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 
(H,SQ;), 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 (l.c.) shows that certain Cryptogams (eg., 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. (115°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 
