The genus Callitriche (Plantaginaceae, Callitricheae) in Australasia and Oceania Author Lansdown, Richard V. Ardeola Environmental Services, 45 The Bridle, Stroud, Gloucestershire, UK. GL 5 4 SQ. & Honorary Research Associate, Royal Botanic Gardens, Kew, Richmond, Surrey, TW 9 3 AE text Phytotaxa 2022 2022-05-24 547 3 243 284 journal article 56760 10.11646/phytotaxa.547.3.3 deb1c3fa-44cf-4574-9b18-ec5c9881e6c5 1179-3163 6577535 20. Callitriche stagnalis Scopoli (1772: 251) Type: — UNITED KINGDOM . CARDIGANSHIRE. ( Lansdown 2006b: 108 ) Aberleri Fields, Borth, Cards 22/61209160, 20 July 1998 , A.O. Chater s.n. ( neotype [designated by Lansdown 2006b: 108 ]: NMW!). Description (after Lansdown 2008 ):—Stem and leaf scales present. Leaf bases connate. Lingulate leaves very rare, usually lacking, expanded submerged or floating leaves variable, from broadly parallel-sided, through obovatespathulate to almost circular, 2.7–21.7 mm long × 1.4–8.3 mm wide; venation simple or complex with numerous loops and anastomosing veins, petiole 0.7–6.5(–7.5) mm long; the apical leaves forming a floating rosette; leaves of terrestrial plants narrowly elliptic 2.6–4.4 mm long × 1.6–3.3 mm wide, petiole 0.8–2.1 mm long. Flowers usually solitary. Bracts falcate, persistent 0.6–2.69 mm long. Styles erect, becoming recurved in fruit, ≤ 6 mm long. Filament erect, becoming recurved and continuing to grow after dehiscence, ≤ 16.2 mm long; anthers quadrilocular, 0.3–0.9 mm diameter; pollen yellow. Fruit not strumose, subsessile or occasionally very shortly-pedicellate, ± as wide as high, greyish when mature, 1.1–1.8 mm long × 1.1–2.0 mm wide, winged throughout. Illustrations: — Figures 10 (a–b) and 10A(a–e) in Mason (1959) ; Figures 1D–F in Orchard (1980) ; Figures on pages 81 and 83 in Lansdown (2008) . Fig. 10j . Recognition: — C. stagnalis can be distinguished from all other Callitriche species in the region except C. platycarpa by the large, pale, broadly-winged fruit which are greyish or pale brownish-grey when mature. It can only reliably be distinguished from C. platycarpa by the pollen which is ellipsoid, compared to the bluntly triangular pollen of C. platycarpa . C. stagnalis also rarely produces linear leaves, has a broader wing to the fruit and is typically more greyish than that of C. platycarpa , however these differences are not diagnostic. Distribution: —Non-native. Within the region Callitriche stagnalis is widespread and abundant throughout much of Victoria and coastal areas into central Queensland, as well as in the west around Perth ( Fig. 17 ). In New Zealand it has been recorded more or less throughout on the mainland, as well as on the Chatham Islands ( de Lange et al . 2011 ). It is native to Europe, where it occurs from the Azores to Iceland and east to the western side of the Ural Mountains in Russia . It also occurs as an alien in North America and South America, including the Falkland Islands ( Lansdown and Hassemer 2021 ). FIGURE 17 . The distribution of records of C. stagnalis held by AVH (inset Chatham Islands) Habitats and Ecology: —In the region, Callitriche stagnalis is the Callitriche species most frequently found growing in permanent water bodies, including rivers, streams, lakes and ponds. It has been described as the most common aquatic species of the genus in wet areas modified by human activity, such as ponds, lakes, ditches and drains ( Mason 1959 , Stanley and Ross 1986 , Webb et al . 1988 , Jeanes 1999 , Bean 2007 ). Within its native range, it occurs in poached ephemeral pools on woodland rides, heathland, wet corners of pasture, seepages, flushes, lake and river margins, and the margins of ditches. 0–1600 m elevation in its native range. There is no information available on elevation in the region. Conservation Status: —Least Concern ( Lansdown 2014b ).