DISEASED WOOD-FIBRES. 141 



at about the same altitude, is 10'1 F. in February, 1895. 

 Hess gives 18 C., or F., as a sufficiently low tempera- 

 ture for frost-crack, and it is evident that frost-crack occurs 

 in the British Isles at temperatures considerably above 13 F. 

 -Tr.] 



Damage by lightning is fully discussed in Vol. IV., and 

 here only some points brought forward by Mayr will be given. 

 E. Hartig has proved that lightning strikes trees more fre- 

 quently than is usually believed often causing numerous 

 little wounds in the cambium, that are eventually occluded 

 but become externally visible as the tree gets thicker. 

 Conifers usually die when struck ; broadleaved trees may 

 occlude the wounds. Usually oaks, spruce, poplars, larch 

 and pines are considered as most in danger, while beech is 

 said to be immune. Hartig, however, has shown that young 

 beeches are struck by lightning but that their wounds are 

 occluded. 



Usually wood struck by lightning is so completely shaky 

 although only one external crack may be visible, yet so 

 many small cracks have branched from the principal one 

 through the wood, that when it is sawn into planks or 

 scantling they eventually fall to pier 



The holes made in trees by woodpeckers may be noted 

 here (cf. Vol. IV.). 



(//) /Jiwrwil Wood-Jil' 



Internal disease in wood, e.g., rottenness in stem, roots 

 or branches, is discernible externally by scientific observa- 

 tion only. Stem-rot is merely a continuation of disease that 

 originated in the roots or branches, or is caused by wounds, 

 or by diseases that are visible externally, such as cankers. 

 The earliest visible signs of disease are discolourations. As 

 long as abnormally coloured wood is still hard it is quite 

 utilisable, provided that by rapidly drying and using it only 

 in dry places (interior of houses, etc.) the growth of the 

 mycelia of the fungi and consequently the spread of decay 

 are arrested. 



* llartig, "Lehrbuch der Buumkrunkheiten," Berlin, 1901, 3rd ed. 



