STEMS, BULBS AND BUDS 51 



aate in the spring growth, and the strengthening cells predomi- 

 nate in the fall growth. These rings are very evident on the 

 cut end of a woody stem. 



Medullary Rays. — The erossrsection of a woody perennial 

 st-em shows a number of lines radiating from the central axis ; 

 they are the medullary rays and are derived from the inter- 

 fascicular cambium (Figs. 26 and 27). Some extend from the 

 pith thi-ough both xylem and phloem, while others originate at 

 various points and extend through a part of the phloem only. 

 The former are known as primary rays and the latter as sec- 

 ondary rays. They consist of thin layers of thin-walled (paren- 

 chyma) or more or less thick-walled cells. They are the im- 

 portant factors in decorative woods, especially in quarter-sawed 

 oak. The direction of the sawing gives various views. If the 

 log is sawed so that the surface of the board is p'arallel to the 

 ray, it is said to be quarter-sawed, and the tissues of the ray form 

 artistic markings. If sawed at any other angle, the figures are 

 much less artistic. 



The Cork Cambium (Fig. 26). — The growth of the cambium 

 causes, an increase in the diameter of the stem and results in an 

 increasing tension of the primary cortex, and would eventually 

 cause a splitting of the outer layer of the stem if it were not 

 for the growth and cell division in this region. In the woody 

 perennials this activity is primarily in the cork cambium or 

 phellogen. This originates in tangential division of epidermal 

 cells or in the cells just below the epidermis. In either case 

 the result is a cork cambium before the end of the first year's 

 growth. This cork cambiuni or phellogen undergoes both radial 

 and tangential cell division, producing cork cells to the outside 

 and a smaller number of thin-walled parenchyma cells to the 



