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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



When a plant with thin membranous leaves is exposed in 

 this way to an atmosphere containing a large amount of free 

 S0. 2 — i.e. an atmosphere, let us say, containing ten times as much 

 SO 2 per cubic foot as is contained in an ordinary severe London 

 fog (which we may regard as 20 milligrammes per 100 cubic 

 feet) — certain very definite phenomena are observable. 



In the course of half an hour the leaves become limp and 

 flaccid, and exhibit a gradually increasing curvature — a hanging 

 down of the distal parts of the leaf similar to that exhibited by 

 plants in want of water. In the case of leathery leaves, and with 

 xerophilous plants generally, this limpness is obscured by the 

 presence of a thick cuticle and of much sclerotic tissue. In process 

 of time, sometimes within an hour, a change of colour is per- 

 ceptible. This begins locally, at the apex or base or margins, 

 and gradually extends until the whole lamina is involved. The 

 colour assumed by most plants is an olive-green, or a brown or 

 yellowish brown. These colour-changes occurred within a few 

 hours of starting the experiments. It must be remembered that 

 we are now treating of strong solutions of S0 2 (200 milli- 

 grammes per 100 cubic feet), far stronger than ever occur in 

 London fog. Speaking generally, it is the oldest and youngest 

 growths— ?'. e. still growing — which suffer first. Those which 

 are in the zenith of their activity appear to offer a greater 

 resistance to the acid, and are the last to succumb. 



It is interesting to note that, in the majority of cases, the 

 leaves of Dicotyledons treated in this way do not disarticulate, 

 but persist, withered, on the stem ; or at any rate disarticulation 

 does not supervene until the parts have become entirely desic- 

 cated. 



The accompanying histological characters are very definite 

 and pronounced ; but they do not differ in any qualitative respect 

 from those occasioned by other acid vapours — e.g. HC1. 



The most marked character exhibited by vertical sections of 

 leaves which have been treated as described is a violent 

 plasmolysis of all the cells. The protoplasmic bodies of the 

 cells are withdrawn from the cell-walls throughout the whole 

 thickness of the leaf. The protoplasm of each cell is massed 

 together in an irregular figure situated at the centre of the cell, 

 or it is attached to the cell-wall at one side. The form of each 

 little mass shows a general resemblance to the form of the 



