4 BORTHWICK—ON THE EFFECT OF 
internal effect of lightning on trees. The results were published 
by him as a first contribution to our knowledge of the subject, 
the facts being too isolated to allow of his drawing a clear picture 
of the whole phenomenon, and further exact investigation being 
required before a satisfactory explanation can be given of the 
various effects of lightning on the same tree at different times. 
Hartig has examined, figured, and described the internal 
effect of lightning on the following trees :—spruce, silver fir, 
larch, scots pine, beech, oak, maple, ash. 
Almost all of the damage done by lightning to those trees 
occurred in the living tissue of the bark and in the young 
immature portion of the youngest annual ring, and it consisted 
in the destruction of the protoplasm of the tissues. There are 
several conditions which modify the ultimate effect of lightning 
on trees, such as the strength of the discharge ; the thickness of 
the bark it has to penetrate ; and also whether the bark is wet 
or dry. Very strong discharges may rive the wood-body of the 
tree into splinters ; while weaker discharges may cause splits or 
fissures in the wood-body which are not visible from the exterior. 
If the path is confined to one side, the result is that the sap- 
wood is more or less splintered, and may be thrown in long 
strips as far as a hundred yards from the tree. Weaker shocks 
do not cause very great damage, but produce peculiar and 
diverse lightning-tracks within or upon the stem, and these have 
remained undescribed as yet to a great extent. . 
In order to understand the phenomena properly it is necessary 
to know, in the first place, the conductivity of different parts of 
the tree. 
Trees with thin bark, for example silver fir, show very 
peculiar lightning-tissue—by which is meant protoplasmic tissue 
which has been traversed and killed by lightning—in the outer 
rind. It frequently occurs as small roundish patches, either 
isolated or connected by zigzag lines, and these patches 
ultimately become cut off from the living bark by cork-layers. 
If the tree has a thick covering of dead bark, then the lightning 
must force its way through this bad conducting tissue to reach 
the interior. 
A distinction must be made between the conducting power of 
the several layers of the living bark, The middle and outer 
