CH. V] 



CAMBIUM. 



75 



overlooked by beginners, is the production of medullary 

 rays. Certain of the cambial cells, instead of developing 

 into xylein or phloem elements, turn into medullary 

 ray cells; in this way the rays which already exist 

 are continued outwards as the trunk thickens, and at 

 the same time new rays make their appearance in each 

 annual ring. This will be understood by a reference to 

 fig. 35 ; it will be seen that only the original primary 

 rays run from pith to bark, while the rest (secondary rays) 

 arise in one of the annual rings, whence they are con- 

 tinued radially outwards by the addition of the medullary 

 ray cells manufactured year by year by the cambium. 



M 



FIG. 35. 



PAKT OP A TRANSVERSE SECTION THROUGH A FOUR-TEAR-OLD BRANCH 

 OF THE CORK OAK. 



(1), a primary medullary ray running from the pith (M ) to the bark. 

 (2), (3) and (4), secondary rays formed in successive years. 

 PC, phloem and cortex. (The medullary rays should be continued 

 into the phloem.) S, cork. 



From Le Maout and Decaisne. 



There remains to be considered the structure of the 

 wood as seen under a high power of the microscope. 



