Introduction

Isoetes (Isoetaceae) is a genus of small herbaceous plants often aligned with Lycopodium and Selaginella. There are more than 150 species distributed worldwide, typically in aquatic habitats (Tryon & Tryon 1982). A particularly intriguing aspect of the physiology of these plants is the presence of Crassulacean Acid Metabolism (CAM) (Keeley 1981, 1982), a photosynthetic pathway commonly associated with terrestrial xerophytes. CAM was selected for in these species by the daytime carbon limitation characteristic of their oligotrophic aquatic habitats (Keeley & Busch 1984; Boston & Adams 1985).

Across its range, Isoetes has radiated into a variety of aquatic as well as some terrestrial habitats and these environments have selected for a number of different structural-functional syndromes (Keeley 1987). Aquatic species occur in lacustrine habitats where they are permanently submerged throughout their life cycle and in amphibious environments where they alternate seasonally between aquatic and terrestrial conditions. In general, all aquatic species so far tested possess a well-developed CAM pathway while under water but lose this pathway when grown in an aerial environment. True terrestrial species of Isoetes are few in number although such species are known from most parts of the world. They readily fall into one of two groups: vernally active, summer-deciduous species at relatively low elevations in temperate latitudes; and evergreen species restricted to very high elevations (> 3500 m) in tropical latitudes. The former species show no CAM activity (even if artificially submerged), possess stomata and presumably depend entirely on C3 photosynthesis.