354 



AMERICAN FORESTRY 



Resin canals in white fir occur in 

 small groups of two to six or more in 

 the early-wood, and usually near the 

 inner boundary of concentric zones 

 (Figs. 5 and 6). These canals are al- 

 ways in direct communication with pith 

 rays (Figs. 5 A, •5B, and 6), and are 

 surrounded by wood-parenchyma fibers 

 that contain starch during the winter 

 (Figs. 5 and 6 st. g.). Transverse sec- 

 tions of twigs cut from the top of the 

 tree show that. young resin canals con- 

 sist of compact groups of cells, the cen- 

 tral portions of which are composed of 



numerous round cells with wide cell 

 cavities, but in other respects are simi- 

 lar to the cells that are filled with starch 

 during the winter (Fig. 7). In the 

 early spring, when the vegetable pe- 

 riod begins, the starch is replaced by 

 a volatile oil, and during the first win- 

 ter there is no trace of resin in these 

 cells. Longitudinal sections show that 

 resin passages are surrounded by wood- 

 parenchyma fibers, the cells of which 

 are from two to three times as long as 

 those in the center of the group. In 

 other respects the inner and outer rows 



of cells are similar. The pits are sim- 

 ple, and only where resin cells touch 

 tracheids do they appear to be bor- 

 dered; such borders are always within 



at.c_. 



^F^.&. 



the walls of the tracheids. The cells of 

 the young resin passages are filled dur- 

 ing the winter with starch (Figs. 7 and 

 9), which is eventually converted into 

 a fluid mass and later into resin. Trans- 



^.^. 



^^ 



7. 



verse sections of two to four-year-old 

 twigs exhibit structures similar to that 

 of one-year-old twigs. There is, how- 

 ever, a difference in cell content. In 



