It's at the end of the abstract: "photographs of the living plants". I found it frustrating to read because of that missing detail too, but I expect it would be obvious to the intended readers.
It would be, it's right there in the title: Aristolochiaceae. From Wikipedia: "The Aristolochiaceae are a family, the birthwort family, of flowering plants with seven genera and about 400 known species[...]"