Arthur G. Ruston 393 



area of Hunslet, it will not flower. Though it will flower in the residential 

 area of Headingley, two miles from the centre of Leeds, it will not fruit. 

 Apparently it will not flower in localities where the annual deposit of 

 soot is more than 200 tons per square mile, nor will it fruit if the annual 

 deposit is greater than 50 tons per square mile. 



Evidently the "General Type of Vegetation" in any one particular 

 district will give some information as to the amount of pollution by 

 coal smoke within the area. 



2. The General Appearance of Individual Plants is also worthy of 

 attention. 



No one coming into Leeds by the Great Northern Railway and 

 looking out of the carriage window near Ardsley station could doubt 

 the fact that he was entering a smoke-infested area; for here almost 

 every tree and every hedge has been killed. In the woods of Middleton 

 and Templenewsam, the trees are to a great extent an index of the 

 smoke pollution from colliery, coke oven and brickworks. Here many 

 of the trees, particularly the ash, oak, elm and beech have been abso- 

 lutely killed, most of them dying from the top. The main drift of the 

 smoke from the industrial area of Hunslet can be easily traced almost 

 to Garforth by the line of dead and dying trees. 



Signs of smoke damage to trees can be discerned quite early in the 

 year, in the appearance of the young shoots and buds, many of which 

 are killed off before opening. 



The leaf perhaps is the safest index. Before May is out, many leaves 

 in smoky districts are showing characteristic brown blotches. In the 

 case of leaves possessing a thin epidermis like the lime and sycamore, the 

 signs of smoke damage are seen as in the case of Figs. 2 and 3, PI. XXIV, 

 all over the leaf, wherever the acid rain drops. In the case of the ribes 

 (Fig. 4, PI. XXV), the leaves of which possess a harder cuticle and have a 

 tendency to offer a convex surface, the damage is shown by a red rim 

 all round the edge. In the case of leaves like those of the laurel and aucuba 

 which possess a hard cuticle and have a tendency to hang downwards, the 

 damage is shown first at the tip of the leaf. 



Evidence as to the amount of smoke pollution can also be obtained 

 by the appearance of trees in the autumn; the greater the pollution the 

 earlier the "leaf fall." Ash trees in the purer parts of Leeds often retain 

 their leaves six or eight weeks longer than those in the more contaminated 

 districts. Thus last year in Hunslet with an annual solid deposit of 539 

 tons per square mile, all the ash trees which remained alive had shed 

 their leaves before the 18th of September, while at Roundhay with an 



Aim. Biol, vii 26 



