XViii PROCEEDINGS OF THE ROYAL HORTICULTURAL SOCIETY. 



Yew sprays affected by this well-known parasite, which causes 

 an arrest of the axis and the leaves to form a rosette. The 

 question was raised as to whether the shoot subsequently elon- 

 gated or not. In Mr. Henslow's garden both male and female 

 Yews are always affected by it, the tufts of leaves and shoots 

 attacked dying every year. 



Injuries to Plants by certain Substances present in Coal 

 Smoke. — Prof. Oliver exhibited several results of his experi- 

 ments in testing the effects of the ingredients of fog and their 

 allied substances upon plants. A frond of Phegopteris trichoides, 

 subjected to the vapour of phenol under a bell-glass, was 

 blackened. White Lilac became of a chocolate-brown colour ; 

 white Cyclamens, however, being unaffected, though killed. It 

 was remarkable that the alcoholic extract of chlorophyll made 

 from the blackened frond was identically like that of the un- 

 injured green leaf, showing that the chlorophyll was " masked," 

 but not altered in character by the phenol. The effects of 

 pyridine were not so marked, as the Lilac and Cyclamen were 

 only slightly tinted with a pale brown colour. Prof. Oliver 

 had tested these and other plants for tannin, but the distribution 

 of this substance in organs which change colour with or resist 

 phenol and other coal-tar products was found to be such that no 

 positive conclusions could be drawn. 



Prof. Oliver also showed the effects of sulphurous acid 

 gas, so prevalent in urban fogs. The above-mentioned white 

 flowers were killed by its action. The spectrum of chlorophyll 

 of leaves subjected to its action showed the usual modifications 

 due to the presence of an acid. It is to be noted that the effects 

 produced were only on living protoplasm, no such results 

 occurring if the protoplasm had been previously killed. 



Dr. Scott said that he hoped Prof. Oliver would be able 

 to throw some more light on the falling of almost uninjured 

 leaves, which was perhaps the most remarkable of the effects of 

 fog. In these cases the leaves, when they fall, are alive, and 

 show scarcely any symptoms of poisoning. He thought it possible 

 that the investigation of this phenomenon might help to explain 

 the immediate cause of the normal fall of leaves in autumn, as 

 to which we at present know little more than the anatomical 

 conditions. 



Prof. H. Marshall Ward, in commenting upon Prof. Oliver's 



