218 Innocent: A Century's Changes in the Sheffield Flora. 
times were largely replaced during the nineteenth century by 
steam-driven machinery, with its coal-smoke, and this is the 
most baneful influence with which the plants have had to con- 
tend. 
Coal smoke is deleterious to the plant in various ways. 
It obscures the sun, and thus decreases the vital activity of the 
green plant; observations in Bradford, Leeds, and Sheffield 
have shown that the sunshine and daylight are from 30 per cent. 
to 60 per cent. greater in the suburbs than in the centres of the 
cities. Smoke also deposits sticky matter on the leaves, thus 
choking them, and it impurifies the rain water ; the remarkable 
results of this have been shown by experiments at the Uni- 
versity of Leeds in continuously watering cultures of grasses 
with rain from different parts of the city. 
It is possible that the impurities from coal-smoke enter the 
soil and attack the roots, and possibly, the deposit affects the 
pollen grains of flowers. 
Smoke is not a direct, but rather an insidious enemy, 
weakening the plant for its individual part in the struggle for 
existence. 
The disappearance of Campanula hederacea from the city 
is probably due to smoke, which, however, seems to have only 
a very small effect on Saponaria officinalis, Artemisia vulgaris, 
Tanacetum vulgare, Matricaria inodora, Linaria vulgaris. 
A century ago the old open-, or common-field system of 
agriculture was surely, if slowly, coming to an end, but the 
effects of its disappearance were not yet evident in those im- 
provements in farming, only possible under the new system ; 
and one of the results of which we see to-day in the disappear- 
ance of many plants of rough cultivation. Few people realize 
the extent to which land-drainage has been carried out under 
the separate ownerships and tenures of the last century ; this 
drainage has affected not only the bogs and marshes, but also 
the woods and fields, and the longest list of extinctions in the 
district is that of the hygrophilous and hydrophilous species; 
The unfortunate water plants have also had to contend with 
the pollution of rivers arising from the increase of population 
and manufactures. 
Water plants exterminated from the district by the two. 
above causes, or others unknown, are :— 
Hypericum elodes. Potamogeton lucens. 
Oenanthe crocata. P, densus. 
Gnaphalium uliginosum. | Juncus uliginosus. 
Comarum palustre. Blysmus compressus, 
Callitviche autumnalis. Isolepis setacea. 
Veronica scutellata. Scirpus pauciflorus. 
Scutellavria minor. Schlevochloa distans. 
Hydrocharis morsus vane. Triodia decumbens. 
and some half-dozen species of sedge. 
Naturalist 
