Exploring native plant curiosities

Tane Mahuta

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When he saw the giant Californian sequoias the much-quoted 19th century American essayist, Ralph Waldo Emerson, noted that ‘the wonder is that we can see these trees and not wonder more’. I suspect he would have said the same about the New Zealand bush. What would he have said if he saw Tane Mahuta?

Last time we looked at why New Zealand plants predominantly have small, inconspicuous, wind-pollinated flowers. Now we explore two other native plant curiosities – the frequency of the divaricating habit and plants with distinct juvenile and adult forms.

In most other shrubs and trees there is a leading shoot at the apex with side shoots below dictating the upright form of the plant. In contrast, divaricating plants consist of masses of tangled branches with no dominating influence of a leading shoot. This can result in springy ball-like shrubs forming dense inpenetratable spheres.

New Zealand’s flora includes a high incidence (about 10%) of plants with a divaricating habit. Plants with this growth habit produce small leaved plants with tough and wiry interlaced branches that grow in a zigzag pattern. Divaricate, literally, means ‘to branch at wide angles’.

 

The small-leaved Kowhai Sophora microphylla displays the divaricating habit in its juvenile form

In the adult form Sophora microphylla adopts a more typical growth habit

Divaricating shrubs can be found in many different situations, including wet lands, salt marshes, and under forests trees. This growth form exists elsewhere in the world but nowhere else with such a high frequency. Why is this?

A number of theories have arisen explaining the penchant for divarication in New Zealand’s plant life, including:

  1. The assertion that the dense and tangled branch growth evolved as a defensive response to grazing by the now extinct moa, a large flightless bird similar to the modern day emu.
  2. The claim that divarication aids plants in protection from dry and windy climatic conditions by acting as a windbreak, reducing transpiration from the plant’s inner leaves and limiting water loss.
  3. Another view that the inner depths of a divaricate shrub provide a hospitable environment for native lizards such as skinks and geckos. In return for acting as agents of fruit and seed dispersal, the lizards are provided shelter and protection by the interwoven branches of the plant.

Any one of the above three theories could be true – no one really knows, and we may never know, why plants have developed the divaricating habit or why divaricating plants are more common in New Zealand than anywhere else in the world.

Another bizarre feature is that a number of New Zealand plants, while beginning life as small-leaved, tangled divaricating shrubs (juvenile stages), ultimately grow into normal leafy trees. This is called heteroblasty, a feature demonstrated in Sophora microphylla shown in the images above. It may be that the evolution of resistance to browsing by moa would explain the juvenile divaricate habit of certain plants, since once they reach a mature height they would no longer be at risk from browsing animals.

Perhaps the most striking example of heteroblasty is Pseudopanax crassifolius or lancewood; in its juvenile stage it has tough strap-like leaves, then as an adult the leaves become broader, more upright and soft.

 

The lancewood has a distinct juvenile growth form
 

As an adult mature tree Lancewood adopts the more typical growth habit of a tree

The Chatham Island Pseudopanax does not have a juvenile form

The genus Pseudopanax gives another hint as to why heteroblasty is a feature of New Zealand plants. The closely related Pseudopanax chathamicus is endemic (only found on) to the Chatham Islands. And guess what? There is no evidence that moa ever lived on the Chathams.  It is possible then that P. chathamicus didn’t need to develop a juvenile form because it never had to withstand browsing pressure from the Moa.

Creating theories and testing them is the hallmark of science. Maybe one day we will know why plants developed the divaricating habit and heteroblasty. As the great Albert Einstein said, “Look deep into nature, and then you will understand everything better.”\

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