GENERAL CONSIDERATION OF PLANT DISEASES 289 



ning never strikes the laurel." In certain parts of the United States, 

 it is held that the beech tree is never struck. 



"Avoid the oak, flee from the spruce, but seek the beech," yet in 

 the Garden Magazine for January, 1916, is given a photograph and an 

 account of a fine beech tree which was struck by Ughtning in Pennsyl- 

 vania about the middle of June. Plummer^ sums up his investigations 

 on the relation of lightning and trees, as follows: 



1. Trees are the objects most often struck by Ughtning because: 

 (a) they are the most numerous of all objects ; (b) as a part of the ground, 

 they extend upward and shorten the distance to a cloud; (c) their 

 spreading branches in the air and spreading roots in the ground present 

 the ideal form for conducting an electrical discharge to the earth. 



2. Any kind of tree is likeh' to be struck by lightning. 



3. The greatest number struck in any locality will be of the domin- 

 ant species. 



4. The likelihood of a tree being struck by Ughtning is increased: 

 (a) if it is taUer than surrounding trees; (b) if it is isolated; (c) if it is 

 upon high ground; (d) if it is well (deeply) rooted; (e) if it is the best 

 conductor at the moment of the flash; that is, if temporary conditions, 

 such as being wet by rain, transform it for the time from a poor conduc- 

 tor to a good one. 



5. Lightning may bring about a forest fire by igniting the tree itself, 

 or the humus at its base. Most forest fires caused by lightning proba- 

 bly start in the humus. 



Experiments on the electric conductivity of various woods shows 

 that this conductivity depends upon the water content of the wood. 

 When absolutely dry none of the specimens showed conductivity, but 

 the resistance of all was practically infinity. 



Effect of Smoke, Soot, Gases and Smelter Fumes on Plants. — The 

 smoke, which is destructive to vegetation under our modern conditions, 

 is derived from four sources of supply: (i) smoke from manufacturing 

 plants, or from large buildings; (2) smoke from special concerns, such 

 as the electric power plants of electric trolley Unes; (3) smoke from rail- 

 road locomotives; (4) smoke from the chimneys of dwelUng houses. 

 Smoke belts have been drawn by students of the problem to determine 

 the area influenced by the smoke. From a survey made for the City 



^Plitmmer, Fred G.: Lightning in Relation to Forest Fires. Bull. 11 1, U. S- 

 Forest Service, 1912. 

 19 



