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. HartigJ with 3o-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. 



f Brown, H. P. Growth studies in forest trees, I. Pinus rigida Mill. Bot. Gaz- 

 54: 386-402. 1912. 



J Hartig, T., loc. cit. 

 Brown, H. P., loc. cit. 



