Llyn Fawr is a disused reservoir, now used mostly for recreation and fishing. It is situated under the imposing north-facing walls of Craig y Llyn, near Rhigos. As a result of the prolonged hot, dry weather we've had for the last several weeks, the shores of the reservoir have receded significantly to expose large turfs of Shoreweed. From a distance it looks like a grassy lawn.
Extensive turf of Shoreweed along shore of Llyn Mawr
Male and female flowers of Shoreweed (Littorella uniflora)
The four, long stringy stamens of Shoreweed male flower
When submerged, the spongy rosettes of Shoreweed resemble those of Quillworts (e.g. Isoetes lacustris) and Water Lobelia (Lobelia dortmanna), with which it often grows. These species exhibit what is called an isoetid growth form and they share similar physiological traits too; all 3 of these species grow in Llyn Fach, the smaller lake that occurs next to Llyn Fawr under the western extension of Craig y Llyn. Unlike terrestrial plants, where the carbon dioxide for photosynthesis diffuses into the leaves from the atmosphere, isoetids obtain most of their carbon dioxide via their roots from the muddy, lake-bottom sediments in which they grow. The carbon dioxide then diffuses from the roots to the spongy, air-filled leaves which have hollow lacunae to facilitate diffusion.
Section of Shoreweed leaf showing the network of hollow lacunae
The isoetid strategy is a remarkable example of convergent evolution, where plants that belong to unrelated taxa have evolved similar morphological and physiological traits to adapt to a particular habitat or environment. For example, Water Lobelia is a member of the Bellflower family (Campanulaceae) and is not closely related to Shoreweed while Quillworts, which are related to ferns, are not even flowering plants.