JOUKNAL OF THE KOYAL HORTICULTURAL SOCIETY. 



amounts of an acidified solution of iodide of potassiuju. Light brings 

 about a liberation of iodine in such solutions in quantities that are 

 roughly proportional to the intensity of the light. The amounts of 

 iodine liberated can then be detemiined by chemical methods. 



The results are summarized in the following Table, which brings out 

 clearly the connexion between smoke and curtailment of daylight. 



Table III. 



Total Suspended Matters brought down 

 Relative Intensity of Light on each acre per annum 



Station No. (Clearest Station = 100) (see Table I.) 



2 61 1565 



3 73 1163 



4 80 849 



5 83 659 

 7 94 399 

 9 100 147 



If the results at the cleanest and dirtiest stations be compared, it will 

 be seen that at the latter the intensity of the light was apparently 

 reduced by no less than 40 per cent. ! 



But curtailment of light is only one of the retarding influences upon 

 plant-growth that the smoke pall exeri.s. Innumerable tarry-soot 

 particles fall upon the leaves of the plant and produce there a black 

 deposit, which not only absorbs some of the already enfeebled light 

 before it can penetrate into the leaf, but, according to our observations, 

 will also choke up many of the stomata or breathing-pores which are 

 so essential to the normal activity of the leaf. In order to demonstrate 

 the reality of these effects the following experiments were made. 



Comparison of Assimilatory Powers of Leaves from different 

 Parts of Leeds. 



Measurements were made of the amounts oi carbonic acid gas that 

 were assimilated by cherry-laurel leaves obtained from different parts 

 of the city. In every case leaves of the current season's growth were 

 taken * and compared with practically clean leaves from the neighbour- 

 hood of Station 9.t 



The results may be summarized as follows, putting the assimilatory 

 power of the clean leaves from Station 9 at 100. 



Table IV. 



Relative Assimilatory Power 

 Station 5 = 53 



„ 7 =42 

 ,, Midway between 1 and 7 = 15 

 „ 4 =111 



The great reduction in assimilatory power in passing from the non- 

 industrial district of Station 9 to the more polluted areas of Stations 1 

 and 4 is very striking. In the case of the dirtiest leaves, a mere 



* The best leaves obtainable in the polluted areas were very much smaller 

 than average leaves from the clean districts. 



t For details of these and other experiments see Crowther and Euston, 

 loc. cit. 



