290 KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 



days preceding wood formation in 1909 had 39.4 hours of sun- 

 shine; the same period in 1911 had 26.2 hours of sunshine. It is 

 very probable therefore that during 1911 the relatively higher 

 temperature for the six days preceding wood formation was 

 important in hastening the inception of xylem formation. 



SUMMARY 



RESULTS OF 1909. (i) Cambium and phloem development. 

 The cambium in the trunk during the resting condition consists 

 of six rows of cells 34 ju, diameter. On April 19 the first material 

 collected exhibited an increase in diameter of the cambium tissue. 

 The cambium cells were all enlarged while the outer cells were in 

 the process of transformation, changing from a rectangular to 

 square shape as viewed in cross section. In the middle regions of 

 the trunk on April 19 an increase in phloem cells was evident. 

 The increase of cambium and phloem was gradual over the entire 

 trunk in tree A up to May 25, the greatest increase being main- 

 tained in the middle region. Similar conditions were found in 

 tree B, though here the increase of new phloem cells was more 

 marked. The greatest growth of phloem occurred immediately 

 after May 25 and was coincident with the greatest development of 

 xylem. 



(2) Xylem development. In tree A no xylem was formed before 

 May 25. In tree B a few xylem cells were formed by May 25 in 

 the middle regions of the trunk. Growth of xylem was almost 

 simultaneous, however, in all parts of the trunk. The greatest 

 growth occurred immediately after May 25 and in tree C the 

 xylem was nearing completion by July 6. 



RESULTS OF 1911. (i) Growth in diameter in the lateral 

 branches begins first in the middle branches and is followed by 

 that in the basal and apical branches. 



(2) In the individual branches growth begins first at the apex 

 and then descends towards the base. 



(3) Diameter increase of the trunk precedes that in the 

 branches and twigs. 



(4) Temperature and insolation conditions in 19-11 induced 

 wood formation five days earlier than in 1909. 



(5) No direct evidence was secured in 1911 concerning the 



