NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 201 



range of the 3.2-cm. terminal bud. They contain the following: i see 

 pre-1941 and I see plus i dee for 1941. The xylem of pre-1941 con- 

 stitutes one-third of the total xylem present. It does not seem possible 

 that the pre-1941 growth layer could be considered the first growth 

 layer of a multiple 1941 increment nor could it be considered to come 

 within the range of what is normally held to be a bona fide tip flush 

 from terminal bud scale scar to terminal bud scale scar. The similarity 

 and identity of position of the dee in sections a and b positively date 

 it as 1941. Add to this the presence of pre-1941 in sections a and the 

 conclusion seems justified that it was xylem in the terminal bud of 

 1940 formed during the season of 1940, even though at cutting date it 

 fell within the range of 1941 tip growth as delimited by terminal bud 

 scale scars and as measured. This is, of course, projecting the lessons 

 of TTP 20-3-c backward a year in time. 



Thus we have the experience of finding a previous year's growth 

 layer within the compass of the succeeding year's tip growth. It gives 

 multiplicity somewhat different from that heretofore illustrated. 



The situation found in TTP 22-1 is repeated in branches 2, 3, 4, and 

 6. In two of them the outer margin of the "1940" or pre-1941 growth 

 layer was somewhat indefinite, as in TTP 20-3-c. The following cases 

 of xylem related to terminal buds have been encountered thus far in 

 our work: 



1. Densewood reaches to the base of the terminal bud, and lightwood falls short 



of it. 



2. Densewood only extends into the terminal bud. 



3. Both lightwood and densewood extend into the terminal bud. 



Secondary xylem fails to reach base of terminal bud. — A marked 

 contrast to the phenomenon described above existed in TTJ 4-3 whose 

 tip growth was measured January 30, 1943, February 5, 1944, and 

 July 27, 1944. On the last date, sections were cut. Sections a were 

 taken 3 cm. out from the base of 1943 tip growth and contained i see 

 for 1943 and i inc for 1944. Although 1943 tip growth measured 13.4 

 cm., and 1944 measured 8.9 cm., the growth layer, measured at a cross 

 section 3 cm. out from the base of 1943, showed 1943 to possess a 

 width only one-sixth as great as that for 1944. The longer tip growth 

 of 1943 had less diameter growth than the shorter tip growth of 1944. 

 Sections b, taken 3 cm. inward from the outer end of 1943 and there- 

 fore necessarily sectioning growth of 1943 and 1944, actually con- 

 tained primary xylem only for 1943 and i inc for 1944. In cross sec- 

 tion, the primary xylem appeared as detailed patches. Based on the 

 small amount of secondary xylem for 1943 in sections a near the base 



