2990 KnupsON: 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. (1) Cambium and phloem development.— 
The cambium in the trunk during the resting condition consists 
of six rows of cells 34 » 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 I9gII.—(1) 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 1911 induced 
wood formation five days earlier than in 1909. 
(5) No direct evidence was secured in tott concerning the 
