28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



results were so striking as to be ample remuneration for the work en- 

 tailed. These results will be detailed in succeeding chapters. In addi- 

 tion to the purely anatomical results so far achieved, the method of 

 artificial freezing has a wide field of application in dating precisely 

 the time of formation of annual, as well as intra-annual, growth 

 layers. Furthermore, the method can be extended from small 

 branches, such as we have used, to limbs and to trunks of trees that 

 are too large to lend themselves to tip-growth measurements or to the 

 effects of natural frosts. 



TIP-GROWTH MEASUREMENTS 



These measurements, in some cases weekly, in others yearly, were 

 used as a third means for the absolute dating of the diameter flushes 

 within that part of the branch which had grown during the measure- 

 ments. The existence of two or more growth layers in the tip growth 

 measured for one year gave clear proof of multiplicity. Correct loca- 

 tions on the branches were maintained by the methods described pre- 

 viously and gave certainty even in those instances where bud scale 

 scars were not developed. In the case of yearly measurements, they 

 were made during the winter for one to several years. Weekly or 

 periodic measurements were begun well before the start of the grow- 

 ing season and were terminated at least a year later. Obviously, sec- 

 tions taken from the branch tip grown during the measured intervals 

 could contain only the diameter flushes grown during the same 

 intervals. 



Annual measurements of tip growth were made on the following 

 species in the Lubbock area : Conifers — ponderosa pine (4 trees, 81 

 branches), loblolly pine (2 trees, 35 branches), a short-leafed yellow 

 pine (MP i; i tree, 3 branches), another short-leafed pine (TTP 

 22; I tree, 7 branches), Arizona cypress (6 trees, 36 branches), and 

 red cedar (3 trees, 11 branches). Dicotyledons — apple (2 trees, 5 

 branches), cottonwood (i tree, 7 branches), Siberian elm (i tree, 6 

 branches), honey locust (3 trees, 15 branches), and silver maple (2 

 trees, 12 branches). At Washington, D. C, annual measurements 

 were made on the following species of young fruit trees: Apple (i 

 tree, 2 branches), cherry (2 trees, 8 branches), peach (i tree, 11 

 branches), and plum (i tree, 2 branches). Through the kindness of 

 Mr. Van Horn, we were able to obtain grapefruit cuttings from 

 Yuma, Ariz., which represented the tip growth for specific years; 

 these included 15 branches probably from four different trees. The 

 total of annual measurements thus included 256 branches from 35 



