48 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



temperature varying from 16° to 20° C. During this period it 

 did not make any perceptible growth, and at the end of the 

 experiment the leaves were quite as rich in starch as at the 

 beginning." 



The Double Action of Fog. 



I have devoted the last few paragraphs to showing that plants 

 undergo a lowering of tone when insufficiently illuminated. This 

 is evident (1) by the occasional development of intumescences 

 and cupping ; (2) by the tendency to drop leaves ; and (3) by the 

 persistency with which residues of starch remain in the chloro- 

 phyll-corpuscles. I could have gone further and instanced the 

 frequency with which more or less etiolated growths are produced. 

 All these things indicate that the plant is in a diseased condition. 

 It is true we know too little of the maladies of plants which are 

 due to environmental fluctuations — light and shade, heat and cold, 

 dryness and humidity. The labours of many who would be 

 plant physicians have too frequently been devoted to unravelling 

 the life-histories of the fungi which prey upon plants, whilst the 

 constitution of the plant and the predisposing causes of disease 

 have perhaps hardly received the attention they deserve. 



If, then, when a period of darkness overtakes a plant, and 

 from this cause its constitution is liable to be undermined, how 

 vastly is the danger aggravated if poisonous vapours are present 

 in the air ! With urban fog these two conditions are fulfilled. 

 In addition to the prevailing darkness, the air is charged with 

 sulphurous acid, hydrocarbons, and other impurities toxic in 

 property. 



Whilst investigating the action upon plants of the known or 

 probable impurities of fog, I was impressed with the difficulty of 

 successfully imitating the main features of fog-injuries. It is 

 sufficient for our purpose to cite what happens with sulphurous 

 acid. If a healthy plant be placed in an atmosphere in which 

 sulphurous acid is present in much greater amount than it is in 

 a severe fog — e.g. if there be ten times as much — we observe the 

 development of certain injuries. The leaves are discoloured, the 

 cells killed ; but the leaves do not readily disarticulate. If the 

 amount of sulphurous acid present be diminished so that the per- 

 centage in the air is approximately the same as in a typical fog, 

 then the killing of the leaf is much prolonged. The change in 



