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Show [Ch. V. AGENCY OF TilE WINDS IN TilE 74 . of sand, of ten roo ve at the rate of abo1u t sweep along grams d if th storm be very violent, at t le • 1 hour an e · l · forty mt es an . ' Th hurricanes of troptca regwns, · tles*. e rate of fifty-stx m d buildings, sweep along at d throw own which root up trees an h that for however short a . t iles an our, so ' the rate of mne Y m en the heavier fruits and .1 th may carry ev time they prevat ' ey f · derable width, and, doubt-f . 1 d seas o const seeds over r1t 18 an f 'ntroducing into islands the f the means o 1 . less, are o ten . t Whirlwinds are also m- . f d' oininO' contmen s. vegetatwn ° a J . 0 1 h vy vegetable substances to l . b rmg a ong ea stru~1enta m. ea s SliO'ht ones may frequently be observed constderable dtstance · t:> • g up haycocks into the air, fi ld · summer, carrym in our e s, m ll f f hay far and wide over the I . f 11 sma tu ts o and then ettmg a . t' es so powerful as to dry up b t they are some tm country ; u b k off the boughs of trees, and lakes and ponds, and to rea . u in a whirling column of atr. . carry them P . f his letters, that he saw, m Franklin tells us, m one o . d whirlwind which began by takmg up t~e ust Maryland,. a d . the form of a sugar-loaf wtth the which lay m the roa d' tn d on after grew to the height . d end downwar s, an so . . I pomte . ent or thirty m dtameter. t of forty or fifty :eet,.bemg ttw ryyto the wind, and although d d in a dtrecuon con ra . . a vance . f the column was surprisingly raptd, Its the rotatory motwn o ufficiently slow to allow a man to keep d progress was s . b k onwar . h . ~ ot F' ran kl'm f ol lowed lt on horse ac ' ce wit 1t on .10 • ·1 d w pa . his son for three-quarters of a mt e, an sa accompamed by ' . . d d turned round large d where tt tw1ste an it enter a woo . '. These were carried up in a spiral trees with surprtsmg fo:ce. . h . . t rrether with boughs d flymg m t e au, ob line, an were seen 1 . 1 . ht appeared hich from t letr letg ' and innumerable leaves, ": f' fl' As this cause operates d h pparent stze o Ies. reduce to t e a . t eat ortion of the at different intervals of time throughou a g~ p t only plants . b th eans of bearmg no earth's surface, 1t may e e m . 'th many other d nd their eggs, WI but insects, lan -testacea a . h h ld never otherwise species of animals, to points whtc t ey cou * Annua.ir~ du Bureau des Longitudes. Ch. V.] DISPERSION OF PLANTS. 75 have reached, and from which they may then begin to propa· gate themselves again as from a new centre . The seeds of some aquatic fresh-water plants are of the form of shells, or small canoes, and on this account they swim on the surface, and are carried along by the wind and stream. Others are furnished with fibres, which serve the purpose of masts and sails, so that they are impelled along by the winds, even where there is no current. They cannot take root until the water stagnates, or till they reach some sheltered corner, where they may live without being exposed to too much agitation from winds and currents*. The above-mentioned contrivances may enable aquatic plants to diffuse themselves gradually to considerable distances wherever there is a great chain of lakes, or a river which traverses a large continent. It has been found that a great numerical proportion of the exceptions to the limitation of species to certain quarters of the globe, occur in the various tribes of cryptogamic plants. Linnreus observed, that as the germs of ·plants of this class, such as mosses, fungi, and lichens, consist of an impalpable powder, the particles of which are scarcely visible to the naked eye. there is no difficulty to account for their being dispersed throughout the atmosphere, and carried to every point of the globe, where there is a station fitted for them. Lichens in particular ascend to great elevations, sometimes growing two thousand feet above the line of perpetual snow, at the utmost limits of vegetation, and where the mean temperature is nearly at the freezing point. This elevated position must contribute greatly to facilitate the dispersion of those buoyant particles of which their fructification consists i". Some have inferred, from the springing up of mushrooms whenever particular soils and decomposed organic matter are mixed together, that the production of fungi is accidental, and not analogous to that of perfect plants :f:. But Fries, whose authority on these questions is entitled to the highest respect, * Rev. Dr. Rennie, Essays on theN at. Hist. of Peat Moss, p. 248. t Linn., Tour in Lapland, vol. ii. p. 282. t Lindley, Introd. to Nat, Syst, of Botany, who cites Fries. |
