39 



search on the completion of the microscope 

 by Hook, to om' own time, that is, to the 

 time of the equally admirable Dutrochet. 



The medullary rays, which appear like 

 the spokes of a wheel, when the stem is 

 cut across, are in fact thin plates, running 

 the whole length of the stem, roots, and 

 branches. They are supposed by Dutro- 

 chet to be of the same nature as the pith, 

 though they appear very different. In 

 width, they increase each year by the 

 width of the new layer of wood, across 

 which they extend to the bark ; and in 

 length, they increase each year by the 

 length of the new shoot of the branches 

 and roots. 



The medullary rays and the concentric 

 rings of pith form interstices in the wood, 

 and prevent its perfect union. What are 

 called shakes in timber arise in them. 

 These shakes are often seen outside the 

 stems of living oaks. They are the con- 

 sequence of diseased medullary rays ; they 



c 4 



