8o YEARBOOK OF AGRICULTURE 1961 may resist germination for long periods. Some species produce pods in which one segment remains indéhiscent— closed—and the seed within it remains Plants Must Disperse dormant for a long time, as in cockle- bur (Xanthium), for example. Their Seeds THE DISPERSAL of seeds is determined largely by the size, shape, and char- acter of the seedcoat or the persisting PAUL G. RUSSELL AND ALBINA F. MUSIL structures of the fruit as, for example, the awns of grasses; the ''fuzz" of cot- ton; spines and bristles of various NOT ALL SEEDS survive the struggle for forms; "wings" on the seeds of certain existence. Any marked change in trees; plumes of dandelion and thistle; environment, moisture, temperature, the forceful opening of the seed pod, amount of sunlight, or soil composition as in witch-hazel {Hamamelis virginica) ; may create conditions under which and a sticky surface when wet. seeds of certain plants cannot germi- Such seeds are dispersed readily by nate. Plants therefore must disperse such, natural means as wind, water, their seeds in such a manner and animals, and birds. in such quantity that some, at least, When structures, such as awns and will survive so that the species may pubescence, have been removed in the continue. process of harvesting and cleaning of Devices for survival among plants crop seeds, such seeds may become are many. widely distributed in any of several The dormant embryonic plant with- ways—with crop seeds, feeds (hay and in the seed of most kinds of plants is grain), common carriers (trucks, auto- protected by a seedcoat until condi- mobiles, wagons, airplanes), farm im- tions are favorable for new grow^th to plements, ships, birds, and insects. start. The seedcoat may be tough, as The natural means of dispersal have in the bean. It may be thin and been lost long since by most of our delicate, as in the peanut, in which it cultivated seed crops, like the cereals, is protected by the shell of the fruit. beans, and peanuts. Many persons are The struggle for existence is re- not aware of that fact. Only the con- flected also in the amazing variety of stant, watchful efí'orts of man enable shapes, structures, and sizes of seeds these seed crops to withstand the effects and fruits among the 300 families of of unfavorable wxather and the on- flowering plants. slaughts of insect pests and diseases A variable proportion of seeds of and continue their roles as valuable many kinds of plants resists prompt sources of food. germination and so assures survival if Another fact is that the quality and conditions are unfavorable for some quantity of production would begin to of the seeds that germinate first. One deteriorate very soon were it not for example is the uneven ripening and the continuous research of the plant shedding of the fruits of some grasses, breeders and geneticists. Through the such as giant foxtail {Setaria faberi). selection and hybridization of plants, Another is the delayed shedding of the they are constantly developing new seeds (achenes) produced from the ray strains and varieties of all the main flowers of certain composites, such as seed crops. The improved species are some species of thistles. increasingly superior to the original Many legumes, such as the clovers, wild types to which all would revert if produce a variable proportion of seeds left to the uncertain natural means of with impermeable seedcoats, which spreading. PLANTS MUST DISPERSE THEIR SEEDS 8i WIND DISPERSAL is the commonest carried by the wind to great distances. means of dispersal. Strong winds dur- Sometimes they float in the air for ing storms may carry rather heavy 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 {Amar- anfhus graecizans)^ a weed that is com- monly 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 promi- nently in songs. Another example is the noxious Russian-thistle (Salsola pestifer), w^hich is not a true thistle but is closely related to the pigweeds. The wind blows the plants in every direc- The witchweed has very liny seeds, only ^ mm. tion, especially in winter when the long, and one plant produces jo thousand to joo ground is frozen, and the small, conical thousand seeds. It is a dangerous parasitic weed (Striga asiática), and attacks corn, sorghum, seeds arc scattered in all directions. sugarcane, and other grasses. Plants of the mustard family, such as the shepherds-purse {Capsella bursa- pastor is), an annual weed, have pods seeds and seedUke fruits, regardless of in which the seeds remain. The seeds structure, for miles. Even a light breeze are attached to a partition between the may transport small, light seeds for two halves of the pod. The partition some distance. and the seeds are blown aw^ay by the Dispersal by wind often is facilitated wind and are distributed widely. One by the small size of certain seeds. large plant may bear 500 seed pods, Possibly the smallest known are those each with about 24 seeds. of the witchweed {Striga asiática)., an In the fieldcress (Lepidium campestre), Asiatic parasitic plant that has been wild peppcrgrass (L. virginicum), and found in two Southern States. The tiny other species of Lepidium, the small, seeds, only 0.0078 inch long, arc pro- round, flat, papery half pods, each duced in enormous quantities—50 with one flattened seed, are blown thousand to 500 thousand on one about by the wind for considerable plant. Because of their minute size, distances. they are easily dispersed by water, Many plants have wdnged seeds or wind, and farm implements. The winged, seedlike fruits by which their witchweed has long been known in distribution is facilitated through wind Old World Tropics and subtropics. It dispersal. They are more likely to is a dangerous parasite that attacks occur on trees, tall shrubs, and high, corn, sorghum, sugarcane, and other woody vines than on low, herbaceous grasses. plants. The orchid family (Orchidaceae) The winged keys of the maples are also has extremely small seeds. Some familiar to everyone. So are the genera have seeds so fine they resemble winged nutlets of the ashes (Fraxinus). dust. Freed from the capsule, they are Many trees of the pine family have 050888°—Gl 7 82 YEARBOOK OF AGRICULTURE 1961 winged seeds that are samaroid—that Many herbaceous plants also have is, they resemble the small, dry, seeds and seedlike fruits that are dis- winged, seedlike fruits of the maple persed by wind with the aid of various 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). 5=^,^^. The elms {Ulmus) and the crapemyrtle {Lagersiroemia indica) also have small, samaroid seeds. The handsome tree-of-heaven {Ail- anthus altissima), popular as a street The airplane seed, borne by an East Lidian cucurbit vine (Macrozanonia macrocarpa), is tree because of its adaptability to city about 6 inches wide, and derives its common name conditions, has large clusters of sama- from the fact that it describes a spiral about 20 feet ras. The seed is in the center of the wide as it falls to the ground from the urn-shaped long, narrow wing. The sweetgum capsule. {Liquidambar styraciflua) has samaroid seeds about one-fourth inch long that are blown about freely by the wind in types of wing. One is an East Indian cu- autumn. curbit vine {Macrozanonia macrocarpa), The seeds in the samaroid group that which has a flat seed an inch across. have only one terminal wing rotate A wing about 3 inches long and 2 briskly in the wind as they travel, and inches wide is on each end. It has been the area of their distribution thus is called the airplane seed, because it greatly extended. describes a spiral about 20 feet wide True yams {Dioscorea), of which when it falls to the ground from its there are more than 600 tropical and urn-shaped capsule on the high-climb- subtropical species, arc climbers with ing vine. winged seeds. The wings are attached The true lilies {Lilium), popular as to the seeds in various ways, but in ornamentals throughout the Temper- most species the thin, papery seeds are ate Zones, have thin, flat seeds, dis- winged on both sides. tinctly winged all around. From the The small, light, flat seeds of many many-seeded capsules, often borne at of the rhododendrons are winged, the summit of the stalk, the seeds are usually all around, and fly readily in blown away a few at a time, especially the wind. The leathery, ovoid capsules after the stalk becomes withered. of the handsome paulownia {Paulownia On the plains of South. Africa, a tomentosa) enclose great numbers of similar method of dispersal may be small, delicate seeds, each of which has observed in certain of the native several wings. gladiolus {Gladiolus), We get quinine from South Ameri- Some of the rockcresses (Arabis) have can trees {Cinchona). Their small, ob- long, narrow pods that release to the long, samaroid seeds are one-fourth to breeze small, elliptic seeds that have one-half inch long and have irregular, marginal wings.