MIGRATION OF PLANTS. The natural movement of plants from one area to another. This term is thus somewhat in contrast to natu ralization (q.v.). Aligration of species is possi ble by reason of variations in the structural adaptations which are found in most plants. Among these perhaps the most important are the structures which are concerned in the dis persal of seeds and spores, which may be seat tered by means of birds, wind, or water to very great distances. Besides this type of dispersal. certain plants may be scattered in a vegetative way for great distances and become established far from the original home. Thus the common water weed Elodea is believed to have spread all over Europe vegetatively from a single plant introduced in the early part of the nineteenth century. It seems likely that after a time a certain equilibrium between the various species of the world will have been reached, and that each particular species will be found in the con ditions best suited to it. When this time conies, further migration would seem to be without re sult, whether the scattering of seeds takes place or not. It must be remembered, however, that various factors enter in to disturb any equilib rium which may be formed. In the first place.
there are changes in the organic world itself: that is, new species of both plants and animals are developed from time to time through evolu tion and new disturbances in the so-called equilib rium must arise. In the second place, the ex ternal world suffers tremendous changes. It is probably this last cause which has been most responsible for the migration of plants. Through the geological ages continents have arisen and passed away, and all the-se changes must have been accompanied by changes in the mutual rela tions of the species then living. Changes in climate have taken place many times in the world's history, and all of these changes must have been attended with great plant migrations. To illustrate: the oncoming of the ice epoch caused a southern movement of the climatic zones, and the species which had become adapted to a particular climate moved south to a greater or less degree pari passu with the climatic move ment. Post-glacial times have witnessed north ern migrations which are necessarily much slower than the northern migrations of the southern zones. Plants must not be regarded as less active migrants than animals. though they make no seasonal migrations.