16 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



traversed by smaller or larger medullary rays, radiat- 

 ing from the central pith, and passing across the 

 cambium to the cortex. Moreover, cracks would be 

 apt to form on exposure, as before ; the opening 

 occurring along the lines of medullary rays lines of 

 weakness. 



Again, if we cut a segment of the wood, like Fig. 2, 

 the chief features would present themselves as there 

 shown, and the lines of demarcation indicating the 

 annual rings would be found to be due to the sharp 

 contrast between the spring wood and the autumn or 

 summer wood, as before. 



On closely examining a transvere section of such a 

 piece of timber, however, we should find differences 

 which at first sight appear profound, but which on 

 reflection and comparison turn out to be of more 

 relative significance, from the present point of view, 

 than might be expected. 



Selecting a given example, that of the beech for 

 instance, the first difference which strikes us (Fig. 6) 

 is a number of relatively very large openings on the 

 transverse section : these are the vessels pitted 

 vessels long tubular structures which are not formed 

 by the cambium of the conifers. Each vessel may be 

 regarded as a tube made by the joining of a long 

 vertical row of tracheides, the lumina of which become 



