Large water-starwort  (Callitriche heterophylla)

The large water-starwort has yet to be classified by the IUCN.

A small, aquatic herb with smooth, branching stems (2) (3), the large water-starwort (Callitriche heterophylla) is unusual in having two distinct types of leaves. When growing underwater, its leaves are long and narrow, with a divided tip, and reach up to two centimetres in length. In contrast, the slightly shorter leaves that develop above or float on top of the water are oval or spoon-shaped, with a narrow base and a rounded tip (2) (3) (4) (5) (6).

All of the large water-starwort’s leaves have smooth edges and grow in opposite pairs along the stem (2) (3) (7), or sometimes form a rosette at the tip (4) (5). Both types usually occur on the same stem (6), with a gradual transition between them (4).

As in other Callitriche species, the minute green flowers of the large water-starwort lack petals or sepals, and are borne in the axils of the leaves, where the leaf meets the stem of the plant (2) (3) (5) (7). Beneath each flower is a pair of whitish bracts (2) (4). The large water-starwort produces a tiny, four-lobed fruit, which is oval in shape and usually measures just over a millimetre in diameter (2) (4) (5) (7) (8).

Two subspecies of the large water-starwort are recognised: Callitriche heterophylla heterophylla and Callitriche heterophylla bolanderi (Bolander’s water-starwort) (1) (7) (9).

The large water-starwort is found in North America, where it occurs from Alaska, across most of Canada, and south throughout the United States (2) (8) (9). It is also found in Greenland (9).

The subspecies C. h. bolanderi occurs along the west coast of North America, from Alaska south through British Columbia, Alberta, Washington, Oregon and California (9).

A largely aquatic plant, the large water-starwort usually grows submerged in shallow water in ponds, pools, ditches and slow streams. Where it has become stranded on muddy shores, it will also sometimes grow in the mud (2) (3) (4) (7) (8).

The large water-starwort is an annual plant, living for just one year (2) (3) (5). It flowers in spring (9), and, as in other Callitriche species, each plant produces separate male and female flowers (2) (3) (5).

The fruits of the large water-starwort usually appear from May to September or October (2) (3) (4) (8). When mature, the fruit of this species split into four, with each section containing a single seed (2) (5) (7).

The large water-starwort is listed as a ‘Threatened’ species in Michigan and Wisconsin, in the U.S. (9). Other Callitriche species are known to be sensitive to specific pollutants (5), but very little information is available on any potential threats to the large water-starwort, and its conservation status has yet to be classified by the IUCN (10).

There are no specific conservation measures currently known to be in place for the large water-starwort.

Find out more about plant conservation in the United States:

• Center for Plant Conservation:
  http://www.centerforplantconservation.org/