26 VEGETATION OF THE PEAK DISTRICT [CH. 



then mingling with the West Riding smoke." Dr Cohen (1900, 

 1909) has investigated the smoke nuisance in great detail, and 

 he emphasizes the view that the smoke lessens the amount of 

 bright sunshine and lessens the temperature, even at some 

 distance from the towns. 



The reduction of light intensity by smoke does not, of 

 itself, affect vegetation adversely, as, at ordinary temperatures 

 and with the available amount of carbon dioxide, the natural 

 illumination is greatly in excess of the amounts which plants 

 utilize in their photo-synthetic activities ; but the reduction 

 of temperature which results from a smoke-laden atmosphere 

 must adversely affect the carbon assimilation of plants and 

 reduce their vitality. The clogging of stomata by grease and 

 soot would also seem to be a factor of importance, as this must 

 lessen the quantity of carbon dioxide which the plants can 

 obtain ; and it is known that plants are able to utilize more 

 carbon dioxide than is ordinarily available. It is also known 

 that in smoke, certain injurious acids, e.g., sulphurous acid 

 (H2SO3), are present, and that these have a poisonous effect on 

 plants. Further, the soil is also injured by smoke on account 

 of the sulphurous and other acids being deposited upon it. The 

 injurious effects of fog on plants have been fully investigated 

 and discussed by Professor F. W. Oliver (1893). 



Whether or not the deleterious influence of smoke is a 

 limiting factor as regards the plant associations of the district 

 is doubtful, as it seems likely that all the associations may be 

 affected to an equally adverse degree. On the other hand, 

 Wilson {I.e.) shows that certain Cryptogams {e.g., Ulota spp. 

 and Orthotrichum spp. and lichens) are much rarer than 

 formerly ; and, in certain cases, he attributes this fact to smoke. 



Temperature 



Judging from the figures in the standard physical atlases 

 (see Bartholomew and Herbertson, 1899), the mean annual 

 temperature of the district is about 49° F. (9-44° C). This may 

 be compared with the mean annual temperature of Penzance 

 (see Davy, 1909: xx) which is 52-68° F. (11-5° C). January, 

 with an average temperature of 36° F. (2*2° C), is the coldest 

 month of the year, and July, with an average temperature of 



