GEOGRAPHICAL DISTRIBUTION OF PLANTS. 565 



sion results most distinctly from the general difference in the species belonging to each 

 of the great continents a difference never called in question in respect to the principal 

 masses of vegetation and the great aggregate number of plants which, by their situ- 

 ation at a distance from sea-coasts, and by the nature of their seeds, are removed from the 

 chances of transportation ; and secondly, from the arrangement of gregarious plants round 

 some particular foci, the individual species being spread out in various directions with 

 reference to the central points." 



In relation to the plants which are diffused through different botanical regions, we shall 

 perhaps see reason to believe that they are not indigenous in all of them, but that each 

 has been dispersed from its original seat, by briefly adverting to the natural provision 

 made to effect the diffusion. This includes the agency of air, water, animals, and man. 



The seeds of many plants are furnished with downy appendages or winglets, by which, 

 under the control of the winds, they are borne to a distance from their birth-place ; and 

 where the breezes are permanent in one direction, as in the case of the trade-winds, their 

 transportation to a far remote site may obviously transpire. The minute and almost 

 impalpable sporules of the cryptogamic species, light as the finest powder, answering to 

 the seeds of other plants, are most completely susceptible of atmospheric impulsion ; and 

 it is exactly this species that we find most extensively spread. De Candolle found two 

 species of lichen on the south-west coast of Bretagne, which had never been met with in 

 France before the Sticta crocata and the Physcia flavicans of Jamaica, which he sup- 

 posed had been brought from that island by the south-west winds. Linnaeus records the 

 striking case of the Erigeron canadense, or flea-bane, which, after being brought from 

 Canada, and introduced into the botanical gardens at Paris in the course of a century spread 

 itself over all France, Italy, Sicily, Belgium, and Germany, the wind scattering the seeds. 

 " The brown linnet, when feeding on thistle-seed, perches on the top of the weeds, and 

 tears the downy head asunder, in order to reach the seeds which are attached to the 

 receptacle. During this act, many of the grains, being loosened, are borne away on their 

 downy wings by the breeze to places far distant from the parent stem the bird being in 

 this case the indirect disseminator of the thistle. Were the head not torn asunder in this 

 manner, ten to one but it would become soaked with the rains of winter, and fall down 

 only a few inches from the original stalk, instead of being transported, as it often is, 

 across many miles of country. What is here mentioned of the linnet may be witnessed 

 in any thistlery, during some fine day in September, when the birds are feeding in flocks, 

 and scattering the down in every direction. The greater part of the seeds is no doubt 

 devoured by them, but a number also escape, a fact which the bird is well aware of, as it 

 frequently ^ives chase to the stray ones as they are borne away by the wind." Even 

 seeds which are comparatively heavy, and not furnished with wings, or downy ap- 

 pendages, are easily uplifted by the stronger aerial currents, and compelled to take a 

 journey' of several leagues by the rush of the tempest. Such storms as the hurricanes 

 that are common in tropical regions, which move at the rate of from fifty to a hundred 

 miles an hour, overturning buildings, and wrenching up the largest trees, and conveying 

 heavy fragments to a considerable distance, are potent agents in the dispersion of plants, 

 introducing the seeds of those that are peculiar to one island into another, bearing them 

 across wide arms of the sea, and assimilating the vegetation of opposite shores. 



The agency of water is another active and influential cause in the dissemination of 

 plants. " The mountain stream or torrent," observes Keith, an able writer on botany, 

 " washes down to the valley the seeds which may accidentally fall into it, or which it 

 may happen to sweep from its banks when it suddenly overflows them. The broad and 

 majestic river, winding along the extensive plain, and traversing the continents of the 

 world, conveys to the distance of many hundreds of miles the seeds that may have vege- 



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