How Long Can a Seed Remain Alive?

CLARENCE R. QUICK.

PEOPLE USED to think that 150 years was about the maximum lifespan of the most durably viable seeds.

Some years ago, however, a Japanese botanist found some viable lotus (waterlily) seeds in a layer of peat under a layer of windblown soil in a dry lake-bed in Manchuria. A geologist classified the peat and loess layers of the lakebed as Pleistocene deposits, but this geologic period the Ice Age in general came to an end 10 thousand to 15 thousand years ago.

The ancient Manchurian seeds are the size of small hazelnuts. They have thick, horny seedcoats. They closely resemble the seeds of Nelumbo nucifera, the East Indian lotus.

Several germination tests were made on the seeds. Almost all of them grew, even after having lain around in museums for a decade or two.

Was the estimation of age of the peat somehow in error, or are these viable seeds actually hundreds of years old?

Preliminary tests on several whole seeds by the residual carbon 14 isotope-method of dating organic carbon residues indicated that the seeds were between 830 and 1,250 years old. More tests on larger numbers of seeds are needed to determine the probable maximum age. The seeds are reasonably common in the peat, but the old lakebed is in China, and there seems to be no immediate way to get more of the seeds.

SEEDS sometimes are divided into three classes according to their lifespan under the best possible conditions.

Seeds may be microbiotic (a lifespan of less than 3 years), mesobiotic (3 to 15 years), or macrobiotic (more than 15 years).

Such a classification is convenient but arbitrary. It assumes that we know the optimum conditions for preservation of viability in many kinds of seeds, but that is not entirely correct, even for the many economically important kinds of seeds.

Many perhaps most kinds of seeds of Temperate Zone wild plants are best preserved if they are carefully and thoroughly dried, placed in airtight containers, and stored under moderate refrigeration.

Many others, particularly seeds of numerous plants of the wet Tropics, would be killed by this treatment. Some seeds are dead if they become thoroughly dry. Immature seeds and seeds from diseased or otherwise unvigorous plants commonly have less longevity than more normal seeds.

Obviously, adequate methods for storage and adequate methods for germination must be known specifically before maximum longevity of seed can be approximated.

Most of our knowledge of longevity of seed comes from germination tests with three general types of stored seed samples: Seeds recovered from planned length-of-storage tests, "finds" of old seed samples, and old herbarium collections. The third method has led to valuable results, but it is not generally recommended, especially by herbarium curators! Collection data and storage conditions, furthermore, are not likely to be known so explicitly for seeds from herbarium samples and from finds of old samples.

Carefully planned length-of-storage tests have been made on many species of seeds and under many conditions of storage. Storage conditions and tests are highly important, especially for crop seeds. In this, chapter I discuss only the tests that resulted in germination of seeds of ages well beyond common lengths of commercial storage of crop seeds.

Paul Becquerel, a French botanist, in 1907 and again in 1934 gave reports on germination tests on about 500 species of old seeds from a storage room in a museum. Of 13 species of viable seeds more than 50 years old, 11 species were legumes. Two species of Cassia (Leguminosae), one with seeds viable after 158 years and one after 115 years, were outstanding.

A. J. Ewart, an Australian, in 1908 reported tests on some 1,400 species and varieties of old seeds. Of the 49 samples of viable seeds more than 50 years old, 37 were legumes. Seeds of one species each of Hovea and of Goodia were viable after 105 years. Both plants are legumes.

J. H. Turner, Kew Gardens, England, in 1933 reported viable seeds in samples of seven species of legumes that were 80 or more years of age. The genera were Anthyllis, Cytisus, Lotus, Medicago, Melilotus, and Trifolium.

The Gardeners' Chronicle, London, in 1942 reported viable seeds of Albizzia julibrissin (a legume) after 149 years of storage and of Nelumbium speciosum (a waterlily) after about 250 years.

Many other germination tests on old seeds have been reported.

Frits W. Went, of the California Institute of Technology, and Philip A. Munz, of Rancho Santa Ana Botanic Garden, Calif., in 1948 started an elaborate longtime longevity test on seeds of more than 100 native California plants. Samples were dried in vacuum desiccators, packed in small glass tubes (20 tubes per seed sample), evacuated in the tubes to 0.1 millimeter of mercury or less, sealed, and placed in an insulated but unrefrigerated storage room. It is proposed that the last set of these samples be tested for germination in A.D. 2307.

We used to read reports that plants grew from "mummy" grains and pease found in ancient Egyptian tombs. It was also suggested somewhat later that viable seeds of Silene (a pink) and of Glaucium (a poppy) may have remained dormant for some 2 thousand years in the soil under heaps of ancient mining and smelting debris in the Laurian area of Greece.

Seed physiologists now consider all such claims erroneous. In all tests of authentic seed from Egyptian tombs, the seeds were dead. They disintegrated promptly.

FARMERS know that weed seeds can remain viable but dormant for many years in clean-cultivated land. Seeds of many garden plants react similarly to burial.

Seedlings of certain plants grow abundantly around old cow pads in pastures, an indication that some seeds can pass through the digestive system of grazing animals and remain viable, possibly to contaminate clean pastures. Sheep, horses, deer, bear, and rabbits are known to pass viable seeds. Some birds scatter many kinds of viable seeds. Other plant-eating and fruit-eating animals are presumed to do the same. Topsoil of fields and wildlands comes to contain considerable populations of viable seeds.

For example, 800 seedlings of field bindweed per acre emerged in 1941 in a 14-acre field, from which the original stand of bindweed had been eradicated in 1921. No bindweed plants had been permitted to fruit during the intervening period.

The plant ecologist, the farmer, and the seed physiologist are interested in these "soil-stored seeds" because of their importance to the long persistence of plant species that are thus scattered in time as well as in space.

The forest floor under stands of mature timber often contains surprising numbers of viable seeds. One study in Maine indicated a total viable seed population of 650 thousand per acre.

A research worker in California found 2,820 thousand viable seeds per acre. Seeds of trees in ecologically climax vegetation are seldom found in abundance in these studies.

Seeds and other plant residues in adobe buildings in the Southwest and sodhouses in the Midwest have been studied to estimate seed longevity and to date the introduction of weeds.

Drastic disturbances to the soil mantle, such as war bombings, often have caused seedlings of plant species to emerge in unexpected places.

The knowledge that some seeds remain viable in soil for decades stimulated several extensive buried-seed experiments. Investigators mixed seeds of known species, age, and condition with sand or soil and packed them in replicated bottles. They buried the bottles, mouth downwards, at one or several depths below the soil surface. They removed one or a set of bottles at planned intervals for testing. The remaining bottles were kept undisturbed.

These buried-seed tests simulate, but do not necessarily duplicate, environmental conditions of seeds naturally stored in the soil. One big advantage of the planned tests is that the age of the seed is known precisely.

Prof. W. J. Beal, of the State University of Michigan, started one of the earliest of these tests. He buried 20 pint bottles of weed seeds mixed with sand in the autumn of 1879. Each bottle contained 50 seeds each of 20 kinds of weedy plants. He buried the 20 bottles, with mouths tilted downward, 18 inches below the soil surface.

After 40 years in the soil, but not after 50 years, seeds of the following five plants were still viable: Amaranthus retroflexus (pigweed), Ambrosia elatior (ragweed), Lepidium virginicum (pepper-grass), Plantago major (plantain), and Portulaca oleracea (purslane).

After 40 and 50 years, but not after 60 years, two additional species grew: Brassica nigra (mustard) and Polygonum hydropiper (knotweed).

After 40, 50, and 60 years, but not after 70 years, Silene noctiflora (catchfly) grew.

And after 70 years, three species were still germinable: Oenothera biennis (evening-primrose), Rumex crispus (a dock), Verbascum blattaria (mullein).

J. W. T. Duvel, of the Department of Agriculture, in 1902 buried multiple sets of seeds of 107 species of wild and cultivated plants near Roslyn, Va. He packed the seeds in sterile soil in flowerpots covered with porous clay lids and buried the pots in the open at three depths-8, 22, and 42 inches. Sets of seeds were removed periodically and tested for germination.

Of the 107 species buried in 1902, 71 grew after 1 year, 61 after 3 years, 68 after 6 years, 68 after 10 years, 51 after 16 years, 51 after 20 years, 44 after 30 years, and 36 after 39 years. The tests were discontinued with the 39-year-old set of pots dug up in 1941.

W. L. Goss, of the California Department of Agriculture, in 1932 buried samples of seeds of 12 troublesome weeds. Seeds of bindweed (Convolvulus arvensis), a nightshade (Solanum elaeagnifolium), and Klamathweed (Hypericum Perforatum) were the only ones viable after 10 years.

FROM THE MANY experiments and observations on seed longevity, we can now draw some generalizations in terms of seed morphology and chemistry and in terms of plant ecology and taxonomy.