New Zealand Plants and their Story
Chapter IX. — The Naturalised Plants
The Naturalised Plants.
Plant colonists—Origin of the naturalised, plants—Method of arrival—Statistics —Distribution—Definition of term "weed" —Origin of weeds—Bracken, manuka, and piripiri as weeds—Weeds and human beings—The horned poppy and marram-grass—History of a pasture—Microscopic weeds—A Chatham Island orchard—The struggle between native and introduced plants —Equipment of aggressive species—Origin of a gum forest—Plant-sanctuaries —Esthetic value of acclimatised plants—Hedgerow plants—Likelihood of new weeds—Eradication of native species.
In the preceding chapters only the native plants, the true New Zealand, aborigines, have been considered. But, besides these, a host of foreigners—of colonists, if you will—have overrun the land. These have not merely settled down side by side with their antipodean relatives, but in not a few instances have driven them from the soil. That characteristic stamp which the native vegetation gives to the New Zealand landscape has frequently disappeared, and another— almost English in appearance—has come in its stead- Newcomers from the Motherland look in vain near most of the cities for any sign of a foreign land, and, judging from the plant-covering alone, might well believe themselves back in Britain. A full account of this plant colonisation and of the bitter struggle between the invaders and indigenous species would be an important contribution to science; but it can never be written, since records as to the arrival of most of the plants or their wanderings in their new home are wanting. Here only certain general principles can be touched upon and, a few illustrative details cited.
Already so widely spread and so abundant are many of the species that a beginner in the study of the New Zealand flora could not possibly tell amongst the specimens he might collect which were indigenous and which introduced. Even experts cannot agree as to the nativity of certain species, and discussions have taken place, as in the case of the wireweed (Polygonum aviculare).
The naturalised plants have come from many lands, but by far the greater part are, as in the analogous case of the human colonists, page 127natives of Great Britain and Ireland.* There are also Australians, North and South Americans, Africans, and Asiatics. Unlike the ancestors of the indigenous species, the aliens were not borne hither by winds or birds, or over ancient land-extensions now submerged; but the ships that conveyed the human immigrants or their goods brought the plants also. Some were purposely introduced for their economic (grasses, leguminous plants, vegetables, trees, &c.) or ornamental value; others came unbidden as impurities in agricultural or garden seeds, in ballast of ships,† in the hay or straw packing of goods, and in other ways. So thoroughly has the acclimatisation of these plants succeeded that there are now more or less firmly established about 530 species, some being abundant from the North Cape to Stewart Island, and quite at home even on the highest mountains.
Moreover, the species here under discussion, leaving out of consideration for the present the non-flowering plants, are a most varied assemblage, since they belong to no fewer than sixty-six families and 287 genera. Certain of these families are not represented in the indigenous flora—e.g., the poppy family (Papaveraceae), the mignonette family (Resedaceae), the valerian family (Valerianaceae), the teasel family (Dipsaceae), and some others. Most numerous of all, as might perhaps be expected, are the grasses (eighty-one species), which surpass in number even the great composite family (seventy species). Then come the pea family (Leguminosae, forty-nine species) and the cress family (Cruciferae, thirty-six species). Other fairly large families are those of the pink (Caryophyllaceae, twenty-six species), sage (Labiatae, twenty species), dock (Polygonaceae, fourteen species), buttercup (Ranunculaceae, thirteen species), rose (Rosaceae, fifteen species), potato (Solanaceae, thirteen species), carrot (Umbelliferae, twelve species), figwort (Scrophularinaceae, eighteen species), poppy (Papaveraceae, ten species), and borage (Boraginaceae, ten species). On the other hand, some families are represented by only one species—e.g., the gentian (Gentianaceae), primrose (Primulaceae), and periwinkle (Apocynaceae).
* Such species are not confined to the British Isles, but are natives of northern and central Europe as well, many being also found in Europe generally, and extending into Asia.
Amongst the introduced plants the most notorious are those known by the opprobious term of "weed." We all must know, though at the present time it is difficult to conceive such a state of affairs, that before the advent of the white man, and even for some considerable time afterwards, there were no weeds in New Zealand. Mr. T. W. Adams tells me how his land on the Canterbury Plain was at first weedless.* A little further thought, and it is plain that in any virgin vegetation weeds must be unknown, for what is a weed but merely some plant growing where it is not wanted? The mere presence of man in a new land, then, creates weeds. There is no occasion for a plant to be in itself useless to man to constitute it a weed. The best of plants, such as potatoes, if growing unbidden in a flower - garden may become weeds for the time being. Even in a New Zealand taxad forest all the trees not required for milling purposes are weeds, or a forest occupying ground intended for grazing is a distinct weed association.
* Of course, there would, be plenty of introduced plants, but he meant those kinds which overrun the land, and so become a nuisance.
It is only in a virgin vegetation that we can actually witness the evolution of a weed. Our own flora has furnished some rather striking examples. The common bracken-fern (Pteridium esculentum) is a case in point. This plant can be transported over great distances by means of its tiny spores, which, light as the finest dust, are carried for many miles by the wind. It has a stout far-creeping underground stem full of nutritious starch. Consequently, when the farmer burns its leaves and the grasses and other plants in its neighbourhood, the stem remains unhurt beneath the ground, and from its store of food can soon construct fresh leaves. Owing to their form and internal structure, these are capable of enduring considerable drought, so that the plant need not be at all concerned about its water-supply; consequently, all things considered, the bracken is a fair example of a potential weed. Nor does it belie this expectation, for at the present time this fern, worthless from the farmers' point of view, is far more abundant than in primeval New Zealand.
So, too, with the manuka (Leptospermum scoparium), thanks to its abundant and quickly germinating seeds, its hard seed-capsules that are not easily destroyed by fire, the early blooming of the seedlings, its leaf-form and structure, its toleration of most varieties of soil, and its extreme plasticity with regard to changes of environment. Miles of manuka now exist where originally forest or a mixed shrubbery flourished.
* Really a dry fruit which contains one seed, and does not split open at maturity. The calyx, which is furnished with several barbed bristles, remains attached to and encloses the fruit proper.
Other native plants, though not actually aggressive, hold their own on grazed land owing to certain qualities they possess—e.g., species of Geranium (low growth, long roots); Oxalis corniculata (low growth, spreading habit, quick germination); species of Cotula (low, turf-forming habit and far creeping and rooting stems); Senecio bellidioides (rosettes close to ground, wind-borne "seed," deep roots); Coriaria ruscifolia* (poisonous, much - spreading underground stem which puts forth shoots when the plant above ground is destroyed by fire).
The term "weed" is evidently merely relative, and depends upon the plant in its relation to man. If we leave the human element out of the question, a weed is simply a living organism, like any other plant or animal, and its habits and structure are entirely for its own benefit, just as are the organs of all animate beings. In itself there is nothing noxious at all, nor in an undisturbed plant society would it react upon its neighbours more than any other plant.
The flourishing "weed" of civilisation, so far as the plants with which it comes into contact are concerned, is much the same as is civilised man in relation to the savage. In an environment of wild nature, as a hunter with rude weapons of stone, bone, or wood, and inured to cold and hunger, the latter is in a far better position than the European under like circumstances. But should the civilised man, armed with the arts and under the surroundings of civilisation, come in contact with the savage, the latter is rapidly displaced.
Nor is the colonisation by introduced plants very different from human colonisation. Some plants, through their special favourable qualities and adaptability—in other words, through their power to make the best possible use of their circumstances—outdistance their fellows, and establish themselves far and wide, living in great security, and growing with a luxuriance not attainable in their mother-land.
* This becomes aggressive in certain localities.
Others flourish, it is true, but, lacking adaptability, are narrowly restricted to a definite and well-defined station out of which they cannot go. On the shores of Cook Strait the horned poppy (Glaucium flavum) is confined to gravelly and stony beaches. On the clayey hillside near Lyall Bay is a fine colony of this striking plant, with its silvery foliage and delicate yellow flowers; but it is strictly limited to a spot where a quantity of gravel and coarse sand has collected. The well-known marram-grass (Ammophila arenaria) can grow only where there is drifting sand. Cut off the supply, and the green leaves quickly assume a sickly yellow hue, and, if no more sand comes, the plant will eventually die. Certain species may become established in a few places, but can spread no farther. Especially is this the case with the plants of old gardens, which may linger for years, but, failing to reproduce their kind, finally die out. Lastly, many species thrive well for a time, but are eventually eradicated by more vigorous competitors.
Nowhere can this last example be better seen than in pasture land. At first the-newly laid-down grass may consist of valuable rye-grasses (Lolium perenne, L. italicum) and clovers. In time, owing to its perennial habit, the first-named will overcome the annual species, which is unable to reproduce itself freely from seed on the closely occupied ground. The adult plant, too, dies at the end of the year. Next, through the action of cattle, sheep, and horses, the perennial rye-grass is eaten to the ground, while those grasses worthless for feed which have crept in, such as the soft brome-grass (Bromus hordeaceus) and Yorkshire fog (Holcus lanatus), increase at a great rate, their flowers being untouched and their reproduction not hindered. Finally, the pasture deteriorates so much that it must be broken up and laid down afresh.
Before leaving the question of weeds, those minute organisms must be mentioned which, settling down upon other plants and living as parasites, damage and not infrequently kill the host. To this category belongs that vast assortment of non-flowering plants commonly termed "blights." Many of these are members of the great family of fungi. For the most part they are more or less invisible without the aid of the microscope, but their presence is often writ large on the unfortunate host-plant. The rusts and smuts, the so-called Irish potato-disease, the organism causing "damping off" in seedlings, and many and diverse causers of plant-diseases belong to the "blights."page 132
Right at the north-east corner of Chatham Island is an old orchard planted before the middle of the last century. At the time of my visit, some years ago, I was astonished to learn that blights of all kinds were quite unknown there, the extreme isolation of the orchard having proved its salvation. And so, too, it was with New Zealand orchards in the early days, before the coming of the microscopic weeds. Many of the ills which afflict humanity are due to the presence of extremely minute introduced plants known as bacteria, the "microbes" of the journalist, and so various diseases now common in New Zealand were absolutely unknown to the Maori.
From the plant-geographical standpoint, of particular interest are any facts bearing on the struggle between the introduced and indigenous species. The primary point to insist on is that so long as the surface of the soil is left intact—that is, if the primitive plantcovering be quite undisturbed—it is very hard indeed for the world's selected weeds—even those best equipped for aggression—to gain a foothold, and it is almost impossible for them to spread. On the Snowcup Range, in Canterbury, there is a, clear line of demarcation, between the meadows which are virgin and those where sheep are pastured and burning is periodical. On the latter are introduced plants in abundance, side by side with tussock-grasses, gentians, and Spaniards; but on the former there are merely the celmisias, buttercups, groundsels, and other mountain plants, the foreign invaders being altogether absent. And yet the native dandelion, which is closely related to the introduced species, is in abundance in many places, and the violent westerly winds must be charged at times with the seeds of certain weeds.
Ruapehu, the upper portions of Egmont, and the mountains of Stewart Island and elsewhere, especially on the west of the South Island, are weedless. So, too, are the virgin forests. Disturb the ground, however, and at once a seed-bed is ready, and the foreigners pour in. Burning makes more space, and they spread and increase, the native plants decreasing in proportion, and going to the wall. Grazing animals assist in the destruction. The native plants having come into being in the absence of such animals, the moa excepted, are little protected against their attacks. At the same time, the special equipment of the more aggressive introduced species of plants makes these formidable. Many are annuals—a great advantage for rapid dispersal—whereas almost all the indigenous species page 133are perennials. Rapid vegetative increase by means of creeping and perhaps rooting stems, the formation of a dense turf, rapidity in germination of seeds, enormous-seed-production, special contrivances for seed-dissemination by the wind, tolerance of extreme changes of soil and climate, leaves pressed closely against the ground, deeply descending roots, immunity against attacks of animals afforded by a woolly covering, unpleasant taste, &c., conspicuous flowers for insect fertilisation—all these and other beneficial characters are frequent amongst the acclimatised species.
A plant may remain quite isolated, for years and be apparently incapable of spreading, but an unlooked-for change of conditions may give it just what it requires. Dr. Truby King pointed out to me a most interesting case. At Waitati, on the land belonging to the Mental Hospital, stands a fine example of the stringy-bark (Eucalyptus numerosa) more than fifty years of age. Originally the vegetation of the place was mixed forest, but this has been replaced by a close growth of manuka heath. Some ten years ago this was burned in the neighbourhood of the tree, and a young forest of gums several acres in extent has sprung up (fig. 59), the new ground and the potash from the fire being eminently suitable for the germination of the gumtree seeds. At the present time the gum-saplings grow extremely closely. Their height is from 40 ft. to 50 ft. Some are half a foot in diameter, while others are extremely slender. Thousands of manuka seedlings sprang up along with those of the gum; and it must not be forgotten that manuka, far more than most of the indigenous plants, can reproduce itself again and again after burning, and can exclude almost all other vegetation. But in this case the greater rapidity of growth gave the gums the victory, and now only a little manuka remains near the margin of this remarkable and quite natural forestgrowth. Nor is the above merely interesting biologically: it is equally important from the point of view of cheap afforestation of unproductive areas covered, with manuka heath.
From what has gone before it must not be inferred that the acclimatised vegetation has no esthetic value. Highly cultivated lands, with their green meadows, waving grain, and stately plantations of foreign trees, are delightful. The gorse, noxious weed though it may be proclaimed, is a glory when its sheets of gold dazzle the eye. The green lines of self-planted willows on many a river-bank; the yellow lupin of the dunes; blue periwinkles by the roadside; white arum lilies on wet ground in the north; stately mulleins on montane riverbeds —these and many more are well worthy of admiration.
A rather curious fact that seems to need explanation is the absence of those hedgerow plants that make delightful the lanes of England. It is not that the prettiest of the flowers of the Mother-land are absent, though many appear to think this is the case. Some perhaps are to be seen only in our gardens—e.g., the wood-anemone, the wild hyacinth, the lily of the valley, primroses, cowslips, the bluebell of Scotland or harebell, and the daffodil. But the germander speedwell, the herbrobert, deadnettles white and pink,. daisies, buttercups, the raggedrobin, the stonecrop, wild roses, blackberries, the, honeysuckle, the forget-me-not, the toadflax, the bird's-foot trefoil, the dog-daisy, the bugloss, and the foxglove, to mention only a few British plants, are now wild in various places. The fact is, our hedges are frequently of gorse, a plant which smothers out most herbs with which it comes in contact. Introduced grasses also grow with a vigour unknown in Europe, and will not permit such plants as the primrose to become established. Cattle, too, graze by the roadside. In short, the conditions are quite different from those afforded by the mixed hedges of the Old Land.
No further details can be given regarding individual acclimatised plants. A full list is given in Cheeseman's excellent Flora (pp. 1063 to 1093), and descriptions of most may be found in any British Flora. For those living in settled parts of the Dominion, where the indigenous plants are scarce, the introduced species will afford much material for profitable study, the purpose of which should be not to find their names merely, but to continue such observations as this and previous chapters may have suggested.
It may be asked whether there may not be introduced some day other plants which might become dangerous pests. So far as animals are concerned, the experience of New Zealand towards acclimatisation has not been encouraging, and it is wise to consider long and care-page 136fully, and get the best advice available, before turning any animal loose. Doubtless the same reasoning applies to plants; but, unfortunately, they do not wait to be set free. Probably all the worst weeds of the earth have already arrived, and we cannot expect newcomers that would rival the couch-grass (Agropyron repens), the sorrel (Rumex Acetosella), or the so-called "Californian thistle"* (Cnicus arvensis).At the same time, any farmer who sees a new plant on his farm should look at it askance, obtain what information he can get respecting it, and eradicate it at once if he has any reason to suspect it of having the weed-assuming characteristics.
Finally, there comes in the question whether any of the native plants are liable to extinction. Personally, I should answer this in the negative. There is nearly always some haven of refuge, and, though, many species will eventually become much more rare, it is most unlikely that any will be entirely eradicated.
* This plant is a native of the British Islands, Europe generally, north Africa, and northern, to western Asia. It is only naturalised in the United States, where, however, it is called the "Canada thistle." In England it is known as the "cornthistle" and "creeping thistle," and is a weed of either cultivated or waste ground.