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).
- Also known as
- Bolander’s water-starwort, differentleaf waterstarwort, diverse-leaved water-starwort, greater water starwort, large water starwort, larger waterstarwort, twoheaded waterstarwort, twoheaded water-starwort, variedleaf waterstarwort.
- Callitriche anceps, Callitriche bolanderi.
- Stem length: up to 20 cm (2) (3)
Large water-starwort biology
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).
Large water-starwort range
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).
Large water-starwort habitat
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).
Large water-starwort status
The large water-starwort has yet to be classified by the IUCN.
Large water-starwort threats
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).
Large water-starwort conservation
There are no specific conservation measures currently known to be in place for the large water-starwort.
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- Lives or grows for just one year.
- Modified leaf at the base of a flower.
- A small, non-woody, seed bearing plant in which all the aerial parts die back at the end of each growing season.
- A floral leaf (collectively comprising the calyx of the flower) that forms the protective outer layer of a flower bud.
- A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
Integrated Taxonomic Information System (ITIS) (July, 2011)
Mohlenbrock, R.H. (2008) Aquatic and Standing Water Plants of the Central Midwest. Acanthaceae to Myricaceae: Water Willows to Wax Myrtles. Southern Illinois University Press, Carbondale, Illinois.
Yatskievych, K. (2000) Field Guide to Indiana Wildflowers. Indiana University Press, Bloomington, Indiana.
Northern Prairie Wildlife Research Center: Aquatic and Wetland Vascular Plants of the Northern Great Plains - Water starwort, Callitriche heterophylla (July, 2011)
Heywood, V.H. (1978) Flowering Plants of the World. Oxford University Press, Oxford.
Deschamp, P.A. and Cooke, T.J. (1985) Leaf dimorphism in the aquatic angiosperm Callitriche heterophylla. American Journal of Botany, 72(9): 1377-1387.
Kozloff, E.N. (2005) Plants of Western Oregon, Washington & British Columbia. Timber Press, Portland, Oregon.
Britton, N.L. and Brown, A. (1970) An Illustrated Flora of the Northern United States and Canada. Volume II (Amaranth to Polypremum). Dover Publications, New York.
USDA PLANTS Database - Twoheaded water-starwort, Callitriche heterophylla (July, 2011)
IUCN Red List (July, 2011)