Tuatara: Volume 11, Issue 3, September 1963
A Comment on Divaricating Shrubs
A Comment on Divaricating Shrubs
One of the most puzzling features of the New Zealand flora is the so-called ‘divaricating’ habit, largely if not entirely peculiar to New Zealand, of many of the shrubs and some of the juvenile forms of several forest trees. The chief characteristics of a divaricating shrub are free branching with the branches more or less at right angles to each other, the end result being a densely interlacing mass of slender twigs with very small, sometimes sparse leaves.
If this distinctive plant form occurred in only one genus in New Zealand then it might be dismissed as a chance abberration. However, in addition to trees and shrubs of normal form, about 23 genera have some divaricating species at either the adult or juvenile stage. There must then have been some peculiar circumstance, or set of circumstances peculiar at least in part, leading to the independant evolution of this form in the genera concerned. Suggestions have been made as to what these circumstances might have been (Cockayne 1911, Rattenbury 1962, Wardle 1963), but in every case the conditions described are not peculiar to New Zealand, but can be found in other parts of the world where they have not been associated with the evolution of the divaricating habit. I feel that it will be some time before the formulation of a completely convincing theory will be possible and when it appears it will probably be based on much greater knowledge than we have at the present time about the mode of growth, physiology and ecology of divaricating shrubs.page 194
Observations on the growth pattern of the juvenile divaricating stage of Carpodetus serratus leads me to suggest that physiological studies of divaricating shrubs could be very rewarding. In the several plants studied there was only main shoot growth from germination until about seven nodes had been formed. At this stage the axillary buds nearest the main apex began to grow out quite strongly with their own axillary buds following suit soon after. Buds further away grew out less strongly and the lowermost buds initially not at all. Growth of the main shoot appeared to be quite weak by comparison with the adjacent laterals. In shrubs or trees of normal form it is the lowermost axillary buds on a main shoot that grow out first and this pattern of development is generally attributed to control by the growth hormone auxin. Auxin is formed in the shoot apex and diffuses from there to the older parts of the shoot. Where auxin concentration is high, i.e. near the shoot apex, the growth of axillary buds is inhibited, while at a point further removed the auxin concentration drops below the threshold for inhibition and the axillary buds are able to develop. The fact that the growth pattern in Carpodetus is the reverse of this suggests some abnormality in auxin production or in its effects. It should be possible to apply auxin to the stem apices of divaricating shrubs to see whether the divaricating habit can be suppressed or, alternatively, to attempt to produce the same result, a scion from a normal species could possibly be grafted on to a stock from a divaricating species of the same genus.
It is well known that many divaricating species cross freely with related, but morphologically very different, non-divaricating species. This suggests that divaricating shrubs could have been derived from normal trees or shrubs by a mutation affecting auxin production or function, which would result in a plant morphologically very distinct from, but genetically still very close to the parent. Why such a mutation should have occurred independantly in so many genera is another, as yet unanswered question.
Cockayne, L., 1911. Observations concerning evolution derived from ecological studies in New Zealand. Trans. N.Z. Inst. 44: 1-50.
Rattenbury, J. A., 1962. Cyclic hybridization as a survival mechanism in the New Zealand forest flora. Evolution 16: 348-363.
Wardle, P., 1963. Evolution and distribution of the New Zealand flora, as affected by Quaternary climates. N.Z. Jour. Bot. 1: 3-17.