In the latter function he was responsible for preparing the Gray Herbarium Card Index. Foster was primarily interested in the Iridaceae and the flora of Bolivia and published a number of papers on those topics as Contributions of the Gray Herbarium. He did not complete his projected flora of Bolivia, but he did publish a catalogue of the ferns and flowering plants of Bolivia as a Contribution in 1958. Foster also published some short notes on historical-bibliographical problems pertaining to botany. The cards are arranged alphabetically by genus, and by species within each genus.
Most cards give name of species, author, and citation of original description.
Sometimes references to or quotes from later publications are also included, and sometimes there are additional notes, such as notes on characteristics of tribes. Most cards are handwritten, some are typed, and some have printed notes pasted on. The index covers a much broader field that the aggregate of Foster's publications on the Iridaceae. The index is arranged systematically by families, and each family section has two parts. The first part contains species cards, which give the name of the species and source of the name, and general bibliographic reference cards.