236 BOTANICAL GAZETTE [march 



number of xylem rays. The secondary xylem forms between the 

 primary xylem rays and under the primary phloem, and it follows 

 that there are as many secondary xylem areas as there are primary 

 xylem rays. In a young root where the secondary thickening has 

 begun we find the primary and secondary xylem areas alternating 

 with each other (figs. 9 and 10). It is usually not until the 

 following year that the segments unite and complete the ring of 

 cambium. 



An unsuccessful attempt was made to find out at just what 

 period in the growing season secondary thickening began in the 

 root. Roots were examined on May 1 1 and again on May 30 with 

 this object in view. No secondary growth was in evidence in 

 either case in the new tissues, even when, as at the last date, 

 elongation had gone on to the extent of 5-6 inches. In every case, 

 however, where the last formed growth of the preceding season 

 was examined, secondary xylem was in evidence between the poles 

 of the primary xylem, and evidence of a resting period was to be 

 seen, so that it must be concluded that secondary growth occurs 

 later in the growing season than May 30, probably during the second 

 period of activity in the autumn. The cambial segments, however, 

 apparently do not unite over the poles the first year, so that second- 

 ary growth the first season is confined to as many separate areas 

 as there are poles. 



The course of secondary thickening in the root, once started, 

 is much more irregular than in the aerial parts. The aimual rings 

 are usually thickest on the Iqwer side of the roots as they en1;er the 

 root crown, but all regularity is lost a short distance from the bole. 

 The rings may be narrow here and broad there, and apparently 

 their position in the ground has no appreciable effect; geotropism 

 is not a factor in annual ring formation. False and double annual 

 rings are often present. As Rubner (28) has pointed out, the 

 cambium may be active on one side of a root and dormant on the 

 other for several years without its vitality being impaired, and this 

 is responsible, in part at least, for the irregularities in growth which 

 arise. Furthermore, the tissues of exposed roots present the same 

 characteristics as those in aerial parts, a peculiarity previously 

 noted by Kny (20). In conclusion, it may be said that roots, 



