OCR Text |
Show 360 GEOGRAPHICAL DISTRIBUTION. CHAP. XI. per diem); on this average, the seeds of ( 0 4 0 plants belonging to one country might be floated across 92~ miles of sea to another country ; and when stranded, If blown to a favourable spot by an inland gale, they would germinate. . Subsequently to my experiments, M. Martens tned similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea: so. that they were alternately wet and exposed to the au~ hke. really floating plants. He tried 98 seeds, ~ostly ~ffere.nt from mine· but he chose many large fruits and hkewise seeds from' plants which live near the sea; and this would have favoured the average length of their flotation and of their resistance to the injurious action of the salt-water. On the other band he did not previously dry the plants or branches with the fruit ; and this, as we have seen would have caused some of them to have floated much' longer. The result was t h at 9188 of l1'1 s seeds floated for 42 days, and were then capable of germination. But I do not doubt that plants exposed to the waves would float for a less time than those protected from violent moven1en t as in our experiments. Therefore it would perhaps be safer to assume. that the seeds of about J.JL plants of a flora, after having been 100 00 il dried, could be floated across a space of sea 9 m es in width, and would then germinate. The fact ~f t?e larger fruits often floating longer than the sma~l, IS Interesti~ g; as plants with large seeds or fruit could hardly be transported by any other means; and Alph. de Candolle has shown that such plants generally have restricted ranges. . . But seeds may be occasionally transported In ~nother manner. Drift timber is thrown up on most Islands, even on those in the midst of the widest oceans ; and the natives of the coral-islands in the Pacific, procure CHAP. XI. MEANS OF DISPERSAL. 361 stones for their tools, solely from the roots of drifted trees, the.se s.tones being a valuable royal tax. I find on examination, that when irregularly shaped ston are embedded in the ·roots of trees, small parcels ~~ earth are very frequently enclosed in their interstices and behind them,-so perfectly that not a particle could be washed away in the longest transport: out of one s~all portion of earth thus completely enclosed by wood In an oak a?out 50 years old, three dicotyledono~ s plants ge.rmina ted : I am certain of the accuracy of this observation. Again, I can show that the carcasses of ?irds, when floating on the sea, sometimes escape .bein~ immediately devoured; and seeds of ma~y l~In~s In . the crops of floating birds long retain their VItahty: peas and vetches, for instance, are killed by even a few days' immersion in sea-water; but some ta~en.out of the crop of a pigeon, which had floated on artifi~Ial salt-water for 30 days, to my surprise nearly all germinated. Livin? birds can hardly fail to be highly effective agents In the t:ansportation of seeds. I could give many facts showmg how frequently birds of many kinds are blown by gales to vast distances across the ocean We may I ~hink safely assume that under such circum~ stances thmr rate of flight would often be 35 miles an hour ; and some authors have given a far higher estimate. I ~ave never seen an instance of nutritious seeds passing through the intestines of a bird· but hard .seed~ of fruit will pass uninjured through' even the digestiv~ organs of a turkey. In the course of two months, I picked up in my garden 12 kinds of seeds out of the excrement of small birds, and these seemed perfect, and some of them, which I tried germinated bB. ut the :D0 II OW·i ng fact is more important :' the crops of· rrds do not secrete gastric juice, and do not in the R |