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Page:Popular Science Monthly Volume 17.djvu/379

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ON THE MODES OF DISTRIBUTION OF PLANTS.

ON THE MODES OF DISTRIBUTION OF PLANTS.
By JOSEPH F. JAMES.

THE study of the geographical distribution of plants over the earth is one of the most profound interest, not only to the botanist but to mankind in general. To the former it is of especial interest on account of the intimate relations existing between it and the origin of the different species of plants. Where we find an isolated example of a group of plants existing in one country, while its nearest congeners are in another perhaps thousands of miles off, we naturally feel interested in trying to discover the cause of this wide separation, and the means by which the plant has reached its present location. It is to the means of distribution that we shall devote this paper.

It is a trite remark that although there may be places identical in temperature, in soil, in humidity, and other circumstances governing the stations of plants in both North America and Europe, and in South America and Africa, still it does not necessarily follow that the species of plants in these identical localities are alike or even at all similar. Indeed, researches show it to be rarely or never the case. In almost every country, however, there seems to be a certain though sometimes a small proportion of plants which are found in other and distant parts of the world. For instance, Mr. Brown found that, out of 4,100 species of plants then known to inhabit Australia, 166 were identical with those of Europe, and that the greater proportion of these were cryptogamous plants, while those that were not were plants common to the intervening regions.[1] There are 359 indigenous plants out of the 2,277 phænogams given in the last edition of Gray's "Manual," which are also indigenous to Europe. A number, too, of the plants of eastern North America are common to northeastern Asia, China, Japan, and India. Out of a collection of 600 plants from the river Congo in Africa, Dr. R. Brown found thirteen which also grew on the opposite coasts of Brazil and British Guiana.[2] No less than one fifth of the algæ from the Antarctic seas, exclusive of the New Zealand and Tasmanian groups, have been identified by Dr. Hooker with British species.[3] A few of the most remarkable cases of distribution of identical species will no doubt be of interest here. Sauvegesia erecta grows in the Antilles, in Brazil, in Madagascar, and in Java; Scirpus maritimus grows in North America, in Europe, in western India, in Senegal, at the Cape, and in Australia;[4] Brasenia peltata grows in the United States, in Japan, in eastern India, and in

  1. Lyell, "Principles of Geology," ii., p. 387.
  2. Lyell, ibid., ii., p. 394.
  3. Lyell, ibid., ii., p. 359.
  4. Jussieu, "Elements of Botany," p. 718.