How Plants Travel
( Originally Published Early 1900's )
RAMBLING over the South Downs, in England, in late summer or autumn you may often see the short, smooth turf veiled with a soft, moving mist. It is the down loosened from the ripened heads of the little dwarf plume thistle floating about in the breeze. So it sails along hither and thither, until it finds a resting place where it may grow and repro-duce its kind. Sitting in this garden, again, which here slopes down to the willow-fringed river, the south wind wafts toward you the soft white down of the willow's ripened seeds. Thus making the wind their travelling car, many plants travel round the globe. The seeds of the locust tree have been carried from Africa to Italy. On April 24, 1897, a thunderstorm passed over the latter country. Mingled with the rain there came the sand of the African desert and the seeds of the locust tree. The familiar dandelion is one of the greatest wind travellers. Each seed, surmounted by a little delicate parachute, is admirably adapted for an aërial voyage. Crossing rivers, seas, and mountain ranges, it has traversed the wide world. With the adventurous Peary it has entered the Arctic regions and with the sailor it has crossed the equator, appearing in both northern and southern hemispheres. The Rose of Jericho utilizes the wind in a peculiar way in seeking "fresh woods and pastures new" for its offspring. It is an annual, growing in dry, sandy places and as its seeds ripen the leaves wither, and the branches dry up and curl inward. A ball is thus formed, inside of which are the seedpods. This is soon loosened from the sandy soil and becomes the plaything of the wind. In the deserts of Syria and Egypt many of these balls may be seen drifting about. When the rain comes they uncoil and the seeds may germinate. The "Wind Witch" of the Russian steppes is a plant which travels in a similar way. This is a sort of thistle which, after flowering, curls up into a ball. Then the stem rots off and the plants start off on the wings of the wind. There they go, careering over the plains, outstripping the swiftest horseman. Now they rise suddenly into the air, hopping and dancing along in erratic and fantastic courses. Some-times two hook on to each other and go spinning off together. A dozen more join them, and the mass rolls lightly along before the brisk east wind. Some day, at the gentle invitation of the rain, they will uncoil and the seeds grow.
As you wander by the clumps of furze and broom in August and September a little sharp crack ! crack ! re-sounds on every side. It is the bursting open of the seed-pods and the scattering of the seeds. Each pod, in fact, is a little gun, and the seeds are the bullets. The contraction of the pods by drying is the firing of the gun. The outside of each division of the pod contracts most and tends to curl outward. In time this force overcomes that which keeps the divisions of the pod together, and they spring apart. The seeds are thus shot out. Cranes-bills, or native wild geraniums, adopt the principle of the sling. You can illustrate the method thus : Hold a stone in each hand, the arms being held down to the sides.
Raise the arms sharply, and at the same time open the hands. The stones fly out to some distance. This is precisely how the geranium scatters its seed, only it has five arms instead of two. Examine a ripe head of seeds. The five seeds are arranged at the bases of five arms round a central axis. The outsides of these arms contract more quickly as they dry than the insides, and this gives them a tendency to curl outward. This is restrained by the attachments at the bases, until the force is sufficient to break them. Then the arm flies suddenly out and throws away the seed. Selecting a seed at the right stage and giving it a little assistance, you may see the slinging away of the seed yourself. The seed of the dandelion may blow anywhere and not infrequently reaches places where the young plant cannot grow. In other plants, however, there are special provisions for getting the seed into a suitable place. The mangrove is one of the most interesting of these. It grows in salt-water swamps and shallow sea water, and the young plants require to be rooted in the mud. If the seeds simply fell and floated on the water they might be carried out to sea and perish. To meet this difficulty the seed begins to grow while still on the tree. It sends out its embryo root in the form of a thick, solid spike, eight or ten inches long, and pointing vertically downward. When the seed falls its weight carries it to the bottom of the shallow water, and the spike is driven firmly into the mud. The young plant has set itself and has simply to go on growing. Two interesting examples occur in the British flora. The pretty little ivy-leaved toad-flax, which grows on old walls, wants to lodge its seeds in the hollows and cavities of the mortar. But it must needs keep its flowers on the outside to bask in the sunshine and spread a banquet for the bee. So, as soon as the flowers give place to seeds, the flower stalks turn right back and carry the seedpods inside, so that they may shed their seeds into the wall. And then there is that pretty water buttercup, or crow-foot, whose white flowers star the ponds in spring. Its seeds must be sown in the mud at the bottom of the pond. So, as soon as the flowering is over, the stalks turn right over and carry the seeds down into the required germinating place.
The mangrove is not the only plant in which the problem of dispersal is solved by the seeds beginning to grow on the parent tree. In East Africa there is a tree known as the Nyika, the seeds of which germinate on the parent plant. Here it is the seed leaves which grow first instead of the root. These form a sort of parachute by means of which the young plants are wafted through the air to suitable spots. Some plants prefer to do their travelling by sea. If they are good sailors, that is, if they can float and stand long immersion in salt water, this is a very effective way of reaching the ends of the earth. Thus the seeds of a West Indian plant have been picked up in the Hebrides, having travelled there on the Gulf Stream. They have even been picked up on the bleak shores of Spitzbergen. In these cases the locality has proved unfavorable, but they show how far plants may travel in this way. Cocoanut palms fringe the shores in tropical regions. Blown from the trees, the cocoanuts float about in the waters. And when, through the labors of the coral polyp, or the exertions of the submarine volcano, a new island rises in the ocean, then, sooner or later, one of these ocean waifs lands on its shores. It takes root and grows, is joined by others, and presently the new land has its fringe of palm trees. The great double cocoanut, the Coco-de-Mer, was known as an ocean traveller long be-fore the discovery of its native land among the Seychelles. Stranded on bleaker Northern lands these travellers from the tropics have not lost their power of germinating. The "sea-bean," a waif from the West Indies, often cast on English shores by the Gulf Stream, has been raised in Kew Gardens from seeds picked up on the Azores. Linnĉus, the Swedish botanist, again knew of cases in which plants had been reared from seeds of tropical plants cast ashore on his own Northern land. The seeds of other plants are provided with hooks or bristles, by which they adhere to the fur of animals. Among English plants, bur-dock, common oats, and goose grass travel in this way. The wool staplers of Elboeuf, in France, used to find seeds of medicaga entangled in the fleeces which they imported from Brazil. On being sown, these seeds, in spite of their long journey, were found to grow. Other plants travel with the birds. And since in their annual immigrations many species cover immense distancesin some cases from North Polar to South Polar districtsseeds may be scattered far and wide by their means. Again, one of the most puzzling facts in the botany of Ireland is the occurrence of one or two American plants. And at rare intervals certain species of American birds are shot in the same country. Put these two facts together, and the former is, perhaps, explained.