11. Callitriche heterophylla Pursh (1814: 3)

Type: — UNITED STATES OF AMERICA. PENNSYLVANIA. Aquatic herb in running water in rocky stream edge, Loyalsock Creek near Ogdonia, Hillsgrove Township, Sullivan County, ca. 800 ft., 10 June 1979, T. Plowman 7836 (neotype [designated by Lansdown & Hassemer 2021: 98] BM 010763506!).

Description (from Lansdown & Hassemer 2021):—Stem and leaf scales present. Leaf bases connate. Lingulate leaves often long and very fine, 4.1–24.7 mm long × 0.3–1.1 mm wide; expanded submerged or floating leaves obovate or oblong, often of a very characteristic shape, with broadly spathulate blade tapering very abruptly or occasionally gradually into petiole 3.7–7.0 mm long × 1.0– 3.5 mm wide, venation complex with numerous loops and anastomosing veins outside the secondary veins, apical leaves forming a floating rosette; petiole 2.5–5.2 mm long; leaves of terrestrial plants 4.1–6.2 mm long × 0.7–1.0 mm wide. Flowers usually solitary or occasionally 1 or more ♀ flowers and 1 ♂ flower in an axil. Bracts caducous, falcate or ovate, 0.1–1.3 mm long. Styles persistent or caducous, erect or spreading, ≤ 6 mm long. Filament ≤ 3.8 mm long; anthers 0.2–0.4 mm long × 0.2–0.4 mm wide, quadrilocular; pollen yellow. Fruit subsessile, not strumose, as wide as long, black when mature, 0.4–1.2 mm long × 0.4–1.2 mm wide, wing absent or only at apex, c. 0.1 mm wide.

Illustrations: —Plates 1167(4), 1168 and 1173(13) in Fassett (1951); Figures 9 (a–b) and (A) in Mason (1959); Figure 3 (a–i) in Bacigalupo (1979); Figure 1k in Lansdown and Hassemer. Fig. 10a.

Recognition: — C. heterophylla can be distinguished from all other Callitriche species in the region by the characteristic small, unwinged, rounded, isodiametric fruit which are black when mature, within the native range of the species some plants may have large fruit but these have yet to be recorded outside the Americas. Some fruit, particularly on herbarium specimens, may appear narrowly winged at the apex. These may resemble fruit of C. palustris, from which they can usually be distinguished by the fruit which are more or less isodiametric, however some herbarium material of these two species cannot reliably be determined. The shape of the rosette leaves, with an abrupt transition from the petiole to the broadly almost circular blade is also a useful character.

Distribution: —Non-native to the region. Within the region, this species has been reported from the Waikato River, where it was first collected in 1900 (Webb et al. 1988) and the Chatham Islands (de Lange et al. 1988). It was thought to be endemic to the Americas, occurring from northern Canada and Greenland south throughout much of North America, through Central America to Chile and Argentina (Lansdown 2009, Lansdown & Christenhusz 2011, Lansdown and Hassemer 2021). See maps on pages 176 and 178 in Fassett (1951). However, it occurs on Sakhalin Island (WTU357850) suggesting that it may also be native to the eastern seaboard of Russia.

Habitats and Ecology: —This species will occur in most water body types, including fast-flowing rivers and streams, backwaters and ditches, swamps, Sphagnum bogs, lakes, ponds, springs and seepages and even seasonally damp soil in shade (Lansdown 2009). Known elevation range 100–4600 m.

Conservation Status: —Least Concern (LC) (Lansdown 2016a).

Notes: —No specimens of C. heterophylla have been confirmed from the region. Given the frequent problems with mis-identification of unfamiliar Callitriche species, even by specialists, it is important that the identity of the populations from New Zealand identified as C. heterophylla is confirmed.