THE EFFECT OF SOOT ON VEGETATION. 17 



Another common condition met wherever there is 

 smoke and gases is the dying of the leaves from the tips 

 down (Fig. 16). This is well marked in the rhododen- 

 drons, pines and many deciduous trees. The leaves of 

 Gingko in the Allegheny Cemetery were found not only 

 dying at the tips but many showed dead spots in the leaves. 



Anatomical study of leaves collected at the different 

 stations did not show any unusual cork formation either 

 in the blades or petioles. In certain plants (e. g., the 

 pines) the stomata are in pits which might be expected 

 to be filled with soot held there by tarry substances. Ex- 

 amination and comparison of leaves collected at these sta- 

 tions, with similar leaves collected on the campus in State 

 College and from plants experimented with in the nursery 

 of the Department of Forestry, Pennsylvania State Col- 

 lege, do not show any marked difference. To be sure, in 

 each instance a few of the stomata appeared to be clogged, 

 but in no case were more than approximately twenty-five 

 per cent, found in this condition. 



CONTROLLED FIELD EXPERIMENTS. 



In order to determine more specifically the effect of 

 soot upon plants, controlled field experiments were per- 

 formed in the following manner. Cases of the sort shown 

 in Fig. 17 were placed over plants of suitable size and in 

 good condition. On the ground within each case and on 

 opposite sides of the plant were placed two crystalizing 

 dishes for the purpose mentioned below. A thermometer 

 was hung in each case. A tin tube connected at one end with 

 a funnel shaped piece was passed through the upper cross- 

 bar of the case. This funnel shaped piece was within 

 the case and was turned upward at an angle of about 30. 

 On the same tin tube and outside of the case another fun- 

 nel shaped piece was attached. The soot was applied to 

 the plants in the following manner : small quantities were 

 placed in the funnel on the outside of the case and sub- 

 sequently driven into the case by means of a small stream 

 of air entering at a high velocity. By this means the soot 



