DAMAGE DONE. 661 



the ground, but in 3 per cent, of the cases observed it passed 

 off to other trees before doing so. 



Neither carbonisation nor tearing open of cells have been 

 observed on trees struck by lightning. 



The injured parts observed on the stem or main branches 

 of a tree are never the first point attacked. The lightning 

 almost always strikes the fine topmost twigs (the best con- 

 ductors), and proceeds thence along the main boughs and 

 stem. It then generally follows the direction of the fibres, 

 the path of least resistance. If the fibres are twisted, it 

 follows a spiral path. The cambium, wood, and pith of a 

 tree struck by lightning become discoloured, and often the 

 topmost leaves turn brown, those below remaining green. 



Very little is known regarding the effect of lightning on the 

 inner structure and technical quality of timber. It is assumed 

 that wood struck by lightning has its hardness and strength 

 reduced. Wood and bark-beetles, wood-wasps and fungi, 

 attack the tree, which soon dies. It should therefore be felled 

 and converted as soon as possible. 



When an unsound tree is struck by lightning it is some- 

 times set on fire, and the fire may then spread to the 

 surrounding forest. 



It has also been repeatedly observed in coniferous forests, 

 that sometimes a whole group of trees may die from the 

 effects of lightning, the marks of which may only be visible 

 on one of the trees. This takes place some time after the 

 occurrence, and leaves an ugly gap in a fine wood. This has 

 been hitherto observed only in crops of Scots pine, spruce, 

 silver-fir and larch. It may be the effect of lack-stroke 

 (p. 658). In such cases the dead trees should be carefully 

 examined, as bark-beetles have been afterwards proved to have 

 caused the death of the unstruck -trees in certain cases. 



The soil may be the cause (p. 664), or small lightning 

 strokes accompanying the principal stroke. Such a stroke is 

 termed group-stroke. 



b. According to Species. 



All species of trees are liable to be struck by lightning, but 

 oaks and other species with deep roots appear to be most 



