Tuatara: Volume 9, Issue 1, September 1961
A Key to the Coprosmas of New Zealand — Part I
A Key to the Coprosmas of New Zealand — Part I
The genus Coprosma has its greatest concentration of species in New Zealand, where there are about forty of the ninety known species. The remainder are scattered through the Pacific, and fall within a line including Tasmania. Eastern Australia, Borneo, Hawaii, Juan Fernandez Islands and New Zealand's subantarctic islands. The genus was first collected by Banks and Solander in New Zealand on Cook's first voyage, but as their descriptions of seven species were never published the name valid to science originates from the Forsters, father and son, who came as botanists on Cook's second voyage. They found two species, one of which forced itself to their notice by its unpleasant smell, strongly reminiscent of carbon disulphide. As a result of this disagreeable first impression they stigmatised the whole genus with the name Coprosma meaning ‘smell of dung’, and applied the specific name foetidissima, ‘extremely vile smelling’, to the offending plant. Possibly this was uncharitable considering that only two species of the genus have a noticeably obnoxious smell, the New Zealand endemics C. foetidissima and C. crenulata. On the other hand, a friend of mine who used to live on Stewart Island remembers that she was taught to avoid mikimikis (Coprosma spp.) completely when collecting firewood, because even the seemingly inoffensive kinds were transformed into ‘stinkwoods’ by heat. The odour of page 32 C. foetidissima has been shown by Sutherland to be caused by traces of methyl mercaptan, and it would be interesting to know whether similar compounds occur in other Coprosma species, or in the related genus Nertera of which N. depressa at least smells like C. foetidissima.
Coprosma belongs to the family Rubiaceae, which includes such useful plants as Coffea and Cinchona, yielding coffee and quinine respectively; and Rubia, the madder, a source of red dye. Other New Zealand genera are Nertera and Galium. Nertera is close to Coprosma from which it is distinguished by its herbaceous habit and bisexual flowers. It grows in mats which would be hardly noticeable except that in season they bear a mass of shining orange-red fruits. A form with the popular name of ‘bead plant’ is sometimes cultivated in rockeries. Galium is a square-stemmed herb with small dry fruits consisting of two nut-like halves. An introduced Galium is well known for its ability to stick to clothes and skin by means of a covering of minute hooked hairs.
Coprosmas are all woody, but range in habit from creeping mat plants through different forms of shrub to small trees, and occur in all kinds of vegetation from the sea coast to the mountains. They have opposite pairs of leaves, interpetiolar stipules and pits or domatia can be found in all but the very small-leaved species. The stipules of Coprosma are not exactly equivalent to the paired lateral structures of the same name found at the base of the petiole in some alternate-leaved plants. In peas and beans, for instance, these are leaf-like, and could be thought of as similar in form and origin to the normal leaflets borne further out along the main axis of the leaf; but a Coprosma stipule bridges the gap between the bases of opposite leaves, and the two stipules and two leaf bases together encircle the node. In simple cases the stipules are more or less triangular, and may be either prominent or minute. They appear at every node on the plant including the cotyledonary node, in the form characteristic of the species. This interpetiolar type of stipule is probably best interpreted according to the theory of Sinnott and Bailey as a fusion of two formerly independent lateral stipules, since its venation is derived from the leaf traces on either side.
It is interesting to note that in Galium, the interpetiolar stipules are prominently expanded and leaf-like, with one or several occurring between the bases of each leaf pair, so that together the leaves and stipules form a whorl of similar organs at the node. So alike are the members of the whorl that the only reliable distinction between them is the presence of buds in the axils of the true leaves.
Figure 1: Stucture of the stipule collar of Coprosma robusta. Drawing by Jean Perry.
Figure 2: Coprosma robusta Raoul
Stipules probably always bear denticles at their upper free margin. These are stumpy or pointed outgrowths (literally ‘little teeth') which appear on close observation to be covered with a shining translucent jelly. Microscopic sections reveal the jelly to consist of elongated glandular cells packed into a dense palisade over a core of soft tissue. When the tips of the glandular cells break, their mucilaginous contents are released as a shining coating easily seen if young growing leaves are prised apart. In time this mucilage dries into a horny film and disappears. After the release of their contents the glandular cells shrivel while the internal tissue darkens and withers, reaching this stage at about the time the leaves of the same node are mature. Thus denticles, like the stipules which bear them, reach their fullest development at an early stage and their secretion probably protects the tender growing leaf tissues from desiccation. In C. serrulata and C. foetidissima the denticles are slim and could be regarded as multicellular hairs with a specialised glandular function. C. lucida and C. repens have similar but fatter, shorter denticles lacking the basal non-glandular portion of the slim ones. However, in C. robusta, C. tenuifolia, C. arborea and C. macrocarpa, the glandular region is the stipule apex itself, and although this is also usually referred to as a denticle, it is not quite the same structure as the hairlike denticles described above. C. australis stipules exhibit a particularly interesting intermediate condition, where the stipule apex is glandular as in C. robusta, but the glandular surface extends downwards and laterally to cover two rows of marginal projections which seem to correspond with the hairlike denticles of other species. The first pair of stipules on an axillary shoot is very compressed during development, and frequently the stipule nearest to the axis has a twinned apex. This is particularly noticeable in species with a simple glandular apex. e.g. C. robusta, C. macrocarpa and C. tenuifolia which have therefore been reported as having 1-2 stipular denticles. Twinned apices also occur in a similar position on axillary shoots of C. lucida and C. repens.
The leaf pits or domatia in Coprosma are shared by its relative Coffea. They occur on the underside of the leaf in the acute angle made by the midrib at its junction with major secondary veins. Their sizes and shapes include variations from wide-mouthed shallow depressions to enlarged cavities entered by a tiny pinhole. They usually produce a raised blister on the upper surface of the blade. Domatia associated with secondary veins have been reported by Shirley and Lambert as occurring very occasionally in C. repens, and have also been observed by the writer on larger shade leaves not only in C. repens but also in C. australis and C. foetidissima.
The function of domatia is still a matter for speculation. Those who hold that they are shelters for small insects would call them domatia, meaning ‘dwellings’, and point out that other genera of Rubiaceae provide domatia for ants, Myrmecodia in a corky basal tuber, and Duroia in hollow stems or leaf swellings. Certainly small insects are often found in the pits, but they could have come upon this convenient shelter by accident. Others suggest that the occurrence of bacterial nodules in the leaves of Psychotria may give a clue to the nature of Coprosma pits. The bacteria of the nodules are beneficial because they fix atmospheric nitrogen, and are carried by the host from one generation to another by means of the seed, whence they invade the apical bud of the new plant and live in a gummy secretion inside the stipular sheath, infecting each new leaf as it develops. This recalls the secretions from Coprosma denticles and led Stevenson to investigate their bacterial content. She reported that the glandular cells as well as the secretion are full of bacteria and taking this together with other evidence that C. robusta at least is able to assimilate nitrogen from the air, concluded that the bacteria might be the agents responsible. However, the demonstration is not yet absolutely conclusive, and the relationship of the bacteria with the pits is not clear. The whole question is probably best left open for further investigation.
Male and female flowers of Coprosma are normally produced on separate plants, and they are in both sexes greenish and inconspicuous, being adapted for wind pollination. The stamens of the male have long threadlike filaments on which the anthers are suspended so delicately that the slightest air movement shakes out the pollen, and the styles of the female are two narrow divergent prongs which expose a considerable area of receptive surface. The dioecious condition associated with wind pollination ensures outcrossing and also favours hybridisation.
The fruits are the most attractive part of the plant. They range from delicate pearly white through bright shades of blue, yellow, orange, red and crimson to black. Cockayne regarded C. brunnea with its translucent blue fruits hanging like beads from the brown wiry stems, as ‘an élite of the flora’ for ornamental garden work. Their colouring appears to depend on the independent occurrence of two different classes of pigment. The fat-soluble carotenoid pigments giving yellow and orange-red colouring are found in the fruit flesh, but the water-soluble anthocyanins of the skin give blue, blue-black or crimson colouring. The fruits are popularly known as berries but are not berries in the strict botanical sense. They are drupes, closely allied to plums and peaches, because each of the two seeds contained in the fruit is enclosed by a hard covering layer, the stone. The seeds are dispersed by birds, passing unharmed through their digestive processes to be planted later in an ideally fertile environment.
Key to Species
|Plant with leaves generally over 25 mm. (one inch) long||SECTION A|
|Plant with leaves about or less than 25 mm. long||SECTION B
(To appear in Part II)
Section A Large-Leaved Coprosmas
|1||Plant has a strong unpleasant smell when broken or bruised (Fig. 10).||foetidissima|
|Plant without a strong unpleasant smell.||— 2|
|2||Glandular surface of stipules not restricted to denticles but spreading well below them to form a clear diagonal boundary when observed from the side (Fig. 3).||australis|
|Glandular surface of stipules restricted to the denticles.||— 3|
|3||Stipules bear numerous glandular denticles which completely surround the stem. When a leaf is removed they can be seen in the axil.||— 4|
|Stipules bear 1 to 3 (occasionally 5) glandular denticles grouped at the stipule apex. When a leaf is removed no denticles can be seen in the axil.||— 5|
|4||Stipule sheath extremely narrow or lacking. Glandular denticles are plump and sessile. Leaf margin smooth (Fig 5).||repens|
|Stipules have a deep axillary sheath. Glandular denticles are slim and hairlike, with the glandular surface restricted to the distal portion. Leaf margin has a conspicuous colourless crenulate border (Fig. 6).||serrulata|
|5||Stipule hairless.||— 6|
|Stipule hairy on surface or margin.||— 7|
|6||Leaf margin has a colourless border fringed with fine hairs, observed most readily in young leaves. Stipules wither and break up irregularly in decay (Fig. 2).||robusta|
|Leaf margin smooth or very slightly irregular, and hairless. Stipules become paper-thin with age, and are finally cut off neatly at a transverse abscission zone (Fig. 7).||macrocarpa|
|7||Stipule hairs are longer than the glandular denticle and surround it in a white tuft. Stipules become paper-thin with age, and are finally cut off neatly at a curved transverse abscission zone (Fig. 8).||tenuifolia|
|Stipule hairs are shorter than the glandular denticle(s) and form a marginal fringe. Stipules wither and break up irregularly in decay.||— 8|
|8||Leaf blade narrows abruptly then gradually towards the base to form a tapered wing on either side of the petiole. Leaf margin sometimes undulate. The leaf is tipped by a group of short furry hairs, often carried on a small apical projection.||— 9|
|Leaf blade narrows evenly and the petiole lacks a wing. Leaf margin plane. * Leaf tip hairless.||— 10|
|9||Young stipules and nodes darkly pigmented. Midrib feels ridged on both surfaces of the leaf. Stipule bears a single glandular denticle and hairs usually occur on the face as well as the margin of the stipule (Fig. 9).||arborea|
|Young stipules have a transverse greenish band about the level of the leaf insertion which contrasts strongly with the dark colour of the stem below. Midrib feels ridged on the upper surface of the leaf only. Stipule bears 3-5 slim denticles on which the glandular surface is restricted to the distal part, and hairs occur on the margin of the stipule only (Fig. 10).||foetidissima|
|10||Midrib feels ridged on the lower surface of the leaf only. Leaf margin has a colourless border fringed with fine hairs, observed most readily in young leaves. Stipule has an axillary sheath and a single glandular denticle. Members of a stipule pair diverge from the central stem when observed from the side (Fig. 2).||robusta|
|Midrib feels ridged on both surfaces of the leaf. Leaf margin has a narrow colourless border lacking hairs. Stipule has no axillary sheath and bears usually 3 glandular denticles. Members of a stipule pair are strongly incurved when observed from the side and the denticles are close to the central stem (Fig. 4).||lucida|
Subscribers who intend to bind copies of Tuatara are advised that an index to the first ten volumes will be issued with Vol. X, No. 3.
* Some specimens of C. lucida have a few delicate deciduous hairs at the leaf tip. As they disappear very early, and are unlikely to be detected without the aid of a powerful lens or microscope, C. lucida should key out satisfactorily under 10. Similar apical hairs sometimes occur on leaves of C. repens.