The shaping of seeds

Forest and Bird, Issue 292, 1 May 1999, Unnumbered Page

The shaping of seeds

— Ann Graeme

early two thousand years ago, the ancients of Rome and China recorded a time of darkened skies and spectacular sunsets. They did not know it, but these were a side effect of a cataclysm far away in the southern hemisphere, the volcanic eruptions that created Lake Taupo about 186 AD. Roller coasters of ash were spewed out to overwhelm and bury the forests of the central North Island. But in the grey desolation after the ash had settled, some trees and some seeds survived. From them grew the podocarp forests of the present day, the most lofty and species-rich forests in the country, nourished on the ash deposits of the past. ‘Podocarp’ is a useful word, bandied about by botanists, but scarcely understood by anyone else, which is a shame, as this is a most convenient collective name for those forest giants, rimu, totara, kahikatea, matai, miro and tanekaha. The word ‘podocarp originates from the Greek podos — a foot — and karpos — a fruit. A podocarp is a plant which bears a naked seed on the end of a swollen, succulent stem — a ‘fruity foot’ or ‘fleshy-footed seed’ — which is a useful description of the seeding peculiarity of these trees.

All these podocarps, including the familiar trees above, and other less common or less widespread species, belong to a predominantly southern hemisphere family of plants called the Podocarpaceae, of which 17 species are native to New Zealand. Podocarps don’t have flowers, because as a group they belong in another ancient class of trees called Gymnosperms (meaning ‘naked seeds’) — all cone-bearing trees. This is confusing for non-botanists because, in common usage, cones are knobbly things that grow on pine trees and make

good fire wood. These woody cones, with overlapping scales laid in a spiral pattern, are the typical fruits of the Gymnosperms of the northern hemisphere, and of one southern hemisphere family, the Araucariaceae, of which we have one species, the kauri. (Other members of the Araucariaceae include the monkey puzzle tree from Chile, the Norfolk Island pine and the Australian Bunya pine). Another predominantly northern hemisphere family in the Gymnosperm Class is the Cupressaceae, which have smaller, woody cones. Our cedars — kawaka and pahautea — belong here.

Compared to these cones, those of the female podocarp seem small and unlikely, because they are only a single seed, and their support structure is fleshy rather than woody. However, male podocarp cones readily show their Gymnosperm relationship. Just as the exotic pine forests around Tokoroa make the streets run yellow with pine pollen, so too our native conifers produce showers of pollen from similar, tiny, male cones, borne in their thousands on the tips of the branches, exposed to any passing breeze. The Gymnosperms are an ancient class of plants, dependent on the wind for carrying their pollen from male to female cone. The wind is a careless carrier, so Gymnosperms must produce enormous quantities of the male pollen to ensure a few grains reach the female cones. In most podocarp species, individuals are either male trees or female trees. This ensures that cross pollination must take place but it lengthens the odds in a lottery whose prize is the achievement of a fertilized seed, for the male and female trees need to be relatively close to each other..

Even by Gymnosperm standards, the Podocarpaceae are a very ancient family, appearing first in the Jurassic period, about 190 to 135 million years ago. Fossil records show that some species growing today are little changed from their ancestors which grew in the forests of Gondwana, the ancient continent of which our country was once a part. Until recently, New Zealand was a rainforest ark for such ancient species, its isolation sparing them from the challenges of invasive species which so profoundly altered other ecosystems of Gondwanan origin. So while flowering plants like tawa and taraire, beech and broadleaf mingle with rimu, totara and matai, it is still the ancient podocarps that lord it over the native forests of New Zealand. Podocarps are our icons, and the pillars of the cathedrals that are our native forests.

PS Miro and matai fruits don’t fit the seed-on-a-foot design. So these species have recently been put into a new genus, Prumnopitys, but still within the family Podocarpaceae.

ANN GRAEME is national co-ordinator of the Kiwi Conservation Club, the junior arm of Forest and Bird. She lives in Tauranga.