Callitriche, von Linne (1753: 969), von Linne, 1753

Lansdown, Richard V., 2022, The genus Callitriche (Plantaginaceae, Callitricheae) in Australasia and Oceania, Phytotaxa 547 (3), pp. 243-284 : 275-276

Key to the taxa occurring in the region

Many of the species treated here are allopatric and identification could therefore theoretically be based in part on location. However, the number of non-native species known from or suspected to occur in this and other regions, combined with their distribution, show that the possibility of discovering a taxon new to a particular geographical area is high. This key is therefore designed to enable identification of any of the species suspected to occur in the region independent of locality. An indication of the known distribution of restricted-range species is given to aid decisionmaking. C. muelleri is the only species which can be reliably identified without flowers or fruit; it is the only species in the genus with toothed leaves.

1. Leaves with a single tooth on each margin.................................................................................................................. 13. C. muelleri

- Leaves entire.......................................................................................................................................................................................2

2. Pollen colourless; style reflexed........................................................................................................................................ 4. C. brutia

- Pollen yellow; style erect or spreading...............................................................................................................................................3

3. Fruit unwinged or only partially winged............................................................................................................................................4

- Fruit winged throughout ...................................................................................................................................................................10

4. Fruit winged at apex, occasionally with narrow wing tapering down the upper parts of the sides ( C. heterophylla fruit may occasionally have a narrow wing at the apex of the fruit. In fresh material, it can be distinguished from C. palustris by the isodiametric fruit, those of C. palustris are longer than wide, however some herbarium material cannot reliably be determined) ... .....................................................................................................................................................................................14. C. palustris

- Fruit unwinged....................................................................................................................................................................................5

5. Vegetative parts fleshy and almost succulent; lower leaves reduced to scarious scales ............................................ 1. C. antarctica

- Vegetative parts not fleshy; leaves either ± homophyllous or lower leaves lingulate, well-formed and green .................................6

6. Fruit blackish when mature ................................................................................................................................................................7

- Fruit pale or golden-brown when mature ...........................................................................................................................................9

7. Fruit> 1 mm high and wide (known only from New Guinea)................................................................................... 10. C. fuliginea

- Fruit ≤ 1 mm high and wide ................................................................................................................................................................8

8. Fruit strumose, wider than high; most axils with both ♀ and ♂ flowers ...................................................................16. C. peploides

- Fruit convex, not strumose, isodiametric; flowers usually solitary (reported in the region only from a single river in New Zealand and the Chatham Islands) ..................................................................................................................................... 11. C. heterophylla

9. Plant dioecious (mainland New Zealand)........................................................................................................................17. C. petriei

- Plant monoecious (known only from the Chatham Islands) ................................................................................ 6. C. chathamensis

10. Mature fruit maroon, chestnut, grey, greyish-brown or golden-brown ............................................................................................11

- Mature fruit blackish ........................................................................................................................................................................14

11. Wing very broad, c. ½ width of fruit; mature fruit maroon/chestnut in colour (known only from New Guinea) ..... 8. C. cycloptera

- Wing narrower, ≤ ¼ width of fruit; mature fruit grey, greyish or golden-brown .............................................................................12

12. Both ♀ and ♂ flowers in most axils; fruit <1 mm high and ≤ 1 mm wide (known only from Rapa and Tauturau Islands, French Polynesia) ...................................................................................................................................................................12. C. insularis

- Flowers solitary; fruit> 1.1 mm high and wide ................................................................................................................................13

13. Pollen ellipsoid; mature fruit greyish, broadly winged; lingulate leaves very rare.................................................... 20. C. stagnalis

- Pollen bluntly triangular mature fruit brownish; lingulate leaves frequent (reported but not confirmed from a single river in New Zealand)....................................................................................................................................................................18. C. platycarpa

14. Fruit higher than wide; flowers solitary ...........................................................................................................................................15

- Fruit ± isodiametric or wider than high; both ♀ and ♂ flowers at least in upper axils ....................................................................16

15. Bracts ≥ 1.5 mm long; fruit not strumose; wing narrow (known only from New Guinea)..........................................15. C. papuana

- Bracts ≤ 1 mm long; fruit strumose; wing broad (south-eastern Australia) .............................................................. 21. C. umbonata

16. Anthers trilocular; fruit 0.5–0.6 mm high ........................................................................................................................................17

- Anthers quadrilocular; fruit> 0.7 mm high.......................................................................................................................................18

17. Mature fruit face flat and strumose; wing narrow and poorly defined (eastern Australia) ...........................................19. C. sonderi

- Mature fruit face convex, not strumose; wing well-defined (south-eastern Australia) ..............................................5. C. capricorni

18. Fruit ± isodiametric (south-eastern Australia) ........................................................................................................... 7. C. cyclocarpa

- Fruit wider than high ........................................................................................................................................................................19

19. Most or all fruit long-pedicellate..................................................................................................................................... 9. C. deflexa

- Fruit sessile.......................................................................................................................................................................................20

20. Leaves fleshy; flowers mainly solitary, occasionally both ♀ and ♂ flowers in one axil (known only from the Auckland Islands ) ... .................................................................................................................................................................................2. C. aucklandica

- Leaves not fleshy; most axils with both ♀ and ♂ flowers (south-eastern Australia) ............................................. 3. C. brachycarpa