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Pterostylis Orchids
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This article was published in The Fringe, November 2014

Greenhood Orchids

About 36 different types of greenhood orchids emerge from the leaf-litter of New Zealand's forests and scrublands, but few are common and most are little known even to keen botanists. Some have been reclassified in recent years so their genus name may have changed from Pterostylis, but they are still greenhood orchids.

New Zealand plants generally have small and insignificant flowers and our orchids are no exception.          But the orchid flower is a fascinating structure and subtle differences can define the separate species.

The green hood from which they get their name forms the bulk of the flower. It covers the labellum which is a tongue that protrudes from the flower and provides a landing pad for pollinating insects. When an insect makes touch down, the labellum is triggered to flick it to the back of the hood, throwing the insect down into the internal parts of the flower. There, the insect deposits any pollen it has collected from a previous flower and then, in the process of crawling up the inside, it collects some more pollen which will be transferred to the next flower it visits.

Greenhood orchids die back after they have seeded so it takes an observant eye to spot their reappearance each year. Underground, food is stored in a tuber which persists throughout the year with the first leaf growing from it and as many as six leaves up the stem before the flower appears. Older clumps spread out from the original tuber with a dozen or more stems creating a spectacular little grove.

The best known of the greenhoods is Pterostylis banksii, because it is probably the commonest, most widely spread and certainly the largest and most visible when it is in full flower in October - December. Called tutukiwi, it can grow up to half a metre tall with the actual flower being up to 5cm.

Two other species, both with the common name kauri greenhood orchid, are found only within a short distance of kauri trees (Agathis australis). But their time of flowering and growth habit make them readily identifiable. One is named Pterostylis agathicola to mark the association with kauri, and it grows 30cm tall with 15cm leaves, and flowers in August - September.  The other was formerly named Pterostylis brumalis but has been renamed Diplodium brumale to note the fact that it has two growth forms. The juvenile is a flat rosette less than 1cm across, then the second stage emerges from its centre and grows to 20cm with 4cm leaves and a 2cm flower from April to August.

The other species are progressively smaller and less common. Their size may in fact be the reason they are uncommon, as they can be very small to the point where they scarcely appear above the leaf litter. Pterostylis venosa can be less than 3cm tall including the 1cm flower and is found in scattered localities throughout New Zealand.

Orchids delight the observant, and once you know the labellum can be triggered to flick to the back of the hood, it is always tempting to get down and artificially set it off by gently touching it with a tiny twig.  Children enjoy seeing a plant move at their command. The labellum will realise it has been tricked if it is not pollinated and will reset the trigger in about half an hour.

But please do not remove the orchids from the wild as they seldom survive and they are already threatened by possums browsing them.

Enjoy them where you find them.



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