202 BOTANICAL GAZETTE [march 



certain limits the number of xylem rays. In general the larger 

 the diameter of a yearling root, the greater the number of rays up 

 to four. A pentarch or hexarch arrangement was not observed 

 in any case, although it may occur, since Van Tieghem {op. cit. 

 p. 7) reports the number as high as 7 in Scotch pine (P. silvestris). 

 The xylem in old roots is comparable to that in the aerial parts 

 of the tree, but differs in several well known particulars. The 

 tracheids in roots have wider lumina and thinner walls and are 

 never as well hgnified as those in the parts above ground. This is 

 especially well seen in cross-sections. In roots late wood formation 

 is not as pronounced, owing no doubt to a decrease in mechanical 

 strain in underground parts. The bordered pits on the tracheid 

 walls, in both roots and stems, are mainly radially arranged. The 

 uniseriate arrangement is here and there interrupted by the pairing 

 of some pits. Further, the bordered pits in roots are larger than 

 in the xylem of aerial parts. It is of interest to note in this connec- 

 tion that wherever an old root becomes exposed it usually presents 

 xylem typical of aerial parts, so that only underground parts 

 exhibit the characteristics above described.'' 



Winter condition'of secondary cortex and cambium' 



The secondary cortex of white pine is very similar to that of 

 pitch pine. It presents the same radial arrangement of the ele- 

 ments, this arrangement becoming less regular as they are pushed 

 to the outside (fig. i). Companion cells are totally lacking, but 

 one distinct row and a few scattered bast parenchyma cells are 

 formed each year as in pitch pine, and these indicate the annual 

 phloem areas in the old cortex. Occasionally the phloem pa- 

 renchyma becomes crystallogenous, but never attains the size of 

 that of pitch pine. The marked differences which exist between 

 the bark of white pine and pitch pine are not present in the young 

 phloem, but are caused by changes which take place subsequently 

 in the outer cortex. 



■• Kny (20) has pointed out the same structure in P. silvestris, and found that it 

 was especially pronounced on the underside of large roots which had been exposed 

 through erosion. 



5 The notes only include observations on the winter condition of aerial parts, as 

 underground parts were not accessible at this time of the year. 



