2 TIMBER AND SOME OF ITS DISEASES, [chap. 



specimen chosen there is a nearly central spot, the 

 pith, around which numerous concentric lines— the 

 "annual rings"— run. Radiating from the pith to- 

 wards the periphery are cracks, the number, and 

 length and breadth of which may vary according to 

 the time the log has been exposed to the weather, and 

 other circumstances ; these cracks are due to the con- 

 traction of the wood as it " shrinks," and they coincide 

 with medullary rays, as lines of weakness. Between 

 these cracks are to be seen numerous very fine radiat- 

 ing lines indicating the course of the uninjured 

 medullary rays, which again will vary in distinctness, 

 &c., according to the species of timber. 



This log of wood, with its annual rings and 

 medullary rays, is clothed by a sort of jacket, consist- 

 ing of cork and softer tissues, and termed the cortex, 

 or, more popularly, the " bark " (an unfortunate word, 

 which has caused much trouble in its time). The 

 largest of the cracks is seen to traverse the whole 

 radius of the face of the wood from centre to circum- 

 ference, and also to pass through the cortex, which 

 gapes widely. 



The remaining cracks, however, stop short at a line 

 which marks on the one hand the inner face of the 

 cortex, and on the other the outer face of the wood : 

 this line also represents the cambium^ a thin sheet of 



