The Kinship of Plant S

Forest and Bird, Issue 314, 1 November 2004, Page 40

The Kinship of Plant

S

Text: ANNE GRAEME

long time ago, that legendary Professor John Morton came to stay with us. We showed him our new garden. He gazed thoughtfully at a southern rata we had just planted. He was far too kind to point out that a southern rata was out of place and ill-suited to the coastal Bay of Plenty, so he said instead: ‘It will be happy beside the feijoa. It wasn’t. It died, but we recall the moment when we first became aware of the relationship between rata and feijoa. For the feijoa and the rata (and the Cae am pohutukawa) are members of the same family of plants, the Myrtaceae, and, like any family, they have similar characteristics. Look at the leaves, particularly of pohutukawa and feijoa. They are similar: oval, grey-green above and whitish underneath. That is a clue, but it is not enough. Neither leaves nor fruits nor even form and stature are a reliable guide to kinship. Look at the flowers. They are the key: not at a superficial level based on colour or perfume, but in the number and arrangement of petals and sepals, of male stamens and of female carpels. How similar the rata and feijoa flowers are with their mass of spiky red stamens. The stamens are arranged around a cup made by the joined sepals. The cup is full of nectar and from its centre rises the column of the carpel, with its stickytipped stigma ready to catch the pollen grains. This is the typical flower of members of the Myrtle family, the Myrtaceae (mer — tay — see). It is a very old plant family which originated in the ancient supercontinent of Gondwana. Myrtles are best represented in Australia and South America, which were linked through Antarctica up until 49 million years ago. Africa, which broke away much earlier, has

only a few members of the Myrtaceae. The family has evolved from more primitive forms in moist, rainforest climates to specialized forms in semi-arid and desert regions. This is reflected in the form of the fruit, which divides the family into two groups. One group has a fleshy berry and the other has a hard, dry capsule with numerous seeds. Rata and pohutukawa are typical of the second group. Both have little capsules that rain masses of tiny seeds. They both belong to the genus of Meterosideros, of which New Zealand has twelve species. Many of our species have bright red stamens, a colour particularly visible and attractive to the lizards and birds that come to sup nectar and thereby accidentally transfer pollen from flower to flower. That hard little seed capsule brings to mind the manuka and kanuka which are also members of the myrtle family. They too have stamens arranged around a cup of sepals, but their petals are much more conspicuous than those of pohutukawa and rata.

The feijoa comes from South America. Its fleshy fruit shows its rainforest origins. The fleshy fruited group of Myrtaceae includes the guava, our native swamp maire, and shrubs like the attractive ramarama with its ‘blistered’ leaves. Members of the Myrtle family are an important part of the New Zealand flora, but we cannot rival their importance in Australia. This is home to the dry-fruited Eucalypts which dominate the landscapes of Australia. There is a Eucalypt for every conceivable habitat, from the snow gums of the Snowy Mountains to the river red

gums that line the ephemeral creeks, to the mallee shrubs of the deserts. In an explosion of evolution, this genus has assumed enormous importance in the ecology of Australia. There are other genera of Myrtaceae in Australia, but their species of Meterosideros are long gone, recorded only as fossils. Our rata species and pohutukawa originally came here from New Caledonia when our countries were linked by a great land mass in Cretaceous times. New Caledonia has an astonishing number of species of Myrtaceae — more than 200 — of which about 70 are species of Meterosideros.

Flowering Relatives

Flower structure reveals the otherwise hidden relationship between ivy and our native lancewood and five-finger trees (at left). They all belong to the Ivy family, the Araliaceae. The flowers of this family are insignificant but they are arranged in a cluster called an umbel, which is easy to recognize and is characteristic of the family.

Odd Bedfellows

Family relationships are sometimes very difficult to see. Who would guess that the violet and the mahoe both belong to the same family? But they do: to the Violet family, Violaceae, which is described as ‘diverse’, its members ranging from herbaceous plants with irregular flowers to shrubs, trees and climbers with regular flowers. The link is in the sophisticated mechanism of their anthers. They have a special flap, so arranged that it is triggered by visiting insects to release a shower of pollen. From the violet family we have two native genera, the white-flowered native violets and twelve species of Melicytus, with which whitey-wood or mahoe belongs.

A Cress is a Cabbage Few plants are so ill-named as Cook’s scurvy grass. It isn’t a grass at all. It is a native cress belonging to the cabbage family, Cruciferae. The flowers of this family are distinctive and constant. There are always four petals, usually white or yellow, and they are arranged like a cross (hence Cruciferae). The six stamens are attached in pairs inside the tube formed from the petal bases. Captain Cook tried diligently to provide green vegetables for his crew whenever the opportunity arose. While he did not know that such plants contained vitamin C, he did know that eating them both prevented and treated the loathsome disease of scurvy. His botanist, Joseph Banks, would have identified the plant called scurvy grass as a member of the cabbage family, and thus known that it would be edible. Subsequently, introduced browsing animals also found Cook’s scurvy grass to their liking, and the cabbage white butterfly recognized this kin to the cabbage, so no wonder the plant is now rare!