Coprosma

The Genus of plants known to the botanist as Coprosma is probably best known to most people by the species bearing the Maori name Taupata Taupata (Coprostnu repens) is a low growing coastal shrub with dark green very shiny leaves and clusters of orange berries. It is very resistant to salt spray and is often grown as a hedge in seaside towns in New Zealand, and even in certain areas overseas where it is highly regarded as an introduced ornamental. In California it is known as the 'Mirror' or 'Looking-glass' plant in reference to the shiny leaves.

It will probably surprise most people to learn that Taupata is only one of about forty New Zealand species of Coprosma collectively encompassing the whole country and adjacent islands in their distribution. Several of the other species can be readily recognized as relatives of Taupata, as they are all shrubs or small trees with leaves of comparable size and form, one of these in fact having the largest leaves in the genus.

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The remaining native species, however, bear little superficial resemblance to Taupata. All except two of them are small shrubs with (in some species) leaves of a very peculiar form. The latter are excessively twiggy with the twigs set at approximately right angles to one another to form a springy shrub which has much in common with a barbed wire entanglement. The leaves are often minute and quite sparse so that the general appearance is quite different from that of Taupata. There is no doubt, however, that the small-leaved and large-leaved species are members of the same genus. A close comparison of the leaves themselves reveals this and the matter is confirmed by the essential similarity of the wind-pollinated flowers and the berries.

The two species remaining are neither trees nor shrubs but small creeping herbs. Their leaves are very small and they are often scarcely visible in the grassy swards of river flat and mountain.

The berries of Coprosma are their most attractive feature. They are usually about the size of red-currants and depending on the species may be various very attractive shades of black, red, orange, yellow, blue or white. Even the twiggy species are quite handsome when covered with sky-blue, deep red, or pale yellow berries.

The unflattering name of the genus ('dung plant') was bestowed by the Forsters (botanists on Cook's second voyage) on a plant which had an unpleasant smell when bruised. This species now suffers under the name of Coprosma foetidisitna ('foul smelling dung plant ') but fortunately only one other species shares this characteristic.

Another interesting aspect of Coprosma is its relationship to the coffee genus. It has even been claimed that the seeds when roasted and ground make quite an acceptable substitute for the real thing.

With forty of the approximately ninety existing species New Zealand can be regarded as the headquarters of Coprosma. Thus it is appropriate that the genus should be studied in this country. One of the immediate problems in such a study is identification of the species. In the larger-leaved forms the problem is not so great, as the points of difference are easily observable and have become familiar to most botanists. In the small-leaved forms the distinctive features are much less easy to observe and as a result many botanists have not made the effort to become familiar with them. It is not surprising then that the status of many of the small-leaved species has become very confused and it is in an endeavour to facilitate their identification that Mrs Taylor of the Botany Department is at present preparing an illustrated key. Such a key involves a series of alternatives for which the unidentified plant is examined. Each decision leads to another alternative and eventually to a name which can be checked by reference to the illustration. In some cases the illustration alone is sufficient for identification, but in other cases where the species exists in a variety of forms identification in this way is more hazardous. Such variations may result from differences in the environment (e.g. a plant growing in the shade will be more spindly and have larger leaves than a plant growing in the open) or to inherent local variations.

In Coprosma much of the confusion has resulted from the occurrence of inter-specific hybrids which in several cases were named as species before their true nature was known. In the best known case a small-leaved and a large-leaved species hybridize page 22 wherever they meet to produce a kaleidoscope of intermediate forms. It is suspected in another case that in some localities three species are involved in the parentage of some hybrids. This aspect of the Coprosma problem has been the subject of two recent studies in the Botany Department. Mr J. W. McEwan made a study of Coprosma populations in the Wellington area, paying particular attention to suspected hybrids. His methods included detailed comparisons of the external features of the stems, leaves, flowers and fruits; investigation and comparison of the internal anatomy of leaves; determination of chromosome numbers; and attempts to carry out artificial crosses between the postulated parental species. As a result of these investigations McEwan concluded that three pairs of Coprosma species hybridize in the Wellington area. In one case already mentioned a great variety of hybrids are produced (known as a 'hybrid swarm'), some of which cross back with one of the parents. In the other two cases only a few hybrid plants were noted and these appeared to belong to the first generation resulting from crossing.

A rather different approach to the study of the hybrid swarm and its parents has been utilized by Mr A. O. Taylor. This method involves a study of biochemical variations in the plants concerned, using in particular the polyphenolic compounds (tannins, flavones, catechins, etc.). In the present case a pattern of polyphenols was worked out for the two parents and the situation in the hybrids was found to be intermediate between them. This may prove to be a relatively quick and reliable method both for deciding parentage of hybrids and detecting other hybrids in the genus.

The studies described represent only a small proportion of the possibilities for research in this interesting genus, and they have been concerned primarily with classification, external morphology and biochemistry. Many other aspects remain to be studied. The anatomy or cellular structure of the various plant organs could be investigated, with particular attention to the development of certain peculiar structures associated with the leaves and stems. Chromosome numbers of the species could be determined and chromosome shapes compared as a possible basis for determining relationships. Ecological study — being a study involving the investigation of the habitats occupied by each species in nature — should be rewarding.

A complete account of the genus would probably occupy a period of many years, but it should eventually provide an account of the course and mode of evolution and migration within the genus — and this, in itself, would be a significant contribution to our knowledge of evolution as a whole.

J. W. Dawson