The three taxa of Acaena occuring in the Wellington area were studied primarily by means of progeny tests, supplemented by field observations, determinations of pollen fertility, germination rates and chromosome numbers.
Acaena novae-zelandiae and A. anserinifolia were widespread in disturbed habitats, and the latter particularly was represented by many forms. Acaena novaezelandiae var. pallida‡ was restricted to coastal sand dunes and was the most uniform of the three taxa. Chromosome counts were obtained for forms of all taxa, and in each case 2n = 42.
The plants comprising several populations were found to have pollen of low fertility and seed of a low germination rate, which in most cases produced some seedlings of low viability. The surviving plants showed marked segregation. It is suggested that these plants were interspecific hybrids and in each case their general similarity to one another, and their low pollen fertility by comparison with that of their progeny, leads to the further hypothesis that they all belong to the F1 generation.
The suspected hybrid progeny were analysed (in one suitable case Anderson's Hybrid Index method was applied) and the data obtained indicated that two types of interspecific cross occur:
(1) Crosses between Acaena anserinifolia and A. novae-zelandiae.
(2) Crosses between A. anserinifolia and A. novae-zelandiae var. pallida.
In view of its comparative abundance and early flowering habit combined with marked proterogyny in all species, it is suspected that Acaena anserinifolia generally acts as the male parent in such cases of hybridism.
Assuming that only F1 hybrids establish themselves in nature, it is concluded that any breakdown of specific boundaries is unlikely to be rapid.
Acaena belongs to the family Rosaceae, sub-family Rosoideae. Within this sub-family it is included in the tribe Sanguisorbae with the familiar, herbaceous northern genera Alchemilla, Agrimonia, Poterium and Sanguisorba, and with the less familiar woody genera—Polylepis (tropical South America), Bencomia (Canary Islands) and Cliffortia (South Africa). Vegetatively it is very similar to both Sanguisorba and Agrimonia.
Estimates of the number of species in the genus have varied from a few dozen to over a hundred. The actual number is probably somewhere between these extremes. Of these, only a handful are found in the northern hemisphere—1 in California; 1 in Mexico and 1 in Hawaii, as well as a few others which extend across the equator in South America. In the southern hemisphere the majority of species (perhaps 50) are found in South America. The remaining 17 or so are distributed as follows. New Zealand, 14 (1 of these extends to Australia); Australia and Tasmania, 3; South Africa, 1; and 1 or 2 in the isolated islands of Tristan da Cunha, Kerguelen and Macquarie.
[Footnote] ‡ In the author's opinion this is quite distinct from A novae-zelandiae, and should be accorded specific status.
The investigation described in the present paper concerned natural hybrids between three lowland species of Acaena in New Zealand—A. anserinifolia (Forst.) Druce (syn. A. sanguisorbae Vahl), A. novae-zelandiae Kirk and A. novae-zelandiae var. pallida Kirk. A generalised description applicable to these three taxa will be followed by a tabulated comparison designed to illustrate their chief differences.
Herbaceous perennials with slender, creeping stems rooting at the nodes; leaves alternate, stipulate, pinnately compound; leaflet margins serrate; stems and abaxial surfaces of the leaves distinctly hairy; peduncles terminal, erect, leafless in the upper part; flowers 100 or more, sessile, crowded into a perfectly spherical, terminal inflorescence; receptacle perigynous, constricted distally; calyx four-lobed, green, persistent; petals absent; stamens 2, filaments exserted, anthers shorter than broad; carpel solitary with a single ovule, style short, stigma markedly feathery, receptive before anther dehiscence; mature fruiting calyx with 4 ribs, prolonged distally into 4 slender, barb-tipped spines exceeding the fruit in length.
The barbed spines provide an efficient means of attachment to the coats of animals. Diploid chromosome number 42 in all three taxa.
|A. anserinifolia||A. novae-zelandiae||A. novae-zelandiae var. pallida|
|Widely distributed and quite common throughout the study area.||Widely distributed but more occasional than A. anserinifolia.||Almost entirely restricted to coastal sand dunes where it may be abundant.|
|Occupies open situations or semi-shade.||Always in open habitats. Will grow under drier conditions than A. anserinifolia.||As above.|
|Cotyledon blade 2–3 mm long.||Cotyledon blade 4–5 mm long.||Cotyledon blade about 6 mm long.|
|First leaf simple.||First leaf trifoliate.||First leaf trifoliate.|
|Seedling growth rapid. Prostrate habit quickly assumed.||Seedling growth rapid. Prostrate habit quickly assumed.||Seedling growth slow. Prostrate habit assumed more slowly.|
|Spreading growth habit. Seedling laterals vigorous.||Trailing growth habit. Seedling Seedling laterals weak or absent.||Trailing growth habit. Seedling laterals absent.|
|Stem diameter 1.5–2 mm at 7th internode.||Stem diameter 2–2.5 mm.||Stem diameter 3.5 mm.|
|Length of terminal leaflet 6–11 mm. Generally 5 pairs of leaflets.||Length of terminal leaflet 11–15 mm. Generally 6 pairs of leaflets.||Length of terminal leaflet about 20 mm. Generally 6 pairs of leaflets.|
|Adaxial surfaces of lower leaflets, at least, more or less pilose. Hairs at serration tips distinctly fascicled.||No hairs on the adaxial leaf surface. Hairs at serration tips sparse, not fascicled.||No upper surface hairs serration tips hairless.|
|Adaxial leaf surface dull. Thin cuticle present.||Adaxial leaf surface shining. Moderately thick cuticle present.||Adaxial leaf surface shining. Thick, wrinkled cuticle present.|
|General foliage colour palegreen, yellow-green or brown-green.||General foliage colour dark-green.||General foliage colour dark-green.|
|Secondary colouration generally pale brown on stem, red-brown on adaxial leaf surface, purple on adaxial surface and brown to reddish on the spines. Hydathodes pink.||Secondary colouration bright-red on the stems and petiole and crimson on the spines. Hydathodes crimson.||Secondary colouration pale-red on stems (except peduncle) and spines. Hydathode white.|
Fig. 1.—Comparable adult leaves from forms of the three species. a–b, Acaena novae-zelandiae.
var. pallida (both Lyall Bay–Seatoun; b, an anomalous form). c–h, Acaena anserinifolia (c, Wainui; d, Southern Tararuas; e, Wainui Reservoir; f, Brooklyn; g, South Karori Stream; h, Prince of Wales Park) i-l, Acaena novae-zelandiae (i, South Karori Stream; j, Wainui; k, Prince of Wales Park; l, Eastbourne). Fig. 2—Lyall Bay-Seatoun Garden Plants. Comparable leaves from one probable parent and progeny of hybrid. A. Acaena novae-zelandiae
var. pallida. Remainder from hybrid progeny.
Fig. 3.—Prince of Wales Park. Garden plants. Comparable leaves from probable parents and progeny of hybrid. A, Acaena novae-zelandiae; B, Acaena anserinifolia. Remainder from progeny of hybrid. Fig. 4.—Prince of Wales Park. Glasshouse plants Comparable leaves from probable parents and progeny of hybrid. A, Acaena novae-zelandiae; B, Acaena anserinifolia. Remainder from progeny of hybrid.
|Achene length 3–5 mm.||Achene length 3–5 mm.||Achene length about 6 mm.|
|Spine length 5–7 mm.||Spine length 8–12 mm.||Spine length about 14 mm.|
|First flowering in September.||First flowering in October.||First flowering in October.|
The species of Acaena in New Zealand have long been regarded as taxonomically difficult, and it has been suggested that this may be due to a relatively high incidence of interspecific hybridism.
Cases of interspecific hybridism have already been reported in the genus both in the field (Buchanan, 1871; Cockayne and Allan, 1926) and in garden cultures (Bitter, 1911). The aim of the present study was to determine whether such hybridism is taking place between Acaena anserinifolia, A. novae-zelandiae and A. novae-zelandiae var. pallida.
Text-fig. 1.—Cotyledons and first leaves. a, Acaena novae-zelandiae var. pallida (Lyall Bay–Seatoun) b–d, Acaena novae-zelandiae (Prince of Wales Park,; Wainui; Eastbourne). e–h, Acaena anserinifolia (Wainui; Southern Tararuas; Prince of Wales Park; Kapiti Island).
The study area comprised the south-west sector of Wellington Province. (Fig. 2.)
Acaena anserinifolia was observed in a number of places in the vicinity of Wellington Harbour, throughout the Tararua Mountains to the north, and on Kapiti Island. Acaena novae-zelandiae was also observed at various localities around Wellington Harbour and at one locality in the Tararuas. Acaena novae-zelandiae var. pallida was observed in sand dunes at Eastbourne, Seatoun and Tapiri Bay.
Suspected hybrid populations were investigated at four localities: Prince of Wales Park, Brooklyn; the south coast from Lyall Bay to Seatoun; Wainui Valley; and the South Karori Stream.