The Resin Sac Pattern in the Needles of Engelmann Spruce and Blue Spruce from Southeastern Wyoming

Abstract

The difference in the number of resin ducts in a cross-section of the needle has been considered by several authorities (Sargent, 1898; Sudworth, 1916; Preston, 1940) to be a reliable specific characteristic for distinguishing between Engelmann spruce (Picea Engelmanni (Parry) Engelm.) and blue spruce (Picea pungens Engelm.). Others have been more cautious concerning the significance of this characteristic. Sharp (1915) indicated that he had experienced difficulty in procuring sections of the needles of either of these species which showed resin ducts. Durrell (1917) avoided the use of the numbers of resin ducts as positive criteria of differences between these two species, but stated that one or both ducts may be missing from Engelmann spruce needles, whereas blue spruce usually has two. Both Sargent (1933) and Harlow and Harrar (1941) refrain from using the resin ducts of the needles in their treatment of these species of spruce. In 1931 Marco seemed to have settled the situation with respect to numbers of resin cavities in the needles of these two spruces. He made two major contributions: (1) that the resin-containing units were not of the usual type found in the genus Pinus, but rather a longitudinal series of short cysts or sacs separated from each other by transverse partitions of mesophyll; and (2) that both Engelmann spruce and blue spruce contain two of these longitudinal series of sacs in the base of their needles, but only in the latter does either one or both of these series extend into the upper half of the leaf. Such cysts or sacs were recognized by PenhalIow (1907) as characteristic of the wood of Picea as well as of that of five other coniferous genera. He does not mention the anatomy of the leaves. The terms cyst, sac, resin passage, and resin duct have all been used with reference to the cavities surrounded by resin-secreting cells in the conifers. We have chosen to refer to these units in spruce needles as resin sacs, because the term seems least ambiguous and conveys the impression of a relatively short, discontinuous unit in contrast with the long, continuous type of resin duct in the needles of the pines. In view of our field experience which seemed at variance with the statements on the subject in the literature, and because Preston (1940) continued the use of the number of resin ducts as a characteristic having taxonomic value in separating these two species of spruce, even after Marco's contribution some nine years earlier, we were motivated to investigate the situation from material collected in Albany County near Laramie, Wyoming.