NO, I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 95 



is impossible to foretell if added growth found during the summer 

 would have gone on to completion. Evidence does exist for the addi- 

 tion of added xylem after the main flush is completed and before the 

 arrival of winter. Therefore, descriptions are based chiefly upon sec- 

 tions cut during the winter. 



Postseasonal growth may be divided into three types : ( i ) Tra- 

 cheids or vessels added outside the normal and sharply bounded outer 

 growth layer — that is, isolated cells or groups of cells (pis. lo; ii, 

 figs. I and 2; 17, fig. I ; 19, fig. i ; 30, fig. 2; 32) ; (2) incomplete 

 maturation of the densewood of the outer growth layer (pis. 16, fig. 

 2; ly, fig. I ; 19, fig. I ; 33); and (3) transition outward of the 

 outer densewood into cells approaching the character of lightwood 

 (pis. II, fig. 3; 12, fig. i). 



The first type may exist as isolated individual cells, as patches or 

 crude lenses, or as distinct but incomplete lenses and entire growth 

 layers. In the juniper, TTJ i-i, cut January 11, 1940, sections a, 80 

 cm. from the tip, carry scattered single immature cells under the cam- 

 bium. Sections h, at 61 cm., and c, at 44.5 cm., duplicate the situation 

 in a. At 24 cm. from the tip, sections d show the merest hint of added 

 xylem with one or two very immature cells. Specimen TTJ 2-1 

 practically repeats TTJ i-i. 



During 1940 a series of branches was cut from the Arizona cypress, 

 TTC 5, at approximately 2-week intervals. No extra xylem had been 

 formed by July 17 when specimen TTC 5-4 was taken. In sections h 

 of TTC 5-5, cut August I, one or two cells had appeared. In TTC 

 5-6-a, cut August 17, postseasonal growth consists of a 100° incom- 

 plete lens one to two cells thick. Sections h show the long lens of a 

 broken down into a series of short, thin, incomplete lenses. In TTC 

 5-7-a, cut September 10, a cell or two gives the merest hint of post- 

 seasonal growth. Sections h, 33 cm. closer to the tip, show two small 

 lenses, one cell thick, of immature cells. In TTC 5-8-a, cut September 

 28, ^2 cm. from the tip, extra xylem exists as scattered cells and one 

 or two short lenses of immature cells. Sections h, 14 cm. from the tip, 

 show a cell or two of added xylem. TTC 5-9, cut November 9 and 

 showing added xylem, indicates that the postseasonal growth which 

 began to be formed soon after July 17 actually was put down as, and 

 remained, postseasonal growth. 



Many examples exist of added xylem present in midsummer, but 

 without sections taken at stated time intervals, no information exists 

 as to whether or not the added growth would have remained such or 

 gone to completion. Practically all our evidence indicates that diam- 

 eter growth is essentially completed by mid-July ; anything present on 



