Shoreweed

Shoreweed (Littorella uniflora) is an interesting, but rather nondescript amphibious plant. It usually occurs submerged along the shallow margins of reservoirs, lakes and pools and it is easy to miss or overlook. It is probably under recorded in South Wales. However, during prolonged dry periods in summer lakes and pools dry up and their margins recede. When this happens, large populations of Shoreweed may be revealed. Summers like the one we are experiencing this year are good times to look for this plant.
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

Shoreweed doesn't flower when it is submerged, but it produces stolons that give rise to rosettes of leaves and allow the plant to build dense colonies by vegetative means. Out of water each plant may produce a single male flower and one or a few female flowers (i.e. it is monoecious). The male flower, which produce 4 stamens with long, stringy filaments, is fairly obvious. The female flowers are produced lower down on the stem (see photos below). Shoreweed is a member of the Plantain family (Plantaginaceae) and like the more common plantains it is wind pollinated.

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.