ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 14D 



volume required to decolourise being recorded. These experi- 

 ments are not yet concluded, but it can be definitely said that 

 increase of temperature, other things being equal, aggravates the 

 poisonous action of the sulphurous acid, a difference of a few 

 degrees of temperature being apparent. 



During the summer these experiments will be continued. 

 An attempt will also be made to estimate the influence of 

 varying amounts of sulphurous acid gas, and other poisons, 

 upon the plant functions : transpiration, assimilation, and 

 respiration. 



VI. Possible Remedial Measures. — It is too soon to speak 

 of these with confidence, until a more complete knowledge of all 

 the conditions of damage to vegetation from this source have 

 been obtained. Many of the results obtained in the physiological 

 inquiry are still obscure, and it will be necessary to carry out a 

 series of experiments before some of these are elucidated. Since 

 October I have had opportunity of inspecting many collections 

 of stove and greenhouse plants, and growers have most freely 

 communicated to me any methods which they employ to combat 

 the fog. The placing of canvas, &c, over the plant-houses in 

 foggy weather has in some instances mitigated the damage, the 

 fog being in this way to some extent filtered. The regulation of 

 the temperature has an important bearing. Some cultivators 

 keep the temperature during a fog as low as can be done, having 

 regard to the safety of the collections in the houses. Others raise 

 the temperature in the hope of, to some extent, excluding the 

 fog. There can be no doubt that a high temperature augments 

 the damage, whilst a lower one, to some extent, hinders it. 



Sulphurous acid acts more violently and immediately in a hot 

 than in a cool atmosphere. Again, in a long, dark fog heat un- 

 necessarily stimulates the plants when (from the prevailing 

 darkness) their transpiring capacity is limited. Everything should 

 be done to tax the vegetative organs of a plant to the least 

 possible extent when any of the vital functions are interfered 

 with, as in dull, foggy weather. In this connection it is im- 

 portant to supply water to the roots with a sparing hand. Heat 

 and moisture at the roots stimulate absorption whilst the leaves 

 are unable to throw off an excess of moisture, as they can in 

 sunny weather. I know that in many establishments these pre- 

 cautions are taken, and, I believe, with relatively beneficial results. 



