Botany: The Science of Plant Life

by Norman Taylor

But with even the greatest ingenuity and the most explosive bursting of pods, most plants could never capture much new ground, and their very existence as a species is often contingent on their ability to spread, if these various methods by which plants shed their seeds were not aided by outside help. Some of these have had conspicuous and almost startling results.

1. Dispersal By Animals, the Wind, and Water

The seeds or fruits of those plants that are used for animal food are often carried considerable distances, while the thrifty squirrels’ burying of acorns is everyday knowledge to those who have seen them busily engaged in the making of winter stores, or the planting of new trees, often many rods from the parent tree. In years of plentiful seed production squirrels have been known to plant great quantities of seeds of the Douglas fir, thereby hastening the establishment of one of the greatest evergreen forests in the west.

While some seeds are destroyed by passing through the digestive tract of animals many are not harmed in the least. From over two hundred and fifty different kinds of plant seeds fed to a variety of birds over 80 per cent germinated perfectly after passing through their digestive tracts. And perhaps the most remarkable case is the seed of a pondweed said to be incapable of germination until it has passed through a bird. This plant grows in fresh water ponds in great quantities and is much eaten by wild ducks. From the stomach of one bird over three hundred such seeds were recovered. In the Eastern States a common feature of our farms is the red cedar or juniper scattered along fence rows, nearly all of which are due to birds roosting on fence rails and dropping the seeds after passage through their digestive tract.

We get some idea of the part birds play in plant dispersal when we realize the enormous number of them that make their flights twice a year, often over great distances. Wild ducks, in untold millions, travel from the far north to the tropics, each carrying their freight of seeds, sometimes as food and often mechanically clinging to their feet. The writer once saw at Gardiner’s Island and Montauk, Long Island, hundreds of thousands of tree swallows which feed on the fruits of the bayberry (Myrica carolinensis). So dense was the flock that they covered nearly every inch of the bayberry patches, and after eating no one can calculate how many million seeds they started off toward their winter home. Stopping as they do each day on their long flight southward, is it any wonder that the bayberry is one of the commonest bushes along the Atlantic coast?

With flights of birds stretching from the Arctic to the Antarctic, sometimes a single species making such a flight during its migration period, and hundreds of species making shorter flights twice a season, it is easily seen how birds can carry seeds for long distances. That they do so carry them is common knowledge and in eleven wild ducks examined by H. B. Guppy, he found nearly 300 seeds of bur reed, forty-one of pondweed, 270 sedges and 222 seeds that he could not identify. Nearly all these seeds germinated when sown, some sprouting more quickly than if they had not passed through the bird’s stomach. Some few seeds which would usually be dormant for one, two or even three years, have their germination unquestionably hastened by passage through birds, who may be looked on in some cases at least as “flying germinators.”

Some sea-flying birds have been captured over five hundred miles from land and seeds of a buttercup and of the sea blite recovered from their stomachs. One student of the flora of Spitzbergen has stated that nearly all the plants of that cold region have come from the northern part of the Scandinavian peninsula through their carriage by birds. One of the commonest plants of that region is the crowberry, which is a favorite food of birds. The plant is found throughout the Arctic regions and on high mountain tops to the south of it, such as the Alps, our own Adirondacks and White Mountains, and many others. There are some islands in the Pacific known to have received a few of the plants now growing on them by birds carrying the seeds from other regions, often thousands of miles away. Several species of birds are known to make sustained oversea flights from Labrador to South America, and one from Alaska to Hawaii. In such flights stopping is impossible, so that the carrying of fresh seeds and spores is always likely and probably more quickly accomplished than by any other means of transport. In New Zealand a northern European bird was once found with the seeds of two species of marsh arrow grass in its stomach, both of which germinated. The plants are native in the cooler parts of the north temperate zone.

Not far from New York, at Montauk Point, Long Island, there was found a few years ago some plants of the cloudberry, a kind of blackberry with amber-colored fruits, which is otherwise unknown in that region but is common enough in the Arctic and on mountain tops northward. That point of land, extending out to sea, is a favorite stopping place for migrating birds and to them was undoubtedly due the introduction of the plant so far south of its true home. Scores of similar cases could be cited which confirm the observations of naturalists all over the world that birds are among the greatest aids to plants in securing wide dispersal. Of course the factors of favorable or unfavorable conditions, once the seed is deposited in the new home, operate to keep plants from the tropics from settling permanently in colder regions and vice versa, but we cannot escape the conclusion that the sky is filled during certain seasons with millions of seed carriers that have, in many cases, populated their stopping places with foreign plants.

Other animals than birds carry fruits and seeds, besides those that eat them. Some fruits like the Martynia ([Figure 60]), are so arranged that no animal with fur can avoid catching some if they come in contact. The prickly fruits of cocklebur resulted in one species of that weed from the steppes of Russia being carried all over southern Europe in a comparatively few years. Tickseeds, Bidens, and all the hosts of plants that have prickles, burs, spines or what not attached to their fruits, have, by the possession of such devices, a better chance for dispersal of their fruits than those not so provided. Others, again, have various coatings of mucilage which stick to animals and thus help plants to overcome their chief drawback to dispersal—their anchorage at the place of birth.