Salix planifolia ssp. tyrrellii, from the Athabasca sand dunes, has been considered to be a recent derivative of the widespread S. planifolia ssp. planifolia. It is endemic to the Athabasca sand dunes of northern Saskatchewan and was documented recently from the Maybelle River and Richardson dunes in Alberta. Salix planifolia ssp. tyrrellii and its putative progenitor are similar in appearance and can be difficult to distinguish. A diagnostic feature of S. planifolia ssp. tyrrellii is the presence of adaxial stomata. In specimens of S. planifolia ssp. tyrrellii from each of the Athabasca sand dunes in northern Alberta, adaxial and abaxial stomatal frequencies were examined. In addition, vessel element (VE) characters were evaluated, including VE density, VE lumen diameter, VE length, and VE clustering. Stomatal frequencies were similar in specimens from each Alberta dune, but some VE characters were variable. These structural data were compared to those of S. planifolia ssp. tyrrellii from the Athabasca sand dunes in Saskatchewan. Similarities among values of adaxial stomatal frequency between specimens from Alberta and Saskatchewan suggest adaptation to this high light intensity environment. Overall, structural similarities described in this study provide additional support for the close taxonomic relationship of this endemic-progenitor Salix pair.

Key words: Athabasca sand dunes, Salix, stomata, structure, vessel element, willow