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Show 76 DISPERSION OF PLANTS [Ch.V. f this argument ·m £a vo ur of th.e old has shown the fallacy 0 . . ,, The sporules of fungi," . cal ueneratlon. . . 'd doctrine of eqmvo o . fi 'te that in a single mdiVI ual says this natural ist ' " are so 111 m 't ed above ten mi.l h·o ns, an d . · a I have coun of Reticulana maxim ' . 'bl often resembling thin smoke; 'l t be scarcely VISl e, . . so subtl e as 0 • d erhaps by evaporatiOn mto h t th y may be ra1se P so light t a e . d . so many ways by the attrac-h e and d1sperse m h the atmosp er ' . . d lasticity adhesion, &c., t at by msects, wm ' e ' tion of the sun, . 1 . from which they may be ex-it is difficult to conceive a P ace eluded." . . the next place, to the instru- . our attentiOn, m In turmng t of dispersion, we cannot do l't f the aqueous agen s . menta 1 Y 0 . t tl wor d s of one of our ablest botanical wnters . better than C1 ~ 1e rrent " observes Keith, " washes ,, 'l.'he mountam-stream or to . 'h 'dentally fall into h d wh1c may acc1 down to the valley t e see st weep from its banks when it h' h 't ay happen o s it, or w IC I m h The broad and majestic rivet·, dd nly overflows t em. . su e . 1 · and traversing the contments winding along the extensi~e Pd~:;~nce of many hundreds of miles of the world, conveys l to t :egetated at its source. Thus the the seeds ~hat m;yth1:v;altic are visited by seeds which grew southern s 1~res o . and the western shores of the in the interiOr of Gh er~any been generated in the interior of Atlantic by seeds t at ave . d' t America and A . '*' " Fruits moreover' m 1genous o menca • ' tl t of the Mi·m osa scan de ns, the the West Indies, sthiCh asl 1a been known to be drifted across h t nd ot ers 1ave cas ew-nu_, a the Gulf-stream, on the western coasts of the Atla~tic bi a state that they might have vegetated ha_d Europe, m sue bl Among these the Gut· the climate and soil been favoura e: . rl mentioned, landina Bonduc, a leguminous plandt,;s padrti~U~~e ~est coast of · b · d from a see 1oun ° as havmg een raise . 1' that the lenticula d s· H ns Sloane lDlOrms us Irel~n t . 11' a b which is frequently cast ashore on manna, or sargasso, a ean n the rocks the Orkney isles, and coast of 1' Irela~d,thgro:: ~s sometimes about Jamaica., where the sunace o e "' System of Physiological Botany, vol. ii. P· 405. t Brown, Append, to Tuckey, No, y, P· 481. Ch. V.] BY WINDS AND CURRENTS. 77 strewed with it, and from whence it is known to be carried by the winds and currents towards the coast of Florida ,y;.. The absence of liquid matter in the composition of seeds renders them comparatively insensible to heat and cold, so that they may be carried, without detriment, through climates where the plants themselves would instantly perish. Such is their power of resisting the effects of heat, that Spallanzani mentions some seeds that germinated after having been boiled in water t . . When, therefore, a strong gale, after blowing violently off the land for a time, dies away, and the seeds alight upon the surface of the waters, or wherever the ocean, by eating away the sea-cliffs, throws down into its waves plants which would never otherwise approach the shores, the tides and currents become active instruments in assisting the dissemination of almost all classes of the vegetable kingdom. In a collection of six hundred plants from the neighbourhood of the river Zaire, in Africa, Mr. Brown found that thirteen species were also met with on the opposite shores of Guiana and Brazil. He remarked, that most of these plants were only found on the lower parts of the river Zaire, and were chiefly such as produced seeds capable of retaining their vitality a long time in the currents of the ocean. Islands, moreover, and even the smallest rocks, play an important part in aiding such migrations, for when seeds alight upon them from the atmosphere, or are thrown up by the surf, they often vegetate and supply the winds and waves with a repetition of new and uninjured crops of fruits and seeds, which may afterwards pursue their course through the atmosphere, or along the surface of the sea, in the same direction. The number of plants found at any given time on an islet affords no test whatever of the extent to which it may have co-operated towards this end, since a variety of species may :first thrive there and then perish, and be followed by other chance-comers like themselves. Currents and winds, in the arctic regions, drift along ice- "'Phil. Trans, 1696, t System of Phil~sophical Botany, vol, ii. ll· 403, |