DAMAGE TO VEGETATION BY SMOKE AND FUMES 1 85 



dioxide on growing vegetation. As much of the country in 

 the smoke zone was devoted to agriculture, the experiments 

 naturally dealt largely with the ordinary grain crops. 



Many serious experimental difficulties had to be overcome 

 in the course of these investigations, and much ingenuity and 

 resource were shown by the staff in devising suitable methods 

 and apparatus. These experimental details are hardly suitable 

 for a paper of this kind, but it may be mentioned that since 

 sulphur dioxide is rapidly absorbed by vegetation, by soil, and 

 by moisture, the fumigation of a growing crop with air contain- 

 ing a definite and constant percentage of sulphur dioxide is no 

 easy matter. As a farm crop is only exposed to the fumes 

 during the growing season, and as the number of experiments 

 was large, the results obtained may be taken as reliable. 



It is different, however, in the case of woods where the trees 

 are exposed to contamination year after year. In this case an 

 amount of sulphur dioxide which might not cause any great 

 effect in a single season might easily have a deleterious effect 

 in the course of a number of years. 



The humidity of the atmosphere was found to have a very 

 important effect, as was to be expected ; in presence of moisture 

 the damage caused by the same amount of sulphur dioxide or 

 trioxide was much greater than when the air was dry. It was 

 also found that the damage was greater in sunshine than in the 

 shade, but this was found to be a much less important factor 

 than the humidity. 



The report is illustrated with an excellent series of coloured 

 plates, showing the effects of various amounts of sulphur dioxide 

 on the leaves. 



The investigations carried out by the plant pathologists were 

 less extensive than the chemical investigations described above. 

 As already mentioned, the country in the smoke zone was 

 largely agricultural, the orchards and native trees not being of 

 particular importance. The methods of diagnosing smoke 

 injury is based on the study of the lesions produced ; the mark- 

 ings, shrinkage, and coloration of the affected parts of the 

 foliage ; the general health of the plant ; the microscopical 

 examination of the injured tissues, particularly the cell shrinkage 

 and the condition of the cell contents, and on certain micro- 

 scopical reactions to determine the nature of these contents. 

 The plant pathologists are of the opinion that there is no 



