410 DR. P. GROOM AND MR. W. RUSHTON ON THE STRUCTURE 
A. Macroscopic FEATURES (naked eye). 
The heart-wood is light brown. The annual rings were 17 to the inch in 
our specimen, 13 in Stewart’s specimen (Gamble). The transition from the 
spring-wood to summer-wood in the same annual ring is gradual. The 
relative proportions of the spring-wood and summer-wood varied widely in 
different annual rings: the latter forming from 1 to more than 4 of the ring. 
The medullary rays are invisible in transverse section but show as thin darker 
bands on the radial section. The resin-ducts, just visible in transverse section, 
are noticeable on the longitudinal section as dark lines; their distribution in 
the annual ring is uneven, as they are scattered and often locally seanty, 
though generally more numerous in the later spring-wood and early summer- 
wood, they are not lacking from the inner and outer boundaries. 
D. Microscoric STRUCTURE. 
Tracheids. 
In transverse section the spring-tracheids are square or oblong-rectangular, 
with the slightly longer axis radial or tangential. The gradual decrease in 
radial diameter and in thickness of wall in the summer-wood culminates in 
the outermost tracheids, which are very compressed thick-walled fibro- 
tracheids. 
The following are measurements of typical tracheids :— 
| | Width of Lumen. Thickness of Wall. 
| — E ———| 
| Radial. | Tangential. Radial. | Tangential. |. 
| | 
| Inner spring-tracheid ...... 94 p | 29 u OO pe S5pu 
‚ Qutermost summer-tracheid. . ён | 21р би Rp 
| 
| 
| 
Length of tracheid 3 mm. 
The tracheids show three sets of deviations from normal shape as regards 
their ends :— 
1. Where they terminate against a medullary ray the straight end may 
be rectangular-truncate or forked, but the end may bend radially inward. 
Bordered pits occur on the bent part or truncate end. 
2. Here and there the tracheids bend radially inwards at their ends and 
dovetail, and on the dove-tailing terminal portions there are pits on the 
tangential walls in the spring-wood. 
3. In the vicinity of resin-duets rectangular parenchyma-tracheids occur. 
