260 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



But the striking feature in both a and h is the condition of the rays 

 where they pass through the cambial region. The individual cells are 

 very much enlarged, full of cytoplasm and conspicuously nucleated. 



In TTE 2-4 -a, taken 13 cm. from the tip, the mature xylem of 1941 

 is surrounded by a broad cambial zone which consists of cambium plus 

 immature tracheids and vessels just set off. Here there was definite 

 growth during midwinter. 



C. D. McGeehee, Superintendent of Lubbock City Parks, reported 

 in February 1942 that the pines, junipers, and other gymnosperms of 

 the Nineteenth Street Park had grown a tip flush during December 



1941. On the pines this amounted to more than a half inch. Our own 

 observations confirm the report. 



It seems clear, therefore, that cambial activity can be stimulated and 

 growth initiated at any time of year, even during midwinter, if condi- 

 tions become propitious. In regard to the particular period of a certain 

 intensity of growth-stimulating conditions during the winter of 1941- 



1942, a definitely specific response occurred ; the Arizona cypress, the 

 honeylocust, and the maple responded least while the pine and the elm 

 responded most. 



One point, having to do with postseasonal growth and temperature 

 effects, remains. It is of major importance because it appears to link 

 the closing phases of regular-season growth and postseasonal growth 

 during the winter. In TTC 30-2-a, 67 cm. from the tip, the outer 

 densewood of the 1937 increment shows the following sequence : (a) 

 A densewood zone sharp on its outer margin, (b) two rows of light- 

 wood cells crushed as if by frost, and (c) two to three rows of dense- 

 wood whose outer contact is slightly ruffled by offsets in alternate 

 radial columns. The increment for 1938 in its outer portion shows 

 the following: (a) The start of densewood, (b) a narrow zone of 

 flattened, unlignified cells, and (c) the remainder of the ordinary 

 densewood. Because of the sequences, as given in both 1937 and 1938, 

 the outer margins become increasingly indefinite when viewed under 

 decreasing magnification. The zones of immature and crushed cells 

 in both increments have the appearance of the effects of an autumn 

 frost which was preceded and succeeded by postseasonal growth. In 

 sections h, 46.5 cm. from the tip, the sequence of 1938 in a is repeated. 

 Sections of TTC 34-1 -a, h, and c differ from the above in lacking the 

 outermost densewood cells ; instead, the outer cells comprise the im- 

 mature zone of crushed and cupped cells with cells added here and 

 there after the crushing. Either the outermost cells of 1938 dense- 

 wood zone were left immature or else postseasonal growth was added 



