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Volume 88, 1960-61
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Natural Acaena Hybrids in the Vicinity of Wellington

Botany Department, Victoria University of Wellington*

[Received by the Editor, April 23, 1959.]

Abstract

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] * This paper represents the results of a study submitted as a thesis for the M. A. degree.

[Footnote] ‡ In the author's opinion this is quite distinct from A novae-zelandiae, and should be accorded specific status.

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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.

Tabulated Comparison
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.
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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.

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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.

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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.

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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.

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Text-fig. 2.—Map of study area. 1, Prince of Wales Park. 2, Lyall Bay–Seatoun. 3, Brooklyn.
4, Eastbourne. 5, South Karori Stream. 6, Wainui. 7, Kapiti Island. 8, Southern Tararuas.

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Prince of Wales Park, Brooklyn

This Acaena population was found within the city area in the vicinity of a football ground known as Prince of Wales Park. The excavations made in 1932 to form this field resulted in steep banks, about 30ft high, along the western side. North of the centre line these appear to have slipped, forming a 30–40° slope with dimensions of approximately 60 yards by 15 yards. It was on this comparatively even surface that the Acaena population was found.

The pattern of the population is illustrated in Fig. 3. The plants referred to as Acaena novae-zelandiae or A. anserinifolia were readily identified in the field; the former by their dark-green foliage, shining upper leaf surfaces and large fruiting heads with long, bright crimson spines; the second by their lighter brown-green foliage, dull upper leaf surfaces and small fruiting heads with short brownish spines. Plants forming the extensive colony to the northern end could not be identified, but evidence obtained from progeny tests strongly suggested that they were hybrids between the two species present.

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Text-fig. 3.—Population pattern, Prince of Wales Park.

As the date of formation of the habitat is definitely known, the population as a whole could not have been older than 21 years at the time of study. An attempt was made to determine the ages of individual plants by means of ring counts but little significant information was obtained for the following reasons:

(a) The annual rings were only weakly developed;

(b) In all cases the oldest woody parts available had broken ends directed away from the growing point. It is possible, therefore, that the first-formed parts of such branches, if they could be traced, would be several years older than those actually obtained. A maximum of 4 annual rings was found in the 2 species and the hybrids.

In many cases apparently separate plants were found to be in connection with one another over a distance of several yards. It seems, therefore, that individual plants spreading vegetatively by this means could eventually cover a considerable area.

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Seeds were taken from one plant of either species and from two plants of the suspected hybrids. In the latter case 9 seedlings of plant A. and only 7 seedlings of plant B. survived the seedling stage. These were eventually transferred to the garden area. A second set of 19 viable seedlings was obtained from plant A. in the second year, which, allowing for deaths of weak seedlings, represented an effective germination rate of only 34%. These seedlings were kept in the glasshouse.

An account follows of those characters of the 2 species in the area which were found to be of value when investigating the hybrids.

Acaena novae-zelandiae.

Germination rate, 100%; cotyledon laminae averaging 5 mm long; first leaf trifoliate; growth habit of mature plants trailing with few lateral branches; average diameter of stems at 7th internode 2.25 mm; stems clothed with unicellular, thick-walled hairs arising from pronounced multicellular bases; mature stems red in colour; adult leaves with 5 pairs of leaflets and a terminal one under glasshouse conditions, or 6 pairs of leaflets and a terminal one in the garden; average length of the terminal leaflet 11 mm; length ratio of one leaflet of the second pair from

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Text-fig. 4.—Frequency distributions of hybrid indices, Prince of Wales Park. 1, Garden plants. 2, Glasshouse plants. N.Z. = Acaena novae-zelandiae. H. = Suspected hybrid progeny A. = Acaena anserinifolia.

the tip / one leaflet of the third pair is 10/7.6; upper surface of leaflets green only, lower surface of leaflets green only, hydathodes at serration tips white in glasshouse plants, crimson in garden plants; upper surfaces of leaflets glabrous; calyx lobes green only; average length of longest fruit spine 9.8 mm; fruit spines bright crimson; pollen fertility 98%; (for method of determination see Owczarzak, 1952) chromosome number n = 21.

Acaena anserinifolia.

Germination rate 96%; average length cotyledon laminae 2.4 mm; first leaf simple; growth habit of mature plants spreading with many lateral branches; stems clothed with unicellular hairs lacking multi-cellular bases; mature stems olivebrown

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in colour; adult leaves with four pairs of leaflets and a terminal one under glasshouse conditions or 5 pairs of leaflets and a terminal one in the garden; length ratio of one leaflet of the second pair from the tip/one leaflet of the third pair is 10: 4.7; upper surface of leaflets with a red-brown colouration at serration tips and along margins; lower surface of leaflets with pink-purple veins; hydathodes pink; upper surfaces of lower leaflets pilose; calyx lobes reddish on the abaxial surface; average length of longest fruit spine 5.2 mm; fruit spines pale-brown to reddish-brown; pollen fertility 94%; chromosome number n=21.

The above contrasting characters of the two species were used according to Anderson's hybrid index method (Anderson, 1949) for the investigation of the suspected hybrids. In each case the Acaena novae-zelandiae character was rated as 0 and the A. anserinifolia character as 1, 2 or 3 depending upon the number of intermediate grades.

(a) Length of cotyledon: 4.5–5.5 mm. = 0 (A. novae-zelandiae); 3.5–4.5 mm. = 1; 2.5–3.5 mm. = 2; 2–2.5 mm. = 3 (A. anserinifolia).

(b) First leaf form: Trifoliate = 0 (A. novae-zelandiae); simple, deeply incised = 1; simple = 2 (A. ansernifolia).

(c) Growth habit: For each plant the total number of visible nodes possessed by lateral branches was determined. 0–10 nodes = (A. novae-zelandiae); 10–20 = 1; 20–40 nodes = 2; more than 40 nodes = 3 (A. anserinifolia).

(d) Stem colour: Red = 0 (A. novae-zelandiae); olive-brown = 1 (A. anserinilolia)

(e) Hair bases: Pronounced multicellular bases = 0 (A. novae-zelandiae); small multicellular bases = 1; no multicellular bases = 2 (A. anserinifolia).

(f) Number of leaflet pairs: 5 pairs (or 6 in the open) = 0 (A. novaezelandiae); 4 pairs (or 5 in the open) = 1 (A. anserinifolia).

(g) Leaflet size gradation: Ratio more than 10.7 = 0 (A. novae-zelandiae); ratio 10.5–10.7 = 1, ratio less than 10.5 = 2 (A. anserinifolia).

(h) Colour of leaflet upper surfaces: No red colouration along upper margins of leaflets = 0 (A. novae-zelandiae); red-brown colouration along margins = 1 (A. anserinifolia)

(i) Colour of leaflet under-surfaces: No purple colouration = 0 (A. novaezelandiae); some degree of purple colouration on veins and surface between = 1 (A. anserinifolia)

(j) Hydathode colour: Hydathode white (or crimson on plants in the open) = 0 (A. novae-zelandiae); hydathode pink = 1 (A. anserinifolia)

(k) Upper surface hairs on lower leaflets: Lower leaflet pair glabrous on upper surface = 0 (A. novae-zelandiae) lower leaflet pair moderately hairy on upper surface = 1 (A. anserinifolia).

(l) Colour of calyx lobes: Green only = 0 (A. novae-zelandiae); reddish colouration on margins and midrib of abaxial surface = 1; general reddish colouration over abaxial surface = 2 (A. anserinifolia)

(m) Colour of spines: Distinct red colouration = 0 (A. novae-zelandiae) brown colouration = 1; (A. anserinifolia.)

(n) Length of longest spine: 9–11 mm = 0 (A. novae-zelandiae); 7.5–9 mm = 1; 6–7.5 mm = 2, 4–6 mm = 3 (A. anserinifolia)

Suspected hybrids in the field.

The plants forming this colony were inspected fairly closely on several occasions and a comparison of specimens from seven well separated points revealed no significant variations in vegetative or reproductive features. There are, therefore, three possibilities: (a) that these plants are first generation hybrids; (b) that they are a clone derived from a single F1 hybrid; (c) that they are a clone derived from an F2 or back cross hybrid. The following data support the first and second possibilities.

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As no seedling stages were available these plants could be scored for the following characters only:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Stem colour—red 0
Hair bases—prominent 0
Number of leaflet pairs—5 1
Hydathode colour—crimson 0
Leaflet margin colour—red-brown 1
Upper surface hairs—present 1
Under surface colour—purplish 1
Spine colour—red 0
Spine length—6–7.5 mm 2
Calyx lobe colour—partially red 1
Total (= hybrid index) 7

For these characters Acaena novae-zelandiae scored 0 and A. anserinifolia 14 so the suspected hybrids were exactly intermediate.

Pollen slides were made from four inflorescences possibly representing four different plants. The percentages of fertile pollen were 63%, 0%, 18%, 48%.

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Text-fig. 5.—Population pattern, Lyall Bay-Seatoun.

Progeny of suspected hybrids.

These were chiefly studied in the vegetative state. The 16 surviving plants from the first sowing were planted out in the garden and the 19 seedlings from the second sowing were kept in the glasshouse. Only 6 of the garden plants came to flower, and their pollen fertility percentages were: 54%, 81%, 80%, 79%, 65%, 93%.

By contrast with the 2 species and the suspected field hybrids these plants were markedly variable. This variability was analysed according to the scheme outlined above with the garden and glasshouse plants receiving separate treatment.

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The 18 glasshouse plants were scored for characters a, b, c, d, e, f, g, h, j, J The extreme scores for Acaena novae-zelandiae and A. anserinifolia in this case were 0 and 17 respectively. The progeny of their suspected hybrid scored as follows: 4, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 9, 9, 9, 10 (Fig. 4.)

The 15 garden plants were scored for characters d, e, f, h, i, j, J. The scores of Acaena novae-zelandiae and A. anserinifolia for these characters are 0 and 8 respectively. The progeny of their suspected hybrid scored as follows: 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 6. (Fig. 4.)

Discussion

A comparison of the forms of Acaena novae-zelandiae and A. anserinifolia in this locality with forms of the two species elsewhere reveals that the first is relatively large and the second relatively small in most measurable characters. The contrast between the two species is therefore particularly marked in this locality.

The hybrid hypothesis advanced in explanation of the unidentifiable colony was based on the following evidence.

(1) The plants possessed a mixture of the distinctive features of the two species present.

(2) Germination rates and pollen fertility were both low.

(3) Progeny obtained showed marked segregation for all characters.

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Text-fig. 6.—Lyall Bay-Seatoun. First leaves from 10 plants of the suspected hybrid progeny.

The supplementary hypothesis that the field hybrids were all F1 plants is based on the following evidence:

(1) The field hybrids appeared to be uniform and intermediate between the two species present.

(2) The average pollen fertility of the field plants is 32.25% and that of their progeny 75.3% Such an improvement in fertility has been observed by other workers in the F2 generation of interspecific hybrids.

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The extent of the suspected hybrid colony appears to contradict this hypothesis, for if the increase in area has been due to the production of new plants, then there should be a significant proportion of F2 plants in the colony. However, the spreading and rooting habit of the Acaenas, which is an effective means of vegetative reproduction, suggests that the colony may have been entirely derived from a few original F1 plants.

The low germination rate of seed from these presumed F1's and the low survival rate of seedlings suggests that F2 plants would find difficulty in establishing themselves under field conditions.

In Fig. 4 it will be noted that the average hybrid index is displaced towards Acaena novae-zelandiae. A. possible explanation is that in the case of three characters scored in the more than two grades A. novae-zelandiae is probably overrepresented— ie, with growth habit inherent lack of vigour may reduce the number and length of laterals formed, and with cotyledon-length and first-leaf form, in respect of which the majority of seedlings resembled Acaena novae-zelandiae, A. novae-zelandiae dominance is probably a factor. The approximately normal curve formed by the hybrid indices does not suggest backcrossing to Acaena novaezelandiae.

Lyall Bay-Seatoun Coast

This area is a strip of coastline approximately three miles long partly within and partly outside the entrance to Wellington Harbour. It consists for the most part of a narrow coastal platform (averaging about 25 Yards wide) backed by steep hills. The platform came into being partly as a result of general uplift during the 1855 earthquake and the habitats so formed were later considerably modified by the construction of a coastal road in 1923. The soil consists mainly of rock fragments mixed with shingle and a small amount of sand.

An extensive area of sand dunes formerly existed at Lyall Bay, but these were later levelled to make way for housing and for Rongotai aerodrome. A comparatively sandy area also exists at the head of Tapiri Bay, and a few sand dunes still remain along the Seatoun foreshore.

The coastal platform supports a rather stunted grass cover with scattered prostrate shrubs, and a similar association is found on the lower hill slopes. On the latter, however, growth is generally more vigorous and in places where there is a certain amount of seepage, almost luxuriant. The major part of the area would be fully exposed to southerly gales.

The population pattern of Acaenas in this area (Fig. 5) is not nearly so straightforward as at Prince of Wales Park. The most common type, indicated by the letter H. on the map, could not be assigned to any known species. The type marked P? was at first identified as Acaena novae-zelandiae var. pallida but later comparisons showed it to be smaller than the common type of that species in all respects. The type marked P. was indentified as Acaena novae-zelandiae var. pallida. Specimens in various herbaria indicate that this variety was also quite common on the formerly existing sand dunes at Lyall Bay.

Only one plant of Acaena novae-zelandiae (marked N) and four plants of A. anserinifolia (marked A) were found and these were at some distance from the unidentified populations.

Ring counts were not attempted in this area, but many plants had quite thick woody parts at the base which resembled those collected at Prince of Wales Park.

Seeds were collected from three Hand one P? colony, from one plant of Acaena novae-zelandiae var. pallida, from one plant of A. novae-zelandiae and from two plants of A. anserinifolia.

The plants of Acaena anserinifolia and A. novae-zelandiae differed in minor details only from the forms at Prince of Wales Park.

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Acaena pallida.

Germination rate, 92%; pollen fertility, 95%; chromosome number n = 21. For other features see tabulated comparison.

The form marked P? on the map was smaller in all respects than Acaena novae-zelandiae var. pallida but otherwise closely resembled that species. When seed of this form was germinated certain unexplained abnormalities became apparent. The germination rate was only 52% and this was later reduced to an effective rate of only 22% as a result of the early death of several seedlings. This last phenomenon followed a uniform pattern. The cotyledons expanded normally, the first leaves appeared as minute, malformed structures, and growth then ceased. A second test using new seed from the field gave similar results. Those seedlings which developed normally showed only minor variations and plants transferred to the garden area in the first year grew vigorously and flowered freely.

Field Hybrids.

Plants from the various colonies marked H on the map were observed to be very similar to each other, and later detailed study confirmed this first impression. As progeny from plants representing all these colonies are extremely variable, the situation here seems to be similar to that obtaining at Prince of Wales Park, and the same explanation suggests itself—i.e., that the plants in question are F1 hybrids.

Progeny of Suspected Hybrids.

Sets of seedlings were raised in the first year from colonies H1, H2 and H4. The H1 seedlings were planted into the garden area, and in the second season 2 of the 11 surviving plants flowered. Glasshouse seedlings were also obtained from the three colonies in the second year. Germination rates for seed from the three colonies were: H1, 42% (10% died soon after germination and a further 8% in the first leaf stages. Effective germination rate therefore = 24%); H2, 52% (20% died soon after germination and a further 20% in the early seedling stage. Effective germination rate = 12%); H4, 48% (12% died soon after germination and a further 6% in the early seedling stage. Effective germination rate = 30%).

Pollen fertility percentages for the two plants that flowered were 87% and 66%. By contrast, percentages for plants from the 3 field hybrid colonies were 47%, 33% and 62%.

As possible parents for the suspected hybrids are rare in this locality, Anderson's hybrid index is difficult to apply. The method of “extrapolated correlates” designed by Anderson for situations where one or both parents are unknown was also considered as a tool for investigation. However, the requirement of at least two characters which could be scored accurately in a number of grades proved a major difficulty. Size characters at first suggested themselves, but the fact that certain plants of the suspected hybrid progeny appeared to be inherently stunted made these difficult to use. Finally it was decided to restrict investigation in this case to a study of those characters of the suspected hybrids that are useful in delimiting the three local species.

The hypothesis that the colonies symbolised by the letter H consist of F1 hybrids is based on evidence similar to that put forward in the section on Princes of Wales Park. The additional hyphothesis that Acaena novae-zelandiae var. pallida is one of the parents involved is based on the following pallida-like characters possessed by the field hybrids and their progeny:

  • (a) Stem diameter approaching that of Acaena novae-zelandiae var. pallida — 4 plants (progeny).

  • (b) Length of terminal leaflet almost as great as that of A. novae-zelandiae var. pallida — 1 plant (progeny).

  • (c) Hydathodes white — 4 plants (progeny).

  • (d) Serration tips hairless — 3 plants (progeny).

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  • (e) Fruit spines pale-red and partly colourless — field hybrids. Characters which suggest Acaena anserinifolia as the other parent involved are:

  • (a) Leaf, stem and fruit spine dimensions of the field hybrids are generally less than those of the Acaena novae-zelandiae and more or less intermediate between those of A. novae-zelandiae var. pallida and A. anserinifolia. (See table.)

  • (b) Simple first leaves — 3 plants (progeny).

  • (c) Stems pale-brown — 2 plants (progeny).

  • (d) Distinct red-brown colouration on the upper margins of leaflets — 3 plants (progeny), also field hybrids.

  • (e) Extensive purple colouration on the under surfaces of leaflets — 2 plants (progeny), also field hybrids.

  • (f) Fascicled hairs at the serration tips — 1 plant (progeny).

  • (g) Some degree of hairiness on the upper surfaces of leaflets — 8 plants (progeny) also field hybrids.

The evidence derived from stem, leaf and fruit spine dimensions can be demonstrated most effectively in tabulated form. Averages for the three local species were based on collections from several localities in the study area.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

(1) Acaena novae-zelandiae var. pallida (Average) (2) Acaena novae-zelandiae (Average) (3) Acaena anserinifolia (Average) Average of 1 and 2 Average of 1 and 3 Field Hybrid Range of Hybrid Progeny Average of Hybrid Progeny.
Length of cotyledon lamma (mm) 5.8 4.2 2.6 5 4.2 2.8–3.9 3.35
Diameter of stem(mm) 3.5 2.1 1.75 2.78 2.6 2.5 2–3 2.35
Length of terminal leaflet (mm) 19.0 13.5 8 16.25 13.5 11.7 10–17.25 12.3
Length of longest fruit spine (mm) 13.8 9.25 5.6 11.5 9.7 8.4

The evidence suggesting Aceana novae-zelandiae var. pallida and A. anserinifolia as the parent species of the hybrid colonies is therefore fairly strong.

The question arises—where did the hybrids originate and how did they become established at such widely separate points? The comparative rarity of possible parents in the vicinity makes speculation difficult. Two possibilities are:

(1) That crossing has taken place outside the area (possibly at Lyall Bay when Acaena novae-zelandiae var. pallida was common there) and hybrid seeds have been carried into the coastal section at various times by people and animals.

(2) That isolated plants of Acaena novae-zelandiae var. pallida have established themselves in sandy spots along the coastal section from time to time and have produced hybrid seed as a result of fertilisation by windborne pollen of distant A. anserinifolia. The inconspicuous flowers, prominent plumose stigmas and elongate stamens of the Acaenas suggest wind rather than insect pollination.

Both suggestions presuppose a selective influence of the environment which discourages establishment of Acaena novae-zelandiae var. pallida seedlings and encourages the hybrids. The majority of habitats available along the coastal strip are certainly unsuitable for Acaena novae-zelandiae var. pallida.

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Wainui Valley and South Karori Stream

Plants which are undoubtedly hybrids were discovered at the above localities, and in both cases Acaena anserinifolia and A. novae-zelandiae were observed in the vicinity. The hybrids were not subjected to detailed study but sufficient information was obtained to suggest that they were first generation crosses between the above two species.

Hybrid seed from the South Karori Stream had a germination rate of 38% reduced to an effective rate of 36% by the death of one seedling.

The first leaves varied in form from simple to trifoliate. Simple — 4 plants; simple, deeply incised—3 plants; trifoliate—11 plants. Adult leaves varied greatly in size, shape and colour.

In the Wainui-o-Mata Valley seed was collected from two plants suspected to be hybrids between Acaena anserinifolia and A. novae-zelandiae in the vicinity. The first had a germination rate of 40% reduced to an effective rate of 16% by the death of several seedlings. The first leaves were as follows: simple — 3 plants; simple deep incised—3 plants; trifoliate—5 plants.

The second seed lot had a germination rate of 14%, reducing to an effective rate of 12%. The first leaves were as follows: simple—3 plants; simple deeply incised—3 plants; trifoliate—6 plants.

Southern Tararuas and Kapiti Island

The Acaenas in these localities belong almost exclusively to one species — Acaena anserinifolia. Acaena novae-zelandiae was observed at only one locality in the southern Tararuas. The form of Acaena anserinifolia on Kapiti Island is similar to those in the Wellington area, but the commonest form in the Tararuas was distinctive in the following respects: relatively large leaves and incised leaflets; distinct pilosity of the upper surfaces of all leaflets; and a preference for the semi-shady habitats of bush tracks.

The few plants of Acaena novae-zelandiae mentioned above were found at widely separated points near the bank of the Tauwharenikau River. In each case a few plants were found in the vicinity which were possibly interspecific hybrids between Acaena novae-zelandiae and the common Tararua form of A. anserinifolia. These possible hybrids had the dark-green foliage, red stems and crimson spines of Acaena novae-zelandiae, but their general dimensions were less and there was a noticeable degree of pilosity on the upper surfaces of their leaflets.

Colour Variation in Acaena anserinifolia

The majority of plants of Acaena anserinifolia observed had red and purple pigments in addition to chlorophyll. At a number of localities there were a few uniformly bright-green plants which lacked the secondary pigments. These plants bred true for this character Some of the plants possessing secondary pigments bred true, but in others a proportion of the offspring had secondary pigments, while in a lesser proportion these pigments were lacking. Figures for the latter phenomenon in the various localities were as follows:—

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Secondary Pigments Present Secondary Pigments Absent
Lyall Bay-Seatoun 84 26
South Karori Stream 84 29
South Karori Stream 39 9
Wamui Valley 34 17
Kapiti Island 45 26
Southern Tararuas 49 17
Totals 335 124
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The overall ratio of progeny with secondary pigments to progeny lacking secondary pigments is 2.7: 1. It seems, therefore, that the production of secondary pigments is controlled by a single genetic factor whose recessive allele is inhibiting when homogygous.

These occasinal plants lacking secondary pigments are probably the form described by Cockayne as Acaena anserinifolia var. viridior (Cockayne, 1916).

General Discussion

As a result of the present study it is concluded that the production of F1 hybrids by the three local species of Acaena is quite frequent in the Wellington area. These hybrids are considered to be of two types: (1) Crosses between Acaena novaezelandiae var. pallida and A. anserinifolia (Lyall Bay-Seatoun); (2) Crosses between Acaena novae-zelandiae and A. anserinifolia (Prince of Wales Park, South Karon Stream, Wainui Valley and possibly in the Tauwharenikau Valley).

With the second type of cross a few plants of Acaena novae-zelandiae were generally found in association with the suspected hybrids. This suggests that Acaena anserinifolia normally acts as the male parent and this is probably explained by the following facts. The generally early flowering habit of Acaena anserinifolia combined with the pronounced proterogyny of all species means that when the first stigmas are appearing on the flower heads of the later flowering species only Acaena anserinifolia pollen is available. In all probability the majority of hybrids are produced in this way, and it follows that Acaena anserinifolia is most frequently the male parent.

The question aries whether the incidence of natural hybridism has always been so high or whether it is largely a recent phenomenon coinciding with the period of Europeans settlement in New Zealand. The latter view is favoured by the author, for, since European settlement, the distribution pattern of the lowland Acaenas must have been greatly altered, and as a result opportunities for hybridism must have increased immeasurably.

A comparison of pre- and post-European times with respect to conditions of dispersal and the extent of available habitats should make this clear.

In pre-Europeans times the only agents for achene dispersal were the Maori and his dog and possibly some of the flightless birds. There are no antive land mammals in New Zealand and the Maori had no form of animal husbandry. In these times, too, the country was largely covered with forest, so open habitats suitable for Acaena were much less common than now. With white settlement came the introduction of domesticated animals, notably sheep, and other animals which became established in the wild—deer, pigs and rabbits. In time the lowland areas were cleared of forest and brought into cultivation, and since that time the creation of disturbed habitats by roading and other construction works has been a recurring phenomenon. Thus, settlement has caused a marked increase in the number of dispersal agents and a parallel increase in the total area of habitats suitable for Acaena.*. The result has been an overlapping of the three species throughout their ranges, and a consequent increase in the opportunities for hybridism.

[Footnote] * With the exception of Acaena novae-zelandiae var. pallida which become less common following obliteration of sand dune areas.

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If the generalisation that second generation hybrids do not survive under natural conditions is correct, then it would appear that the increased hybridism is unlikely to result in a general merging of the lowland species. There is always the possibility, however, that a first generation hybrid might backcross with one or other of its parent species and perhaps produce viable offspring, but there is no evidence of this having occurred.

References

Anderson, E., 1949. Introgressive Hybridisation. Wiley & Sons, New York.

Bitter, G., 1911. Die Gattung Acaena. Bibliotheca Botanica, Stuttgart.

Buchanan, J., 1871. On a supposed Hybrid Acaena. Trans. N.Z. Inst., 3:208.

Cockayne, L., 1916. Notes on New Zealand Floristic Botany Including Descriptions of New Species (No. 1) Trans. N.Z. Inst., 48: 193–202.

— and Allan, H. H., 1926. Notes on New Zealand Floristic Botany, including Descriptions of New Species. Trans N.Z. Inst., 56. 21–53.

Owczarzak, G., 1952. A. Rapid Method for Mounting Pollen Grains. Stain Tech., 27: 249–251.

Dr. J. W. Dawson

,
Botany Department,
University of Wellington.
P. O. Box 196,
Wellington.