Plate 17 



Fig. I. TTP 2S-2-a. Branch section of Piiiiis pondcrosa, cut November 29, 1941. 



Xylem of 1937-1941. Proved multiplicity; irregular margin under cambium; 



lensing; psg. 



1937: see. 



1938: Slight natural frost effects; see. 



1939: nisce. 



1940: Effects of natural frost; msce. 



1941 : see; irregular, inc. lenses, as psg, outer tangential walls of outer 

 xylem cells not lignified. Compare with figure 2, below, and figure i, 

 plate 19. 

 Fig. 2. TTP 2^^-4-c. Branch section of Piiiiis pondcrosa, cut November 29, 1941. 



Xylem of 1938-1941. Atypical gls ; weak dw ; weak annual. 



1938 : see with near normal dw. 



1939: Single gl with dw so weak as to be nearly nonexistent. 



1940: 3-4 cells thick radially, some radial columns have normal sequence of 

 Iw and dw, others all dw : rest all Iw (inward on branch 1940 re- 

 duced to ddw of 1939 in one or two places and a single row of dw 

 cells elsewhere) : under low powers 1940 invisible or present as 

 tiny lenses. 



1941 : inc. gl ; very irregular margin ; cambium killed by artificial freezing of 

 Alay 25, I 941. 



Compare with figure i, above, and figure i, plate 19. 



