KNUDSON: CAMBIUM DEVELOPMENT IN AMERICAN LARCH 291 
region in the trunk of first cambial activity. Indications pointed 
again to the middle and basal regions. 
DIscussION OF RESULTS 
In a number of anatomical text books it is stated that the 
xylem development precedes that of the phloem. This idea is 
conveyed rather ambiguously by Stevens.* In the American 
larch the development of phloem certainly precedes that of xylem 
and its most rapid development is coincident with that of the 
xylem. Brown’sf figures indicate that in Pinus rigida a similar 
condition prevails. 
The results obtained by the writer do not agree with those of 
Th. Hartigt with 30-year old European larch, wherein diameter 
increase near the base of the trunk was two to four weeks later 
' than that in the twigs and branches. The factors which may 
operate to cause this difference are considered subsequently. My 
results agree with those of Brown§ who finds that in Pinus rigida 
the first diameter increase of xylem begins a few meters below the 
apex. The work of Brown was done during the same period as 
that of the writer and on trees in a plot adjacent to those used in 
this investigation. 
Respecting the date of diameter increase Buckhout states that 
in European larch it is coincident with leaf formation. It is 
very probable that the diameter increase at this time is due mainly 
to a swelling of the tissues. In my investigations the development 
of xylem began a month later than the beginning of leaf formation. 
From observations made during the past two years with a consider- 
able number of trees and from the results of other investigators it 
seems probable that in general growth in diameter does not begin 
until the leaves have been fully developed and have been suffici- 
ently active in food making to supply the requirements of rapid 
cell formation. The reserve foods stored up in the fall are prob- 
ably largely utilized in leaf and also in blossom formation, when 
the latter precede the formation of leaves. 
* Stevens, W. C. Plant anatomy, 2nd Ed., p. 170. 
1910 
+ Brown, H. P. Growth studies in forest trees, I. Pinus rigide Mill. Bot. Gaz, 
§ Brown, H. P., loc. cit. 
