491 



Fig. 102 shows a cross-section through the bark of a small spruce trunk 

 injured by artificial lightning. The trace of lightning (b) shows, first of all, 

 a central brown strip-like kernel of swollen parenchyma. ' This kernel is 

 surrounded by a broad, clear zone (k) which consists of radially arranged 

 rows of ver}' thin-walled, nearly empty cells, often containing air. 



Toward the outside, this zone adjoins a tissue ring (kk) of plate-like 

 cells, rich in cyptoplasm, the walls of Avhich give a cellulose reaction. These 

 cells gradually pass over into the normal bark parenchyma (r/>) with its 

 large lumina. The resin ducts (g) lying outside the trace of lightning but 

 pretty near to it, are, as a rule, uninjured ; the living cells at times projecting 

 into the resin ducts are light-walled. This vesicular outpushing of the Hn- 

 ing cells is a normal phenomenon; for in branches of healthy spruce in 

 winter, resin canals are often found completely filled by tylose-like enlarge- 

 ments of the lining cells. Resin ducts also occur isolated in the immediate 

 proximity of a trace of lightning in which the cells filling them are changed 

 to brown, swollen, resinous masses. 



The dead tissue kernel in the centre of the lightning trace consists fre- 

 quently only of dead bark parenchyma. Often, however, it can be noticed 

 that some bast groups (k') have participated in this. The fact should be 

 emphasized, that the dead parenchyma cells are often entirely collapsed and 

 dried. In my opinion, the production of the light colored circular zones, 

 composed of thin-walled cells with broad lumina which are found to be actual 

 cork cells and constitute the difference from the frost wound, is due to the 

 drying up of the cells. 



I conceive of the production of this difference in the two forms of 

 wounds as follows : The electric spark causes a rapid drying out of the 

 dead tissue. Since this, like frost, does not cause any slowly spreading, 

 subsequent death of the adjoining tissue, vigorous cells, capable of reacting, 

 directly bound the dead tissue centres. A reaction to the wound stimulus 

 sets in at once if the vegetative activity makes itself felt in the bark. The 

 parenchyma around the dead tissue responds to the wound stimulus by cell 

 elongation and increase. The cell groups dried by lightning, allow the sur- 

 rounding cells to elongate greatly. The more rapidly the process takes place, 

 the more material is used up. If this is not present in sufficient amounts 

 only a formation of cork will take place and thus the fact is explained that 

 after the electrical discharge the bark parenchyma surrounding the dried 

 tissue must elongate and divide to fill out the large spaces ; then there is a 

 formation of cork. 



When frost kills an area of tissue, lying in the bark parenchyma, at first 

 no drying of the tissue takes place. The dead, swollen cells retain their 

 size, and are still turgid. Also the pressure of the dying frost-injured tissue 

 on the healthy surrounding tissue is not essentially increased. The sur- 

 rounding cells have no incentive whatever to the great elongation and 

 division which were necessary in the drying out of the lightning traces. 

 Therefore, there will appear around the dead centre of the frost wound the 



