Plants Must Disperse Their Seeds

PAUL G. RUSSELL AND ALBINA F. MUSIL.

NOT ALL SEEDS survive the struggle for existence. Any marked change in environment, moisture, temperature, amount of sunlight, or soil composition may create conditions under which seeds of certain plants cannot germinate. Plants therefore must disperse their seeds in such a manner and in such quantity that some, at least, will survive so that the species may continue.

Devices for survival among plants are many.

The dormant embryonic plant within the seed of most kinds of plants is protected by a seedcoat until conditions are favorable for new growth to start. The seedcoat may be tough, as in the bean. It may be thin and delicate, as in the peanut, in which it is protected by the shell of the fruit.

The struggle for existence is reflected also in the amazing variety of shapes, structures, and sizes of seeds and fruits among the 300 families of flowering plants.

A variable proportion of seeds of many kinds of plants resists prompt germination and so assures survival if conditions are unfavorable for some of the seeds that germinate first. One example is the uneven ripening and shedding of the fruits of some grasses, such as giant foxtail (Setaria faberi). Another is the delayed shedding of the seeds (achenes) produced from the ray flowers of certain composites, such as some species of thistles.

The witchweed has very tiny seeds, only 1/5 mm. long, and one plant produces 50 thousand to 500 thousand seeds. It is a dangerous parasitic weed (Striga asiatica), and attacks corn, sorghum, sugarcane, and other grasses.

Many legumes, such as the clovers, produce a variable proportion of seeds with impermeable seedcoats, which may resist germination for long periods.

Some species produce pods in which one segment remains indehiscent-closed and the seed within it remains dormant for a long time, as in cocklebur (Xanthium), for example.

THE DISPERSAL of seeds is determined largely by the size, shape, and character of the seedcoat or the persisting structures of the fruit as, for example, the awns of grasses; the "fuzz" of cotton; spines and bristles of various forms; "wings" on the seeds of certain trees; plumes of dandelion and thistle; the forceful opening of the seed pod, as in witch-hazel (Hamamelis virginica); and a sticky surface when wet.

Such seeds are dispersed readily by such natural means as wind, water, animals, and birds.

When structures, such as awns and pubescence, have been removed in the process of harvesting and cleaning of crop seeds, such seeds may become widely distributed in any of several ways with crop seeds, feeds (hay and grain), common carriers (trucks, automobiles, wagons, airplanes), farm implements, ships, birds, and insects.

The natural means of dispersal have been lost long since by most of our cultivated seed crops, like the cereals, beans, and peanuts. Many persons are not aware of that fact. Only the constant, watchful efforts of man enable these seed crops to withstand the effects of unfavorable weather and the onslaughts of insect pests and diseases and continue their roles as valuable sources of food.

Another fact is that the quality and quantity of production would begin to deteriorate very soon were it not for the continuous research of the plant breeders and geneticists. Through the selection and hybridization of plants, they are constantly developing new strains and varieties of all the main seed crops. The improved species are increasingly superior to the original wild types to which all would revert if left to the uncertain natural means of spreading.

WIND DISPERSAL is the commonest means of dispersal. Strong winds during, storms may carry rather heavy seeds and seedlike fruits, regardless of structure, for miles. Even a light breeze may transport small, light seeds for some distance.

Dispersal by wind often is facilitated by the small size of certain seeds.

Possibly the smallest known are those of the witchweed (Striga asiatica), an Asiatic parasitic plant that has been found in two Southern States. The tiny seeds, only 0.0078 inch long, are produced in enormous quantities-50 thousand to 500 thousand on one plant. Because of their minute size, they are easily dispersed by water, wind, and farm implements. The witchweed has long been known in Old World Tropics and subtropics. It is a dangerous parasite that attacks corn, sorghum, sugarcane, and other grasses.

The orchid family (Orchidaceae) also has extremely small seeds. Some genera have seeds so fine they resemble dust. Freed from the capsule, they are carried by the wind to great distances. Sometimes they float in the air for long periods. Seeds of some orchids are equipped with thin, tiny wings, which add to their buoyancy.

Tumbleweeds when they are dry and ripe may be torn loose by the wind and blown over the ground. Seeds drop along the way. Tumbleweeds are known on deserts, prairies, and steppes throughout the world.

An example is an amaranth (Amaranthus graecizans), a weed that is commonly found here and there in drier areas throughout the United States, especially in the western plains. The tumbleweed is so characteristic of our West that the name figures prominently in songs. Another example is the noxious Russian-thistle (Salsola pestifer), which is not a true thistle but is closely related to the pigweeds. The wind blows the plants in every direction, especially in winter when the ground is frozen, and the small, conical seeds are scattered in all directions.

Plants of the mustard family, such as the shepherds-purse (Capsella bursapastoris), an annual weed, have pods in which the seeds remain. The seeds are attached to a partition between the two halves of the pod. The partition and the seeds are blown away by the wind and are distributed widely. One large plant may bear 500 seed pods, each with about 24 seeds.

In the fieldcress (Lepidizim campestre), wild peppergrass (L. virginicum), and other species of Lepidium, the small, round, flat, papery half pods, each with one flattened seed, are blown about by the wind for considerable distances.

Many plants have winged seeds or winged, seedlike fruits by which their distribution is facilitated through wind dispersal. They are more likely to occur on trees, tall shrubs, and high, woody vines than on low, herbaceous plants.

The winged keys of the maples are familiar to everyone. So are the winged nutlets of the ashes (Fraxinus). Many trees of the pine family have winged seeds that are samaroid that is, they resemble the small, dry, winged, seedlike fruits of the maple and elms. Most of the pines (Pines) have seeds of this type, as do the spruces (Picea), the firs (Abies), the cypresses (Cupressus), the tamaracks (Larix), and the true cedars (Cedrus). The elms (Ulmus) and the crapemyrtle (Lagerstroemia indica) also have small, samaroid seeds.

The handsome tree-of-heaven (Ailanthus altissima), popular as a street tree because of its adaptability to city conditions, has large clusters of samaras. The seed is in the center of the long, narrow wing. The sweetgum (Liquidambar syraciflua) has samaroid seeds about one-fourth inch long that are blown about freely by the wind in autumn.

The seeds in the samaroid group that have only one terminal wing rotate briskly in the wind as they travel, and the area of their distribution thus is greatly extended.

True yams (Dioscorea), of which there are more than 600 tropical and subtropical species, are climbers with winged seeds. The wings are attached to the seeds in various ways, but in most species the thin, papery seeds are winged on both sides.

The small, light, flat seeds of many of the rhododendrons are winged, usually all around, and fly readily in the wind. The leathery, ovoid capsules of the handsome paulownia (Paulownia tomentosa) enclose great numbers of small, delicate seeds, each of which has several wings.

We get quinine from South American trees (Cinchona). Their small, oblong, samaroid seeds are one-fourth to one-half inch long and have irregular, terminal wings. They are so light that they are easily carried for considerable distances by even gentle breezes.