Plants

PLANTS, Migration of. Plants need a constant change of situation because they may exhaust the amount of the special food of each species in the soil; or impart to that soil injuri ous elements ; or for the sake of increased vigor. They accomplish the needed change by many methods. Very low forms, such as the molds, spread by a creeping action; some higher plants are torn up by the roots and rolled long distances, to survive and renew their fixture and growth when favorable conditions are reached. Those that have < like the strawberry, reach out, take root at a distance and so advance by long steps, as it were; while the grasses, ferns and others make similar progress by means of underground root-stems. Dissemination is accomplished more generally, however, by means of fruit (seeds). In many cases the seed-pods are so strained by ripening that at last they burst, hurling the seeds to a considerable distance, or the pods open and the seeds are tossed out by some agitation of the plant, usually with elastic force. No sort of this machinery, however, advances the plant species more than a few feet; and far more effective is dispersal by wind and water. To this end seeds of a large variety of trees and herbs are provided with wings, like those of the maple and tulip-tree, or with long feathery attachments, like cotton and dandelion, which serve as parachutes to sustain them in the air while wafted away to new habitats. These windblown seeds are always minute and oily, the latter feature giving them the necessary lightness of weight. Floating is a means of

dispersal important not only to aquatic plants, but to others. Many river and pond-plants move bodily and are carried here and there by currents; and floods often transport living land plants to new regions. Some send abroad re generative parts, as buds; or, in more cases, seeds in capsules or little bladders, or air-tight pods. The wide distribution of the cocoanut palms throughout the tropics is due to the ability of the tight nut to float long distances. Mangrove seeds are especially fitted to do so; and that a great many other land-growing trees and plants can make long voyages, as seeds, is shown by the vegetation clothing remote islands and the way in which island floras are related to prevailing oceanic currents. That animals assist plants to migrate is well known. Many seeds arc provided with hooks that catch in hair and wool and are carried onward by pass ing animals. Birds and mammals eat fruit and drop the uninjured seeds perhaps miles away. The planting of nuts by squirrels,jays and so forth, is useful in this direction. Finally, man has been distributing plants by his operations ever since he began to move about, by the use of fruit and seeds for food, by passing them on for new orchards and grainfields from country to country; and also by the accidental carriage of them in his ships and otherwise. Every country now has many introduced species. See PLANT GEOGRAPHY.