ANATOMY. 



701 



This closure of resin-ducts, when the wood becomes con- 

 verted into heartwood, is of special importance for fixing the 

 quantity of resin that is removed from a tree by resin- 

 tapping, and for adjudging the effects of this practice on the 

 quality of the wood. 



Vertical resin-ducts do not exhibit, as has been maintained 

 frequently, an uninterrupted course through the stem ; 

 according to Mayr, the longest resin-duct in spruce does not 

 exceed 0*7 m. in length at the base of the tree, nor 0*4 m. in 

 its upper portion. For larch, 

 the corresponding figures are 

 0-3 m. and 0'15 m. The 

 shortest ducts are near the 

 branches ; the central course 

 of a duct is deeper than its 

 two ends, in the annual zone, 

 the ends being formed later 

 than the central part of the 

 duct. Many ducts end with 

 the annual zone close to the 

 cambium, but without continu- 

 ing in the following year in 

 the new zone of wood. Verti- 

 cal ducts, that adjoin one 

 another, place their lumina in 

 communication by means of a 

 group of parenchymatous cells, 

 with adjoining thin walls. The 

 number of vertical ducts in- 

 creases slowly with age, on certain cross-sections of the 

 wood; the south side of a tree contains more ducts than 

 the north side. 



Horizontal resin-ducts are always narrower than vertical 

 ducts ; they lie in the middle of medullary rays and continue 

 in them into the bast, where they end abruptly. Their 

 number is considerable ; in spruce, there are 50 to 110 ducts 

 on a square centimeter, but there are fewer ducts in the 

 central parts of a stem than above or below it. During the 

 season of rest, a horizonal duct is separated by the compact 



Kijr. :*71. Cross-section through a 

 vertic.il duct, which is sur- 

 rounded by the expansion of a 

 cell (a), b Thick-walled paivn- 

 chymatous cells, c c Cells of 

 medullary rays containing 

 starch and drops of turpentine. 



