NATURAL SCIENCE: 



A Monthly Review of Scientific Progress. 



No. 16. Vol II. JUNE. 1893. 



NOTES AND COMMENTS. 



Fog and Vegetation. 



AMONG the most interesting of last month's reports is that issued 

 in the Journal of the Royal Horticultural Society (part i, vol. xvi.) by 

 Professor F. \V. Oliver, who publishes the second instalment of his 

 observations on the effects of urban fog on plants cultivated under 

 glass. The effect is twofold ; in the first place, we have to consider 

 the serious loss of light ; in the second, the poisonous substances 

 present in the air. 



The general tone of plants, especially of those fond of sunlight, 

 must be considerably lowered by the often long persistent, dull winter 

 weather, with its frequently-recurring fogs. The assimilation of 

 carbonic acid is interfered with, while the transpiration of water 

 vapour from the leaves is almost at a standstill. The roots are not 

 affected, and continue to absorb water from the soil, especially in the 

 case of stove plants, where they are kept by the warmth in a state of 

 marked activity. Hence the cells of the plant become unduly dis- 

 tended with water. Finally, owing to faulty circulation in the 

 intercellular passages, access of oxygen from without is impeded, 

 while the supply normally received from the decomposition of carbonic 

 acid in assimilation is almost entirely cut off; respiration is checked, 

 and substances tend to accumulate in the cells from want of complete 

 oxidation. Thus the whole leaf-mechanism is out of gear. In this 

 enfeebled state the plant is exposed to the attacks of sulphurous acid, 

 hydrocarbons, and other noisome fog constituents, and the result is 

 lamentable, often, indeed, fatal. 



Professor Oliver distinguishes two classes of injury, produced by 

 distinct causes. First, cases in which the leaves show local discoloura- 

 tions, particularly at the tips and margins, while the unaffected 

 parts remain fully functional, and the leaf does not fall ; secondly, 



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