ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 9 



fogs. Some of these correspondents, who are within the range 

 of the London fog-area, contrast the hurtful nature of the latter 

 sort of fog with the harrnlessness of the former. Purely nega- 

 tive evidence of this character could be indefinitely extended, 

 but the above is perhaps sufficient to establish the fact that 

 country mists are harmless. 



We may now return to injuries due to fogs. I have limited 

 this inquiry to plants cultivated under glass. This I have done 

 because this class of vegetation affords very striking examples of 

 the effects of fog ; because greenhouse plants are always under 

 observation, and information concerning them is readily obtained ; 

 finally, because outdoor plants are liable to injury by frost, and 

 the problem is thus complicated. 



Nature of the Injuries. 



General Characters. — The injuries to foliage are of wide 

 occurrence amongst Dicotyledons, both stove and greenhouse 

 plants. Many so-called hard-wooded, as well as the softer, and 

 apparently more delicate, herbaceous plants are reduced after a 

 severe spell of foggy weather to an unsightly residue of almost 

 bare stems, blotched and discoloured leaves, and fallen foliage. 

 Amongst certain groups even the soft stems disarticulate at the 

 nodes. 



For the purposes of this report we may distinguish broadly 

 two principal classes of injury to foliage, produced by distinct 

 causes. 



A. Cases in which the leaves exhibit local discolorations, 

 particularly at the tips and margins. A speedy disarticulation 

 of the leaf does not occur ; but when the fog is of long duration, 

 and the local discoloration gradually involves a considerable 

 portion of the area of the leaf, it may do so. But in an average 

 case the unaffected parts of the leaf remain fully functional, and 

 the leaf is retained. Instances are very common. Amongst 

 Ferns I have noticed various species of Pteris and Gymnogramme 

 schizophylla ; amongst Monocotyledons, Odontoglossum crispiim, 

 Freesia, Areca lutescens ; amongst Dicotyledons, from an almost 

 endless list I will only instance two very pronounced examples, 

 Dalechampia Boezliana and Pavonia Wiotii. In a great many 

 cases injuries of this purely local character co-exist with others 



