NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 97 



they grew various amounts of xylem which we called the Conserva- 

 tory growth layer. 



The second example included a group of seven branches cut off 

 February 21, 1942. The three weeks prior to the week of cutting had 

 been unseasonably warm and had had one slight rain. In all sections 

 taken from the branches, the cambium was in excellent condition. 

 TTC 33-8 shows no growth which could be associated with the warm 

 spell. In TTCw 1-4 the cambium had just begun activity and con- 

 stituted a zone, three to four cells thick, rich in protoplasmic con- 

 tents. In TTE 2-4 an active cambium had just set off immature 

 vessels and tracheids. The cambium in TTL 3-5 may possibly have 

 been active; that of TTM 1-5 was likewise questionable. TTP 20-17, 

 a, h, and c, shows scattered cells of postseasonal growth in addition to 

 an excellent cambial zone made up of large cells, three to four rows 

 thick radially. Only a much-expanded bud revealed the effect of warm 

 weather on TTP 24-9. 



The third example concerns specimens from citrus trees grown on 

 the Experimental Farm at Yuma, Ariz. In all branches, growth was 

 actively in progress on November 26, 1940. The presence of vessels 

 and tracheids in some branches suggests rather rapid growth. The 

 climatic regime at Yuma is, of course, different from that at Lub- 

 bock, Tex., and apparently growth occurs during the winter. Ac- 

 cording to the Superintendent of the Farm, tip growth occurs after 

 each irrigation. 



The second type of postseasonal growth — incomplete maturation of 

 the densewood of the outer growth layer — does not occur so com- 

 monly as the first type. Also, it is less easily detected. Some question 

 may be raised as to the propriety of referring to immature densewood 

 as postseasonal growth. If wall thickening and lignification occur 

 after the normal season of growth, even during the following season, 

 then we seem justified in calling the processes and the resulting ma- 

 terials postseasonal growth. The specimen TTC 30-2 was cut De- 

 cember 15, 1939. In sections a, Gy cm. from tip, the increment for 

 1939 contains one mostly sharp, complete, entire growth layer and one 

 incomplete, entire growth layer. The outer growth layer of 1939 does 

 not have a sharp outer contact, and the outer cells of the densewood 

 are partially immature in that the walls are thin and unlignified. At 

 the time of cutting, growth was not complete as shown by the im- 

 mature cells ; the growth layer itself was not complete as shown by its 

 indefinite outer contact. It may be worthwhile to point out that the 

 margins of 1937 and 1938 bear an indefiniteness quite similar to that 

 of 1939. Perhaps the circumstances surrounding the formation of the 



