OCR Text |
Show 472 ISLAND LIFE. [PART II. -not a bl y m. th e case of tho Azores and Bermuda. The .c ha-. racter of the plants is also just what we should ox?ect' for a b ou t t wo-thn. · d s of th e m beloncor to oa enera of world-wide. ra.n ge in the temperate zones, such as Ran'ttncullts, Drm~eTa, Ep~lobLwn, G z z· s · n 0nvolv1dtts AtTiplex Luzula, and many napt~a ~um, enecw, u, , . ' . . .. so cl o·es an d g.r asses, wh ose exceptional1y w1de distnbutwn. shows tha~ they possess exceptional powers of disp~rsal and v1¥our of cons tI' tu t1' 0n, enab lI' noa tllein not only to reach distant countnes, but also to establish themselves there. Another set of plants belong to especially Antarctic or south temperate groups, such ~s Colobxntlw. s, A cam a, Ga/ultheTia, PernettJ;a, and }jl[ uhle~beckw, and these may in some cases have reaehecl both A.us~raha and N~w Zealand from some now submerged Antarctic Island. Agmn, about one-fourth of the whole are alpine plants, and these possess two advantages as colonisers. !heir lofty. stations place them in the best position t~ have tbmr seeds earned a~vay by winds; and they would in this case roach a country -v~hlCh, havincr derived the earlier portion of its flora from the s1de of the t;opi.cs, would be likely to have its higl1cr mountai?s and favourable alpine stations to a great extent unoccupwcl, or occupied by plants unable to compete with specially ac.bpted alpine groups. . . Fully one-third of the exclusively Australo-New Zealancl species belona to the two great orders of the sedgeR and the grasses ; and there can be no doubt that these have great facilities for dispersion in a variety of ·ways. Their seeds, often enveloped in chaffy glumes, would be carried long distances by storms of wind, and even if finally dropped into tl1e sea wonld have so much less distance to reach the land by means of surface emrents; and :M:r. Darwin's experiment'3 show that even cultivated oats germinated after 100 days' immersion in sea-water. Others have hispid awns by which they would become attachoLl to the feathers of birds, and there is no doubt this is an effective mode of dispersal. But a still more important point is, probably, that these plants are generally, if not always, wind-fertilised, and are thus independent of any peculiar insects, which might ~o wanting in the new country. lVhy easily-dispeTsed plants have often Testrietccl ranges.- CIJAP. XXII.] TUE FLOHA O:B' NE\V ZEALAND. 473 This last consideration throws light on a very curious point, whi ch has been noted as a difficulty by Sir Joseph Hooker, that plants ·which h:we most clear and decided powers of dispersal by wind 9r other means, have not generally tho widest specific range ; and he instances the small number of Compositro common to Now Zealand and Australia. Dut in all these cases it will, I think, be found that n.lthough the species have not a wide range the genera often lw,ve. In New Zealand, for instance, the Composita:J arc very abundant, there being no less than 148 species, almost all belonging to Australian genera, yet only nine species, or less than one-sixteenth of the whole, are identical in the two countries. The explanation of this is not difficult. Owing to their great powers of dispersal, the Australian Compositro reached New Zealand at a very remote epoch, and such as were adapted to the clim.ate and the means of fertilisation established themselves ; but being highly specialised plants with great flexibility of organisation, they soon became modified in accordance 'vith the new conditions, producing many special forms in different localities ; and these, spreading widely, soon took possession of all suitable stations. Henceforth immigrants from Australia had to compete with these indigenous and wellestablished plants, and only in a few cases were able to obtain a footing; whence it arises that we have many Australian types, but few Australian species, inN ew Zealand, and both phenomena are directly traceable to the combination of great powers of diRpersal with a high degree of specialisation. Exactly the same thing occurs with the still more highly specialised Orchidero. These are not proportionally so numerous in New Zealand (thirty-eight species), and this is no doubt due to the fact that so many of them require insect-fertilisation often by a particular family or genus (whereas almost any insect will fertilise Compositre), and insects of all orders are remarkably scarce in New Zealand. This would at once prevent the establishment of many of the orchids which may have reached the islands, while those which did find suitable fertilisers and other favourable conditions would soon beco)ne modified into new species. It is thus quite intelligible why only three species of orchids are identical in Australia and New Zealand, although their minute |