Chelsea, though the Ferns do not enjoy such an 

 absolute immunity from fog, the injuries are slight compared 

 with those obtaining amongst other groups of plants. 



The foliage of Monocotyledons at Kew does not suffer. Mr. 

 Watson, who has given this matter his earnest consideration, 

 tells me that he can recall no instance of damage to the 

 vegetative organs of any Monocotyledon at Kew. At Chelsea 



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