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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.
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.