September 9, 2020

Seeds, the Time Capsules of Life

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A time capsule that keeps things along with memories is opened after a long time. The first time capsule was buried at the New York World’s Fair 1939, and is to be opened 5,000 years later (in 6939).

Among living things, there are time capsules that endure long suffering and put forth life: the seeds. Whereas the oldest time capsule is less than 100 years old, a palm sprouted from a 2,000 year-old seed found in Israel. How are solid seeds of plants made?

Flowering plants [angiosperms], which are the most diverse species of plants, produce seeds when pollen reaches ovule. Unlike animals, immobile plants are helped by water, wind, insects, and birds to move their mature pollen to the stigma—the head of the pistil.

Although a pollen grain adheres to the stigma, it has still a long way to get to the ovule. So it grows a long pollen tube. When the pollen tube reaches the ovule, the nucleus of the pollen goes down the tube and reaches the ovule for fertilization. Then the flower soon withers, and the plant makes ready for producing seeds.

The biggest seed in the world is “coco de mer” (sea coconut) because it is washed up on the beach while floating on the sea. It’s sometimes referred to as Seychelles nut. As big as a basketball ball, it weighs 20 kg [44 lb]. On the contrary, the smallest seed is produced by orchids, which is as small as dust, weighing one-100,000th of a gram.

Each species is slightly different, but in general the seed is made up of the embryo that will become the plant body, the endosperm that retains the nutrients required for the seed germination, and the shell that protects the seed. The seed looks like a baby sleeping in a cradle with a feeding bottle. Unlike people, who are under the care of their mothers for a long time, plants are separated early from their mothers, so they come out to the world by receiving their mothers’ love at once.

Most seeds that fall off the plant body are sleeping without sprouting for a certain period of time. It’s called seed dormancy. If seeds fall to the ground and sprout right away, they will freeze in the coming winter. That’s why they wait for the proper environment to come. Seeds are programmed to wake when the seasons, temperatures, and soil humidity are appropriate, so when the conditions are good, they begin to sprout. Some dormant seeds need to undergo physiological maturation like temperature changes. Some have a variety of germination inhibitors or cannot absorb water due to hard integuments.

For farmers who grow crops, it is desirable to make seeds sprout quickly and evenly. So they wake up the sleeping seeds by changing the environment. They make the seeds know that it is time to sprout by mixing the seeds with wet sand and burying them in the soil, or keeping them in the refrigerator or freezer for a certain period of time to feel the temperature difference. In case seed shells are very hard, although it seems harsh, farmers mix the seeds with sand and pound them in a mortar to scratch the shells, or even dissolve the integuments with chemicals such as concentrated sulfuric acid. Sometimes they lower the temperature to -190°C [-310°F], using liquid air or store the seeds at a high temperature of 100°C [212°F] or higher for a while to inactivate the germination inhibitor.

The seeds before germination are slow in their metabolic reactions with less than five percent moisture. The dry seeds do not show any sign of life, so it seems as if they are dead. However, when they absorb water, they swell up and rupture their shells and prepare to sprout. Water is an indispensable element for seed germination. Each species of seeds has a certain temperature for sprouting. Seeds in the germination phase do not yet undergo photosynthesis but only breathe like animals, so oxygen supply is essential. The seeds of lettuce and fig trees germinate only in the light. On the contrary, the seeds of cucumbers and tomatoes sprout when there is no light. Like this, seeds sprout where water, temperature, oxygen, and light are in proper harmony.

Utilizing the characteristics of seeds, the Food and Agriculture Organization [FAO] of the United Nations has constructed a global underground seed vault on the Svalbard archipelago in the Arctic Circle and is managing it. The storage facility is referred to as the “Doomsday Vault” or “Modern Noah’s Ark,” which was built to prepare for future global disasters such as climate change. There are now more than 1,400 short and long-term seed storages around the world.

A small seed looks very weak. However, once it germinates after overcoming cold and harsh winter, the strong power hidden in the small seed is revealed. The force of germinating seeds is more powerful than we think; a wooden boat loaded with rice was broken in two as the grains sprouted, and a barn stocked with crops was destroyed as they germinated.

When germination begins, the embryo grows by using the endosperm as its nutrients. Because there is no leaf that photosynthesizes, the embryo breathes but cannot produce nutrients by itself. As the embryo grows, the nutrients in the seed gradually decrease. However, when yellow cotyledon grows and the green chlorophyll is formed, it begins to photosynthesize gradually and the root grows strong enough to absorb nutrients directly. A small and humble seed changes little by little and begins to resemble its mother plant.

In California, U.S., there is the largest living tree on the earth. It is the giant sequoia, nicknamed General Sherman. The tree, named after a general at the time of the Civil War, is about 83 meters [272 ft] high and its bottom part is about 31 meters [101 ft] wide. The General Sherman tree, which is larger than six jets put together, began with a small seed of just 1/6,000 gram, 2,000 years ago. The books we read and the wooden chairs we sit on, too, began with very small seeds.

It is wonderful and miraculous that seeds, which seemed to be dead, awaken from a sleep of long patience, bud, and blossom into a huge tree. Just as time capsules keep old things over time, seeds contain the embers of life that will not go away even after a long time. The mystery of life in small seeds is indeed profound.

References
William G. Hopkins and Norman P. A. Hüner, Introduction to Plant Physiology, John Wiley & Sons, Inc., 2009
Silvertown Jonathan, An Orchard Invisible: A Natural History of Seeds, University of Chicago Press, 2009
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