THE PINE 117 



But some rows of cells (called medullary rays) running from 

 the central part of the stem toward the outside remain thin- 

 waUed, and, except that they grow somewhat larger, do not 

 become very different from the cambium cells themselves. 

 The medullary rays thus cut across both the wood and the 

 bast ; but except for these rays, the wood forms a continuous 

 zone of increasing thickness just \\'ithin the cambiimi, and 

 the bast forms a continuous zone just outside the cambium. 

 Within the newly formed wood, resin passages develop like 



Fig. 68. — Part of a cross section through a portion of a pine stem 

 which has been growing in thickness for several years. The small Ught 

 spots in the wood represent resin passages. 



those in the cortex. Although the process of growth in thick- 

 ness can best be studied in thin sections of the stem, we must 

 bear in mind that the zones of wood, cambium, and bast which 

 we see in such a section really represent cylinders running up 

 and down within the trunk. 



142. Annual Rings. — The new wood cells formed from the 

 cambitim in the spring grow large and remain thin-waUed. 

 Later in the season, smaller, thicker- waUed cells are formed, 

 and those produced at the close of the growing season (in 

 late stmmier or early fall) are smallest and thickest-waUed of 

 all. No divisions occur in the cambium during the winter ; 



