TOWN SMOKE AND PLANT GROWTH. 



467 



striking, the total numbers o^f bacteria diminishing rapidly with in- 

 creased acidity, this being reflected also in diminished activity as 

 regards ammonia-production, nitrogen-fixation, and, above all, pro- 

 duction of nitrates in the soil. 



Knowledge gained in recent years regarding the functions of soil 

 organisms in connexion with plant growth enables us to assert con- 

 fidently that this reduction of bacterial activity is the main cause of 

 the reduction of growth of the grass brought about by the acid waters. 



Further Experiments in Progress. 



Our experiments have thus demonstrated clearly that the plant 

 growing in a smoke-polluted area is afflicted both at its leaves and at 

 its roots. What this may mean in actual plant growth is being tested 

 by an experiment, not yet completed, in which plants of the same 

 kind, variety, and age are being grown simultaneously in five different 

 parts of Leeds in soil taken from the same place. At each centre 

 three boxes of this soil have been placed, and a succession of crops 

 is being grown therein. 



The first crop taken was radishes. These were followed by lettuce 

 and then by winter cabbage. The cabbages were planted about the end 

 of September, but did not do well at any centre. There were, however, 

 very significant differences. At the centre most badly polluted by 

 smoke all the plants were dead before Christmas. At the cleaner 

 centres most of the plants survived until severe frost was met with 

 in February. This proved fatal to nearly every plant except at the 

 centre where smoke-pollution was lowest. Here, out of the nine 

 plants started with, eight survived the winter. At the next cleanest 

 centre two plants survived, but in all other cases not a single one. 



In the spring of 1912 fresh cabbage plants were introduced, grown 

 throughout the summer, then cut and weighed. Up to the present we 

 have thus obtained in this experiment three crops, the results being 

 as follows : — • 



Table VII. 



station * 



(see 

 Table II.) 



Total suspended 

 matters brought 

 down by rain on an 

 acre per annum 

 (Table II.) 



Weight of crops obtained 



Radishes 



Lettuce 



Cabbage 





lbs. 



grams 



grams 





grams 



2 



1565 



226 



44 





505 



4 



849 



242 



56 





1250 



7 



399 



297 



104 





3056 



7-9 



273 



449 



120 





4167 



9 



147 



496 



140 





3425 



* The centres at which these vegetation tests are being carried out are, with 

 one exception, in close proximity to the stations at which the rain samples were 

 collected. The exception is the centre marked 7-9, which is roughly midway 

 between Stations 7 and 9 of Table II. The total suspended matters are given 

 as the mean of those given in Table II. for these two stations. 



H H 2 



