NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 49 



The Arizona cypress, TTC 30, gives an interesting situation because 

 it grew in a watered flower bed whose soil moisture undoubtedly did 

 not drop below the wilting coefficient. In the densewood of 1939 in 

 TTC 30-1 (cut November 4, 1939), the outer cells narrow somewhat 

 (pi. II, fig. 3) but are followed outward by wider, thinner-walled, 

 green-stained cells. The transition is so gradual that the complete 

 growth layer is highly diffuse. The outer cells, postseasonal growth 

 assuredly, suggest that growth had occurred right up to the time of 

 cutting and that perhaps a second growth layer was being deposited 

 by reason of a warm autumn and constant irrigation. As a matter of 

 fact, 1939 in TTC 30-2 (cut December 15, 1939) does show two 

 growth layers but also an amount of postseasonal growth equal to 

 that of TTC 30-1. The inner growth layer has a sharp outer contact. 

 The densewood of the outer growth layer grades outward into the 

 postseasonal growth which is like that of TTC 30-1 and thus has a 

 diffuse margin. Growth was incomplete in the sense that the cambium 

 had just divided and that the outermost cells did not mature; the 

 growth layer is incomplete in the sense that densewood was not norm- 

 ally developed and that the contact under the cambium was not sharp. 

 Since the outer margins of 1937 and 1938 are also indefinite, it may 

 well be that the situation in 1939 explains the margins of the former 

 years. These specimens, as well as many others, indicate that differ- 

 entiation and maturation do not necessarily have to be completed dur- 

 ing the season when the cells were set off. 



The loblolly pine, Con T 2 (pi. 2, figs, i and 2), is of particular in- 

 terest, first, because it was moved from the Texas Agricultural Ex- 

 periment Substation east of Lubbock to the Conservatory of Texas 

 Technological College on November 16, 1939, and back outdoors to 

 the grounds of a private residence on April 24, 1940, and second, be- 

 cause the sections came from the trunk. These sections were taken at 

 20 different levels from near the soil up along the trunk for a span of 

 240 cm. to the growing tip of the leader. In the lowest sections the 

 outer contact of the growth layer for 1940 was diffuse, and within the 

 span of the next 90 cm. the outer contacts of both 1939 and 1940 

 varied from diffuse to definite longitudinally as well as around the 

 circuit. The margin of 1940 was wholly sharp around the circuit of 

 sections taken 161 cm. up from the basal sections or 78.3 cm. down 

 from the growing tip of May 18, 1944. This brought the sharp mar- 

 gin within the length of tip growth formed during 1940. Xylem 

 whose appearance would classify it as showing a reverse sequence 

 gives a diffuse margin to 1939 in the sections cut 10 cm. above the 

 basal series where the trunk had been frozen artificially. The first 



