578 TRANSACTIONS OP SUB-SECTION B. 



grasses and upon the micro-organic flora of the soil. The results direct attention 

 especially to the following points : — 



(a) The high amounts of suspended matters in town air. — This is not only 

 directly injurious to vegetation in blocking up a large proportion of the stomata 

 of. leaves, but exercises also a considerable influence upon the amount and intensity 

 of the solar radiation placed at the service of town vegetation. The injury due 

 to the former cause is greatly aggravated by the sticky nature of much of the 

 suspended matter (tar, &c), and is most markedly seen in the case of evergreen 

 plants, conifers being the most susceptible. The characteristic sunk stomata of 

 these latter plants, whilst serving admirably for the restriction of transpiration, 

 act as traps for the solid matters suspended in the atmosphere. Some leaves 

 have been found to have as many as 80 per cent, of their stomata so blocked. 

 The well-known detrimental effect of the gaseous products of combustion of coal 

 upon the growth of these plants is thereby intensified. Conifers cannot be grown 

 with even moderate success in Leeds, except upon the northernmost verge where 

 the atmospheric impurities reach their minimum. 



(6) The relatively high acidity of town rain, especially in the industrial 

 districts.— The investigations show that the rain falling in practically every part 

 of the city is distinctly acid. The injurious effect of this acid upon vegetation, 

 as illustrated by the growth of grass, is brought out clearly by the experiments. 

 The injury is probably partly direct, but it is shown to be due in part to the 

 effect of the acid upon the micro-organic flora of the soil. The reduced yield, 

 lower protein-content, and increased fibre-content of the grass grown under acid 

 conditions is a matter of serious import for the farmers in semi-urban districts. 



2. Some Troublesome Diseases of the Potato Tuber. 

 By A. S. Horne, B.Sc., F.G.S. 



Some years ago a disease of the potato tuber, due neither to Phytophthora 

 infestans nor Fusarium solani, was described by Frank under the name Buut- 

 werden or Eisenfleckigkeit, which possessed only internal symptoms, taking the 

 form of brownish blotches or streaks in the flesh. Corresponding to Buntwerden 

 are the types known in Britain as internal disease and sprain (streak-disease), 

 the flesh being marked with blotches and streaks respectively. The markings 

 in both cases are constant for large samples of potatoes of a known variety and 

 have received separate consideration, since it has not yet been shown that they 

 are due to the same cause. No hyphal organism is present in typically affected 

 tubers — if pathogenic organisms are responsible for the disease, they are pro- 

 bably bacteria. Diseased seed-tubers are liable to propagate the disease. 



Internal disease is often associated with Phytophthora infestans (from the 

 soil). The tubers in a given sample affected with blotches only show no external 

 marks, but others of the same sample with the complication can be detected 

 at a glance. When the tuber is cut open there is generally a peripheral zone of 

 the fungus, whilst the central portion is blotched. 



In spite of Frank's description of two types of an attack of Phytophthora, 

 that type which consists of an attack upon the tuber from the soil, which, in 

 the north of England and in Scotland often takes the form of a joint incursion 

 of Phytophthora and bacteria, is considerably neglected in some parts of the 

 country, although responsible for extensive damage to crops. Spraying, although 

 perhaps beneficial to the plant, is not a proper remedy, since the disease is as bad 

 under healthy as under unhealthy tops. Affected tubers when examined imme- 

 diately after removal from the soil show a peripheral zone of disease, but the 

 stalk end is frequently quite clean. In microscope sections the fungus and motile 

 bacteria can be seen in the intercellular spaces and the points of entry of the 

 organisms concerned can be traced. 



Another form of disease which appears towards the middle of the storage 

 period is locally known as bruise. No pathogenic organisms have yet been recog- 

 nised, but, in any case, the disease leans strongly towards the physiological side, 

 since it seems to be eliminated by a suitable change of soil and climate. 



