Research Associate, Biology, Kansas State University, 1988-1992.
Assistant Professor, Botany, Drew University 1977-1979, 1980-1986.
Taxonomist, Jardin Botánico Nacional, Santo Domingo, Dominican Republic, 1979-1980.
Associate Curator (Bryophyta), New York Botanical Garden, 1969-1976.
Leader and participant in expeditions to arctic Alaska, Brazil, Japan.

Systematics and geography of mosses, particularly Polytrichaceae/floristics of Great Plains mosses/New Zealand Hepaticae.

Current research projects include preparation of treatments of Polytrichaceae (10 genera, 48 secies) for Volume 13 of Flora of North America (currently work is concentrated on the genus Atrichum); an outline of a revised and updated classification of the family; study of collections of Polytrichaceae from central America and elsewhere sent to me for identification; and a monographic study of the Australasian hepatic genus Telaranea (with J. Engel).

My research has been concerned chiefly with the Polytrichaceae, a family of mosses of world-wide distribution, composed of 24 genera and an estimated 300 species. The family represents an isolated group, with no close living relatives, and fossil record extending back to the Late Cretaceous (ca. 80 million years ago). The group is notable for the structural complexity of both gametophyte and sporophyte generations, and a number of features not found in any other group of bryophytes. They are the only mosses with specialized internal conducting tissues, comparable to those of simple vascular plants.