248 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



definite. The cells just under the cambium give no indication that the 

 date of cutting was as late as November 21. This, combined with the 

 fact that the annual increments cannot be separated, suggests that 

 water fluctuations cause the growth pulsations as expressed in the 

 growth layers. In fact, the trees are known to grow after each irriga- 

 tion.^ Tip flushes are given as 2 + , 5 + , and the like, in YCt 2 because 

 the branch material available to us was cut so as to include only that 

 part of the branch length which had grown during 1939. 



YCt 2-2-a {1939-1940) 



3 see + 5 to 6 sL + psg — i or i'^ tfs — 2 years. 



Vessel and tracheid succession is much more definite in YCt 2-2. 

 Definite terminal bud scale scars did not exist between 1939 and 1940 

 growth. As before, the cutting date was November 21, 1940. Typical 

 vessels had been formed in contact with the cambium. In fact, many 

 of the vessels had differentiated so rapidly on the outer margin of the 

 xylem that the cambium protruded outward over the vessels. As a 

 manner of speaking, vessel formation in this ring-porous wood had no 

 regard for the time of year. 



YCt 2-3-a (1939-1940) 

 8 to 10 see -(- 4 to 6 sL -{- psg — 2 or 2* tfs — 2 years. 



Growth was not quite so active outward on the branch, YCt 2-3, at 

 the time of cutting, November 21, 1940. 



YCt 2-4-a (1939- 1940) 



4 to 5 see -f- 2 to 3 L's + psg — 2 or 2* tfs — 2 years. 



YCt 2-5-a (1939-1940) 

 6 see + 6 L's -{- psg — 5 or 5* tfs — 2 years. 



Sections a to ^ were taken from YCt 2-5, so that a and h came from 

 the oldest tip flush of 1939, c from the second, d from the third, and e 

 from the fourth or youngest. Because we did not know how many tip 

 flushes grew in 1940 on this branch, one or one-plus is added to the 

 four for 1939. The important feature of YCt 2-5 is the increase in the 

 number of see outward, even an increase outward within the oldest 

 tip flush of 1939 by the sudden insertion of entire growth layers and 

 lenses. This high localization of, and rapid transition among, growth 

 layers in the Yuma area appears to be simply an exaggeration of the 



9 Personal eommunieation from C. W. Van Horn, Superintendent, Yuma 

 Farms, University of Arizona Agricultural Experiment Station. 



