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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:
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(a) Stem diameter approaching that of Acaena novae-zelandiae var. pallida — 4 plants (progeny).
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(b) Length of terminal leaflet almost as great as that of A. novae-zelandiae var. pallida — 1 plant (progeny).
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(c) Hydathodes white — 4 plants (progeny).
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(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:
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(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.)
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(b) Simple first leaves — 3 plants (progeny).
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(c) Stems pale-brown — 2 plants (progeny).
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(d) Distinct red-brown colouration on the upper margins of leaflets — 3 plants (progeny), also field hybrids.
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(e) Extensive purple colouration on the under surfaces of leaflets — 2 plants (progeny), also field hybrids.
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(f) Fascicled hairs at the serration tips — 1 plant (progeny).
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(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.