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.

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 (Lepidium 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 (Pinus) 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 styraciflua) 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.

The horseradish-tree (Moringa oleifera), grown throughout India, has curious, wind-dispersed, three-angled, winged seeds, which yield a valuable oil. The seeds are discharged from a large ribbed pod 9 to 18 inches long.

Many herbaceous plants also have seeds and seedlike fruits that are dispersed by wind with the aid of various types of wing. One is an East Indian cucurbit vine (Macrozanonia macrocarpa), which has a flat seed an inch across. A wing about 3 inches long and 2 inches wide is on each end. It has been called the airplane seed, because it describes a spiral about 20 feet wide when it falls to the ground from its urn-shaped capsule on the high-climbing vine.

The true lilies (Lilium), popular as ornamentals throughout the Temperate Zones, have thin, flat seeds, distinctly winged all around. From the many-seeded capsules, often borne at the summit of the stalk, the seeds are blown away a few at a time, especially after the stalk becomes withered.

On the plains of South Africa, a similar method of dispersal may be observed in certain of the native gladiolus (Gladiolus).

Some of the rockcresses (Arabis) have long, narrow pods that release to the breeze small, elliptic seeds that have marginal wings. These low, herbaceous alpines, of the mustard family, have terminal clusters of attractive pink, white, or purple flowers.

The ovoid capsules of an attractive native white-flowered perennial, the turtlehead (Chelone glabra) enclose numerous small, flat, squarish seeds, which are surrounded by a broad, membranous wing, well suited for wind dispersal when it is released from the upper part of the plant.

The large carrot family has two principal means of seed dispersal. Each fruit consists of a pair of round or oblong carpels, which are attached to a slender axis, or columella. Each contains one seed, is usually indehiscent, and often is furnished on the outer side with several ribs that are sometimes elevated into wings. These fruits usually are borne on a tall stalk, which in some species is several feet high. When the whole plant becomes dry and brittle, the light, corky fruits are easily dislodged by the wind and carried away. The wings enhance the buoyancy.

Fruits of the wild carrot have developed bristles, with or without hooks, that enable the fruits to adhere to the fur of passing animals. Thus they get distributed.

Among the most serious pests of western ranges, the halogeton (Halogeton glomeratus) is especially dangerous, because it is poisonous to livestock. Its small, seedlike, winged fruits are dispersed easily by wind.

The Russian-thistle (Salsola pestifer), another noxious weed, also has small winged seeds.

A number of plants have seeds that are not actually winged but are flat and membranous, borne in capsules, and easily carried by wind. One example is the tulip (Tulipa). Its light, flattened seeds occasionally have narrow marginal wings. Another is the yucca (Yucca), native mostly to the more arid regions of our country. Its large, flat, black, windborne seeds are produced in great abundance.

The wind disperses the "seeds" of many grasses, although generally there is no special structural adaptation for this purpose except the lightness of the chaffy lemmas and paleas that enclose the grain and act as a wing. Many also have silky hairs on the callus. One is the common reed (Phragmites communis), whose long hairs enable the floret to fly away.

Plumed seeds and seedlike fruits generally fly much farther than winged ones. Some travel hundreds of miles. They usually are very light. They can rise and fall with the wind and even rise again from the ground. Most of them are weeds.

The common dandelion (Taraxacum officinale) has plumed achenes, which float gracefully in the breeze. Closely related genera in the chicory family that have similar structures are the sowthistles (Sonchus), wild lettuces (Lactuca), salsify (Tragopogon), and several others.

Among the grasses, the spike pappusgrass (Enneapogon desvauxii) has plumose awns, which enable the wind to carry off the mature florets.

In the aster family are a great many genera with seeds (achenes) that have bristles or hairs light enough to permit the wind to broadcast them. Some are pernicious weeds. The Canada thistle (Cirsium arvense) is one of the worst weeds in various parts of the North Temperate Zone because the plumed seeds are easily carried aloft. The knapweeds (Centaurea), ironweeds (Vernonia), and the erigerons (Erigeron) are other troublesome weeds that are similar to the common thistle in their adaptations for dispersal by wind.

Most of the milkweed family (Asclepiadaceae), many genera of the dog-bane family (Apocynaceae), and some genera of the evening-primrose family, such as the fireweed (Epilobium angustifolium), have comose seeds—that is, with a tuft of hairs attached to one end.

An example from the dogbane family is the Strophanthus from tropical Africa and tropical Asia. The apex of the seed is prolonged into a long, slender beak that is densely feathered along the upper half with thin, white hairs. From the seeds of certain species of Strophanthus, sarmentogenin is derived; it contributed to the synthesis of cortisone, a drug used in the treatment of arthritis.

In the common milkweed (Asclepias syriaca), the large coma, or tuft of hairs, helps the wind carry the seed away.

Almost any type of seed can be transported by some kind of water movement—rainwash, floods, and the action of streams. Very small seeds, those that are light in proportion to their size, and those with flat shapes that make for greater buoyancy, are moved greater distances than other kinds.

Many of the light, corky seedlike fruits of the carrot family float easily, some for several months, as do also seeds of some of the gourd family, which also owe their buoyancy to their light, corky structure. Seeds of both these groups are mostly transported by streams.

The thick-walled, buoyant achenes of the silverweed (Potentilla anserina) have been known to float in streams for 15 months. The weed therefore is widespread along riverbanks and in swampy meadows in the Northern Hemisphere.

The jimsonweed (Datura stramonium) has flat, corky seeds that float easily. In South Africa, and doubtless elsewhere, they are carried downstream by floodwaters and deposited on the riverbanks, where they germinate.

The seedlike achenes of the buckwheat family (Polygonaceae) are often dispersed by water, as shown by their common occurrence along riverbanks. In the docks (Rumex) that have winged seeds, the wings apparently serve for dispersal by both wind and water.

Certain sedges (Carex), of which there are about a thousand species, have seeds (achenes) that contain air pockets and float on water, sometimes for several months before the seed may find conditions favorable for germination.

Some seeds and seedlike fruits are carried by ocean currents. Such seeds are of the kind that can float for a long period without absorbing sea water and establish themselves finally in tidal mud or on beaches. They must have a hard, waterproof seedcoat and the ability to respond to external influences. Of the immense number of seeds floated down into the sea by streams all over the world, only a few fulfill these and other conditions. They represent relatively few species, compared with those dispersed by wind and animals.

Several genera of the bean family appear among those successfully established by this means in widespread areas in the Tropics. They include Mucuna, Guilandina, Entada, Erythrina, and Vigna. In most of them, buoyancy is maintained by a relatively large cavity between the cotyledons. Seeds cast up on the Florida beaches and along the gulf coast picked up rather frequently by tourists nearly always belong to these genera. Some of them apparently have come from as far as eastern South America.

ANIMALS and birds disperse seeds, too. We often see them in late summer and early autumn with numerous seedlike fruits adhering to their hides or feathers—as happens also to the clothing of those of us who walk in the fields and woods. A large number of plants, including many weeds, owe their distribution to this type of seed dispersal. Hairs, bristles, or hooks on the surface of their fruits and mucilaginous seedcoats in some instances help them attach themselves to fur, feather, and fabric.

The bur-ragweed (Franseria tomentosa), a pernicious weed in the Western States, has burlike achenes that are armed with several rows of hooked spines, which adhere readily to passing animals. The barbed bristles of the beggarticks (Bidens) are well adapted to adhere to animal fur, bird's plumage, and clothing. The genus is known almost throughout the world.

The tick-trefoils (Desmodium) are equally adhesive. Their small, flat, in-dehiscent pods are covered with hooked hairs.

The wild carrot (Daucus carota) and some other species of Daucus have seeds (mericarps) with hooked bristles. The same is true of the species of the weedy snakeroots (Sanicula).

The seedlike nutlets of one stickseed (Lappula echinata) are armed with a double row of hooked prickles, which become embedded in sheep's wool in the Western States and are thus likely to lower the grade of the wool.

Another troublesome weed, the houndstongue (Cynoglossum officinale), closely related to the stickseed, also has nutlets with short, barbed prickles.

THE GLUEY SEEDCOATS of some species make it relatively simple for the seeds to become widely dispersed.

Seeds of the butterfly-pea (Clitoria mariana), a leguminous plant with showy, pale-blue flowers, are so viscid that they adhere to any passing animal. Several other species of Clitoria from tropical America and Asia also have sticky seeds.

Seeds of another rather large group of plants are viscid only after being wetted. The plantains (Plantago), with species, mostly weeds, all over the world, belong in this group, as do also many of the rushes (Juncus), some of the flaxes (Linum), and several of the members of the mustard family, including the garden cress (Lepidium sativum), the common falseflax (Camelina sativa), and the shepherds-purse (Capsella bursa-pastoris), all more or less weedy. Several genera of the phlox family have seeds that when wetted emit mucilage speedily in the form of fine threads. The seeds in this group do not become viscid until wetted, so that they may be blown about, when dry, until they reach a spot that is damp enough for them to exude mucilage and also to germinate.

Another way in which viscid seeds are spread is to become attached to dry leaves that the wind may carry off.

FORCEFUL dehiscence sometimes disperses seed.

One example is the squirting-cucumber (Ecballium elaterium), an annual vine whose fruit is an oblong berry about 2 inches long. When thoroughly ripe, the fruit detaches itself from its peduncle. Increasing tension within the fruit breaks the tissue, weakened at the point of detachment, and the seeds are violently ejected, together with the mucilaginous liquid with which they were surrounded. The explosion may throw the seeds as far as 20 feet. Sometimes these adhere to some passing animal that had set off the explosion by touching the ripe fruit.

The small mistletoe (Arceuthobium pusillum) of New England is parasitic on the black spruce. When its berries are ripe in September, the seeds are violently expelled. The mucilaginous matter on the seeds causes them to stick to other parts of the spruce or to nearby trees, upon which they germinate.

Another annual vine (Cyclanthera explodens) has small, spiny, gourdlike fruits about 1.5 inches long. They burst open into three lobes when they are ripe, and the seeds are thrown out.

In some species of Oxalis, as O. stricta, often a weed in gardens and elsewhere, the small, cylindrical capsule suddenly explodes when touched and scatters its seeds.

The pale touch-me-not (Impatiens pallida) and other species of Impatiens have narrow capsules that open suddenly to broadcast their seeds.

The violet-flowered Chinese wistaria (Wisteria chinensis) is a stout climber with woody pods. The hard, rounded seeds, when the pod opens, can fly to feet across a room. The native witch-hazel (Hamamelis virginica) bears explosive capsular fruits.

Seed pods of many leguminous species open explosively when ripe. The large, thick pods of the West Indian swordbean (Canavalia gladiata) snap open, and the seeds are thrown 10 to 20 feet.

The sandboxtree (Hura crepitans) is known in the American Tropics for the force that accompanies the bursting of its round, 3-inch capsules, a force said to be enough to break open a small wooden box in which a capsule was kept.

TURTLES and tortoises often eat fleshy fruits. The box turtle of the Eastern States feeds on wild strawberries and other succulent fruits, and the seeds pass uninjured through its digestive system.

Lizards, particularly the larger ones that are found in the Tropics, feed freely on all sorts of fruits, especially those of certain cactuses. They also disperse the seeds effectively.

Many freshwater fish feed on vegetation, including fruits of water plants like waterlilies, and may swim for long distances before expelling the seeds. The ability of some fishes to pass overland, usually through wet grass, from one body of water to another, is by no means uncommon, especially in some parts of the Tropics. Seeds are dispersed in their travels.

One curious method of seed dispersal was mentioned by Charles Darwin in his Origin of Species. Herons and other birds, he reported, have eaten fish in whose stomachs were viable seeds of the yellow waterlily. In this way the herons often carried the seeds many miles from their source.

Land crabs, which feed on fallen fruits, have increased in this way the distribution of the Malayan leguminous tree known as Inocarpus edulis. The large garden snail in England has been responsible for the dispersal of strawberries by eating the fruits.

The digestive tract of earthworms has been found, on dissection, to contain a very wide variety of small seeds.

SEEDS and seedlike fruits often are unusual and bizarre.

Probably the largest seed known among the flowering plants is the two-lobed seed of the so-called double coconut (Lodoicea maldivica), a tall, handsome palm native in the Seychelle Islands. One of them may weigh about 40 pounds and be more than a foot long and nearly 8 inches thick.

Among the smallest known seeds are those of Striga, which we discussed earlier, and certain orchid seeds.

Cuzco corn, from Peru, has large, flat kernels nearly an inch wide. Another grass, one of the Indian bamboos (Melocanna baccifera), has hard, woody seeds 4 inches long and nearly 2 inches in diameter.

A native oak (Quercus corrugata) in southern Mexico bears enormous acorns with brown, wooly cups nearly 2.5 inches across. The scales are modified into concentric rings. A relative of our well-known catalpa that grows in India (Oroxylon indicum) is a tree with flat pods 3 feet long, filled with delicate, silky-papery, winged seeds about 3 inches wide.

The bizarre capsule of the unicorn plant of the Gulf States (Martynia louisiana) resembles a large hummingbird with two long, slender, upright tail feathers. These appendages have small recurved points, which readily attach the capsule to the fur of passing animals or to a person's clothing.

A similar method of dispersal is employed by the Malagasy plant Uncarina peltata, whose dry fruits, 3 inches in diameter, have numerous long, slender bristles, each terminated by four small, recurved hooks. One of these fruits attached to fur or clothing is very hard to dislodge.

One of the Chinese waterchestnuts (Trapa bicornis) has blackish fruits 3 inches across that closely resemble a bull's head, with two stout, curved, sharp-pointed horns. These fruits float for some distance before sinking to the bottom of the shallow river or pond, where they may germinate.

The dipterocarps (Dipterocarpus), tall trees from tropical Asia, are a source of the commercial gurjun balsam. Their fruits are small and globular and have a persistent calyx. Two of the sepals become greatly elongated up to 7 inches long, and the other three remain short. The elongated, winglike sepals enable the fruits to be wind propelled for long distances.

THE RECEIPT of a shipment of cabbage seed from Russia in February 1898 marked the beginning of a continuing program of plant introduction by the Department of Agriculture, which collects plant materials, mostly as seeds, from all parts of the world for experimental purposes.

The introductions are received from the Department's agricultural explorers, foreign institutions, and other sources. They usually represent special strains or varieties of existing crop plants, or they may be wild relatives of cereals, forage plants, fruit, vegetables, fiber plants, oil-yielding plants, and plants for special projects.

Seeds of weeds may come into the United States with imported crop seeds and on ships and airplanes. Most of the noxious weeds in this country were introduced from foreign sources in this manner. Familiar examples are field bindweed (Convolvulus arvensis) and Canada thistle (Cirsium arvense).

Some kinds of plants that are not important weeds in their native land sometimes become extremely objectionable under certain conditions in parts of this country—as, for example, giant foxtail (Setaria faberi) and St. -Johns-wort (Hypericum perforatum).

It is highly important therefore that all imported seeds, whether crop or weed, be correctly identified and evaluated before planting.

THE IDENTIFICATION of seeds and seedlike fruits usually is based on external morphological features, such as shape, size, color, and surface configuration or texture. Sometimes the internal anatomy of the seed—the size and position of the embryo or the nature of the endosperm, for example—furnishes useful clues to its identity.

For seed identification of special groups, particular attention is paid to the features that are characteristic of the group. For example, in determining species of the genus Cucurbita, which includes pumpkins, squashes, and cushaws, special attention is paid to the shape of the hilum (seed scar), the character and color of the seed margin, and the color of the seed face.

The identification of seeds of the genus Phaseolus, the cultivated beans, takes into account the position, shape, and size of the hilum, the nature of the caruncle and micropyle, the nature and color of the seedcoat, and the nature of the parahilum, which is a small area next to the hilum and is characterized by two small tubercules.

Seeds of two species of Hibiscus, the kenaf (H. cannabinus) and the roselle (H. sabdariffa), both of economic importance, are almost alike, except that glossy, pale-brown spots occur on the surface of H. cannabinus.

Seeds of the mimosa family (Mimosaceae), with one or two exceptions, are easily separated from the seeds of other legumes by a horseshoe-shaped design on the seed faces. A continuous elliptic or oval design on the seed faces would place the seed in the family Caesalpiniaceae.

A SEED HERBARIUM is essential for reference in identifying seeds, since published descriptions of plants rarely include the necessary morphological details of the seed and mature seeds are usually not present on specimens in plant herbariums. Newly introduced plant materials frequently must be identified promptly in order to be handled effectively.

The Department of Agriculture maintains a large general seed herbarium at the Plant Industry Station at Beltsville, Md., consisting not only of seeds, ranging in size up to 2 inches or more in diameter, but also seedlike fruits and certain types of dry fruits. The herbarium is used chiefly for identifying or checking the identification of newly introduced plant materials.

Another large seed herbarium, at the Agricultural Research Center, Beltsville, that consists mostly of actual and potential weeds and seeds of crop plants, is used mainly for identifications in carrying out the provisions of the Federal Seed Act.

Together, these seed herbariums total about 100 thousand samples, which come from all parts of the world. More than 250 plant families are represented, with 17 thousand species.

These herbariums may be used by anyone who cares to consult them.

Some persons have learned that the study of seeds is an absorbing and profitable hobby. It involves little expense and a small amount of time and energy. It is a field in which there are satisfying and practical returns. For the farmer or nurseryman, a collection of seeds of authentically identified plants is of value in checking new or little-known seed materials. The introduction of new and possibly dangerous weeds could very well be forestalled if authentic samples of them were on hand for comparison. Small glass vials and gummed labels from a pharmacy are ideal for filing small samples of seeds.