LIGHTNING-STROKE ON TREES. 5 
layers, which are poor in fatty material, conduct well ; the inner- 
most (youngest) layer, including the cambium, which is rich in 
protoplasmic content and contains as a rule much fat, conducts 
badly. The innermost layer of the living bark is thus protected 
and may remain untouched, even when the middle and outer 
layers are partially or completely killed. In very strong 
discharges the electricity may also pass here and there through 
the cambium, so that a lightning-wound is made which may be 
ultimately closed by occlusion, in which case a lightning-track 
remains visible in the wood-body. 
The best conducting tissue in the whole tree appears to be the 
young wood which is found in the stem from the time of the 
beginning of vegetative activity in spring until August, and 
represents the as yet unlignified portion of the year ring. It is 
very rich in water and contains very little air. The protoplasm 
lines the cell-walls as a thin layer, and there are only traces of 
fatty oils present. It has been proved experimentally that this 
protoplasmic layer is a good conductor, but it may be killed if 
the current is sufficiently strong. Ifthe lighning-track has been 
in the young wood the cell-walls remain unlignified, and they 
are subsequently crushed together by the later formed tissue. 
In deciduous trees lightning-tissue in the wood is indicated by 
this collapse of the cells. 
The alburnum being rich in water conducts the current better 
than does the duramen, but not nearly so well as the young 
wood and bark. At the same time the air-content of the sap- 
wood and of the splint-wood influences the conductivity of each. 
The heart-wood of conifers, which contains no liquid water, is the 
worst conductor of all. 
In the Royal Botanic Garden, Edinburgh, an evergreen oak, 
Q. Ilex, stood for many years on the east side of the Palm 
House. Outwardly it showed very little sign of having been 
struck by lightning. There were here and there a few short 
furrows visible on the outside as they are shown upon the 
photograph (Plate VIL). The tree was so damaged by the gale of 
November, 1901, when H.M.S. Active was wrecked at Granton, 
that it was cut down. When the stem was sawn across, several 
well-marked concentric rings were seen, which proved on 
subsequent examination to be zones of lightning-tissue (Plate 
