PLANT ACCLIMATISATION IN NEW ZEALAND.

Otago Witness, Volume 26, Issue 2419, 26 July 1900, Page 8

PLANT ACCLIMATISATION IN NEW ZEALAND.

% The following is the concluding poition of Mr G-. ,-M. Thomson's paper on' the.abo.ve subject read before the Otago Institute:' —

I 3. In regard to - the introduction of ■ naturalised- - plants by . accidental means I—viz1 — viz, (1 along with seeds, 'hayv, straw, ;,<etc.j in the -soil surrounding other introduced \ plants, by animals,, .in ballast of ships, or in {.any other chance manner, this is certainly the source or mode in which by far the greatest number of such alien's have come to these islands. Agricultural seeds are especially responsible for the majority of our most common weeds, and I have given an example of this in the first paper read by me before 1 this institute. (N.Z. Institute Trans., vol. IX, p. 538.) It is evidently the case that weeds of cultivation, such as duckweed, shepherd's purse, and groundsel, must have developed their special characteristics within comparatively recent times. That is to say, they have developed them pari passu with the development of cultivation by the human race. This, from the naturalist's point of view, does not point to any great antiquity.

An examination of any list of the naturalised plants of a district — for example, that of the plants of Port Nicholson made in 1878 by the late Mr Kirk (Trans. N.Z. Insfc., Vol. X, p. 362), or that of our own immediate neighbourhood issued a couple of years ago by the Dunedin Field Club, — reveals certain interesting general facts. Thus all, or nearly all, are capable of self -fertilisation, if they are not habitually self-fertilised. If one looks at the weeds in any ixnkept bit of garden ground at the present midwinter season (July), they will probably find some or all of the following species j>roducing seed in abundance from flowers which never open, and which are more or less imperfect in structure : Shepherd's purse (CJapsella bursa-rjastoris),' winter, cress (Barbai'ea, vulgaris), bitter cress (Cardamine hirsuta), hedge mustard (Sisymbrium ofEcinale) , - wart - cress (Senebiera didyma), duckweed (Stellaria ... media), groundsel (Seveeio vulgaris), sowthistle. (Sonchus oleraceus), 'spurge (Euphorbia pephis), and ' others'. ~ This faculty of producing more or less imperfect self-fertilised flowers is almost an essential feature in all suoh plants," many of which are thus enabled to produce fruit at all seasons of the year 5 , and almost independent of ' the weather. It is perhaps the most characteristic feature about them. Another point is that most of them produce very small and very numerous seeds ; and still another that a largo proportion of them come to maturity very rapidly, and that their Feeds germinate quickly. These characters are all retrogressive from one point of view — that is to say, the plants exhibiting them have tended jo become less, instead of more, specialised in their development; but by this degradation of their reproductive organs they have really become better adapted for the peculiar conditions which are imposed upon them in their struggle with the gardener and agriculturist.

4. I am not aware that any plants natviralised in New Zealand within historic times have been introduced by means of either wind, birds, or insects, though there is no inherent improbability in this mode of introduction. Many of us can recall the sultry day a few years ago when the sky was darkened, and the sun became lurid, from the dense smoke of Australian bush fires, and it is quite clear that light Seeds and spores might very easily be carried along at considerable elevations by a similar westerly wind. Many of our native species of flowering plants are either identical with or closely allied to Australian forms, and this is "particularly the case with "such plants as the pappus-bearing composites—e.g., Cehhisia longifolia, "Craspedia fimbriata, and C. alpina, several species of Erachtites, etc, — and the various species of Epilobium (E. confertifolium, E. glabellum, E. junceum, et?e.), which have a tuft of hairs on the seeds.

A large proportion of the plants which have succeeded in establishing themselves in this country belong to what Sir Joseph Hooker has called the Scandinavian Flora, the aggressive and colonising power of which has been dwelt upon by him, by Charles Darwin, and by A. R. Wallace. Darwin's explanation, it may be remembered, is that this power of colonising is due to the development of these plants in the most extensive land area, of the globa, where competition has bceri 'most severe and long-continued. A discussion on this subject will be found in tho chapters ■dealing with the flora of New Zealand in Wallace's " Island Life " ; said much additional information is also given in hij3 "Darwinism. '>* > . ' -

_ There are one or two aspects of .this question of plant naturalisation whioh have not yet received much "• v^tigation, though they are very interest!

Often when a species is first introduced into a country or a district, it exhibits most extraordinary vitality for a time, and then appears gradually to lose its exuberance of growth, and to assume a more normal rate of individual development. It is as if the restrictions which formerly kept it within certain limits had been removed, and it sprang with a bound to a height of vigour which it was not able to maintain, and then had gradually fallen back to a level at which it could maintain itself. Anglers will recall the marvellous rapidity with 'which trout grew when they were first put into our streams. Tlie food supply was practically for them- unlimited, and they increased most remarkably in size and weight. Bufc succeeding generations have not been able to keep up the. same phenomenal rate of growth, for not only is the food supply diminished, but the young fish have to' run the gauntlet of the old ones, which are their worst enemies. This factor does not, of- course, enter to anything like

the same extent into 'i.o relations of plants to one another 1 , bufc it is to be- taken into account in considering that well-established plants- are most formidable rivals to' the seedlings of their own kind round about them. A somewhat analogous carfe seems to be that of Iho humble bees. During the first few years after their liberation in Canterbury these insects mci eased enormously in numbers, p.nd beekeepers frequently expressed the opin- ' hi that they would soon crowd the hive bee out of existence. But as far as I can. make out, this rate of increase has not been maintained, and these insects are now by no means troublesome on account of their numbers.

The same phenomenon has been witnessed in the case of some plants. The marvellous growth of watercress in the streams of the Canterbury Plains, producing as it did stems of 12ft in length and. three-fourths' of an inch in diameter, has often been adduced. Biit I do not think that this huge type of growth has been maintained, though I am open to correction on this point. As far as I have seen it about Christchurch, the plant seems to grow larger than the parent, plants in Britain, but it does not attain its former recorded dimensions.

When the Oamaru country was fir&t ploughed, the common thistle (Carduus lanceolatus) took absolute possession of the soil. I remember in 1872 walking through many hundreds "of acres on the Balruddery- and Elderslie properties, between, the Waiare'ka and tha Kakanui, and the, only available track wa;s~on the -dray ruts, and even there the thistles were waist deep, and on both sides they formed a wall 6ft or 7ffc high. During the first year or two of occupation not a blade of grass or other plant could show itself, but afterwards the ground seemed to become somewhat sick of thistles. Meanwhile the soil was enriched by the plentiful crop of vegetable matter which' was produced •on it and ultimately worked in again, the subsoil was penetrated by the roots which decayed in it, and thus helped to decompose and break it up, and on such soil 50 and 60 bushels of wheat were taken off per acre immediately after the thistle crop. No doubt numerous other cases of the same thing could be adduced, and it would be interesting to find out whether such individual development anywhere tends to be maintained, and if so whether it tends to the production of any permanent variety. I think it possible that the development in the pant of such large spe_cies of plant as Myosotidium nobile, Aciphylla squarrosa, and A. eolensoi, Ranunculus lyallii, Liqusticum litifolium, and Pleurophyllum criniferum, all of which are giants as compared with their nearest relations, is due in great part to their isolation in these islands and the comparative absence of severe competition. '

Another curious change which has been noticed as taking place is the adaptability of some of our indigenous plants tc the changed conditions brought about by settlement." Some, of the native species appear to be able to hold their own, and even to benefit by these altered circumstances. I have already recorded the fact that, with the increase of blackbirds and thrushes, many succulent-fruited plants have become widely dispersed. ° This is true also of some native species. Fuchsias are increasing, not diminishing, in numbers in our Tawn Belt, and in my garden I find a species of Coprosma (o. ' robusta) and the common cabbage tree (Cordyline Australis) coming- up where none were sown, and where I do nofc remember any growing naturally for "some 25 years. The fruit of the latter is often eaten by starlings, and thus distributed. Again, some creeping plants furnished with rooting stems or underground stolons are able to .spread in cultivated ground. and in pasture. I have noticed three species particularly aggressive — -viz., Epilobium nummularifolium. Hydrocotyle Asiatica, and H. muscosa.

* At p. 29 (2nd edition, 1889), Wallace quotes Mr W. L. Travers, and apparently on his authority states that " the most noxious weed in New Zealand appears, however, to be the Hypochaaris radicata, a coarse yellow-flowered composite not uncommon in our meadows and waste places. This has been introduced with grass seeds frorrr England, and is very destructive. It is stated that excellent pasture was m three years destroyed by this weed, which absolutely displaced every other plant on the ground. It grows in every kind of soil, and is said even, to drive out the white clover, which is usually &o powerful in taking possession of the &oil." This is a rather over-stated case. The Hypochasris or Cats-ear, usually misnamed Cape weed with us, is only troublesome where &hcep cannot get at it. But on sheep runs and farms on which sheep are fed, the plant quickly disappears, as these animals eat it right to the ground, and so completely eradicate it.