296 DISEASES OF TREES 



on dry ground intermixed with a silicious moor-pan, to such an 

 extent that the cortical and other living tissues beneath the 

 bark exposed to the drying winds became completely withered. 

 This occurred on the south and west sides of the trees, and 

 especially at a height of from three to six feet, although 

 portions both above and below these heights were also affected. 

 The Weymouth pine is found naturally in marshy situations, 

 and, adapting itself to the natural habitat of the tree, its 

 cortex is but poorly protected by periderm and bark. It 

 is thus easy to understand that on a dry soil and in a hot 

 dry year the wood is unable to furnish the cambium and 

 cortical tissues with sufficient moisture. It follows therefore 

 that this species of tree should not be cultivated on excessively 

 dry ground, especially where water cannot be expected to ascend 

 from the subsoil. 



Of quite another character is the pathological phenomenon 

 appropriately called " sun-crack," which is sometimes met with in 

 late winter or spring in the beech, hornbeam, Acer, and oak. 1 In 

 spring, fissures varying in length form in the cortex, which 

 separates from the wood for an inch or more on both sides of 

 the wound. In the case of the beech, with its thin cortex, the 

 rind * not only becomes detached but also dies. Owing to the 

 vigorous formation of callus, such a sun-crack frequently heals 

 up after a few years, whereas in the case of bark-scorching it is 

 very seldom that healing occurs. Fig. 159 represents, in one half 

 the natural size, the cross section of the upper part of an oak 

 taken from the south side of the stem. The tree, which was 

 about 1 70 years old, and was taken from a light pole-wood of 

 beeches on a fairly steep north slope, showed that numerous 

 sun-cracks had been formed all over the stem, at various periods 

 of its existence. 



The cold ground, which in spring was hardly affected by the 

 sun even at midday, must have kept down the temperature 

 of the wood of the oak to a low point, even when the stem 

 was intensely heated by the sun's rays. It is probable that 

 the cortex had become so warm at certain places under the 



1 Untersuchungen, I. p. 141. 



* [The word " rind " is here used in a general sense to denote all the tissues 

 outside the cambium. ED.] 



