282 KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 



While in trees A and C the region of the first growth of xylem 

 could not be definitely located, more fortunate results were ob- 

 tained with tree B. No xylem was formed in tree B until May 25 

 and then neither at the top nor at the base of the trunk, but in the 

 middle region. By May 25 then the xylem in that part of the 

 tree between the fourth and fifth whorls of branches was four cells 

 in thickness, while between the sixth and seventh whorls it was 

 five cells and between the seventh and eighth it was two cells in 

 diameter. The most marked development again occurred im- 

 mediately after May 25. The detailed figures are given in TABLE 



IV. 



Season and duration of diameter increase. In order to bring 

 out more clearly the period of greatest diameter increase, the 

 combined values for increase of phloem, cambium, and xylem, 

 in trees A and C are given in TABLE v. 



From the table it is at once evident that very little growth 

 took place previous to May 25, and the growth which occurred 

 was confined, as before indicated, entirely to the cambium and 

 phloem. From May 25 to June 3 a very marked increase occurred, 

 and during that time in the middle regions of the trunk nearly one 

 half of the total diameter increase was completed. In TABLE vi 

 is given the daily average increase in rows of cells and also the 

 daily diameter increase in trees A and C. 



TABLE VI 



By July 6 the ring of xylem was almost complete, judging from 

 the extent of the summer wood which was then in the process of 

 formation, though not yet completely developed. Unfortunately 

 no further series of cuttings were made after July 6 to determine 

 the succession with regard to growth cessation. 



In TABLE vn is given the combined diameters of the phloem, 

 cambium, and xylem tissue in trees B and D, as well as the total 

 number of cells. 



* For cutting nos. 9-10 only. 



