The Lycopodiales of the British Isles (Isoetes, Lycopodium, Selaginella)


L. Watson and M. J. Dallwitz

Isoetes lacustris L.


Morphology. Stems short and tuberous, with sheathing leaves crowded in dense rosettes; with anomalous secondary thickening. The old leaf bases not persistent. Leaves ligulate; 80–200(–450) mm long (x 2–3mm, stiff, subulate, subterete, with four longitudinal septate tubes, without stomata); spreading.

Heterosporous. Sporophylls resembling the foliage leaves; in fertile zones tending to alternate with sterile zones along the stems, rather than in well defined terminal cones (the earliest leaves of the annual increment each with a basal, sunken megasporangium containing several hundred megaspores, these followed by leaves with one containing up to a million microspores, and the last-produced leaves sterile). The sporangia very large, transversely and longitudinally septate and embedded in the leaf bases. The megaspores covered with short, blunt tubercles.

Ecology and distribution. Aquatic. Upland and montane (in lakes and tarns with water poor in dissolved minerals). Wales, Shropshire, S.E. Yorkshire, Lake District, Scotland from Perth Northwards to Shetland, in Ireland mainly western.

Classification. Family Isoetaceae.

Ilustrations. • I. lacustris: Sowerby and Johnson (1859). • I. lacustris: Eng. Bot. 1826 (1886). • The British species of Isoetes (Sowerby and Johnson). • Isoetes moorei: as I. lacustris var. moorei, Eng. Bot. 1826 (1886). • Isoetes spp., morphological details.

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2007 onwards. The Lycopodiales of the British Isles (Isoetes, Lycopodium, Selaginella). Version: 1st September 2009.’.