483 



lightning paths to the unequal conductivity of the tissues and to the degree 

 of moisture present in them. If rain has fallen, weak flashes of lightning 

 cannot penetrate into the interior of the trees, but only tear off pieces of 

 bark, lichens and dry branches. Trees which have a very delicate cork 

 layer, as, for example, the pitch pine, display sometimes very noteworthy 

 traces of lightning only in the outer bark tissue. Often only small, round, 

 isolated places in the bark or others connected by zigzag lines are killed, 

 which later loosen from the living bark of the tree, often after a preceding 

 formation of cork. In trees with a heavy periderm, the lightning must first 

 strike through this poor conductor in order to reach the inner bark, which 

 is a good conductor Hartig considers the outer layers of the inner bark 

 "which are poor in fat" as especially good conductors, while the tissue rich 

 in protoplasm and. as a rule, containing much fat in the newest layers of the 

 inner bark, is a very poor conductor on account of its fatty contents and 

 often entirely escapes the lightning. The best conductive tissue is the young 

 wood, having only scanty cytoplasmic wall coatings. This is also found to 

 be very susceptible to frost injuries. If (in powerful discharges) the 

 cambial layer is also injured, there results an "internal healing." 



The theory of the influence of the fatty contents on the conductivity of 

 the tissue is based on the works of Jonescu\ He found that the electric 

 spark struck through fresh wood the more poorly, the richer this was in 

 fatty oil. In the same plantation the beech is rarely struck by lightning, 

 while the oak is most frequently injured. A microscopic investigation 

 showed the reason : the wood cells of the beech contained oil ; those of the 

 oak were almost free from it. Other "fatty trees" (in which in winter and 

 spring all the starch is turned to oil), as for example, Juglans regina, Tilia 

 parvifolia, Betula, Pinus, were also found to be bad conductors when com- 

 pared with starchy trees (Acer, Corylus, Fraxinus, Ulmus, Crataegus, etc.). 

 If the oil was removed from the fatty trees by ether, sparks penetrated the 

 fresh pieces of wood just as easily as that of typical starchy trees. It should 

 not be forgotten, however, in judging from these conditions, that the oil 

 content in the different tree varieties changes in different seasons of the year; 

 from this it is evident that their electrical conductivity varies. Jonescu found 

 with equally large pieces from the trunk of Tilia parvifolia that in February, 

 when wood and bark are rich in oil, a much higher electrical tension was 

 necessary than at the end of March, when the young wood was filled with 

 starch and glucose. The converse is true in the beech, which from January 

 to April, is rich in starch, but in May is rich in oil, as also the pine, spruce, 

 hornbean and common oak. The pine is pretty often struck during our 

 summer storms. At this time it contains glucose in its wood, bark and pith 

 and starch in its medullary rays. But in winter, the tree contains very finely 

 distributed oil and it is seen that in countries with winter storms (Ireland, 



1 Jonescu, Dlmitrie, tJber die Ursachen der Blitzeschlag-e in Baumen. Jahresb. 

 d. Ver. f. vaterl. Naturkunde in Wiirtemberg-. 1892. Schweizerbartsche Verl. — 

 Weitere Untersuchungen liber die Blitzschlage in Baumen. Ber. d. Deutsches Bot. 

 G. 1894, p. 129. 



