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Contributions to Botany"An informative, easy to read, and entertaining book covering many topics relating to plant seeds."
—Lee Luckeydoo, Sida, Contributions to Botany, September 2005
Those honeysuckle seeds sat through all sorts of indignities, yet within each thin seed coat, living tissue managed to survive. The embryo, surrounded by the endosperm, put its life processes on hold. Within a state of suspended animation, the seed's contents still clung to life.
Seed storage proteins, deposited within special cellular organelles called protein bodies, stood side by side with glycoproteins, amino acids, neutral lipids, triacylglycerols (once called triglycerides), proteins, fats, carbohydrates, and other known (or perhaps to be discovered) substances — all in a little brown nucleus about an eighth of an inch wide. Under the direction of the nuclei of individual cells, simple chemicals were organized into complex chemical units, replete with energy, ready to be released when the proper time came.
The seed coat, or testa, is the wall that stands between the seed embryo and the outside world. Considering how important the contents are, the package varies quite a bit. In fact, many seed coats are so individual in character, they have been used to differentiate between different genera and species.
Seed coats are often impregnated with fats or waxes that lend additional resilience and strength to their construction and that often lead to very effective waterproofing. In addition to the outer layer are one or more layers of thick-walled, protective cells.
The seed coats of water lilies, mallows, many legumes, and several morning glories have such tough seed coats that the seeds are virtually impermeable to water and must be nicked by the gardener or soaked in warm water for twenty-four hours before they germinate. If these jackets are not broken, scratched, or eroded, water never enters and germination never begins. Gardeners are a patient lot, but after a time, they'll attack such seeds with a file or soakings, where in nature, time and exposure to the elements does the trick. Hard seed coats are softened by alternately freezing and thawing or drying and wetting. Microorganisms in the soil can infect seed coats and will eventually rot them off.
The seeds of red and white clover (Trifolium pratense and T. repens) have a seed coat that is impermeable to water when the moisture content of the seed reaches a low level. There is actually a fissure along the groove of the hilum (the sear that marks the seed's attachment to the parent plant), which acts like a hygroscopic valve. When the seeds are surrounded by dry air, the fissure opens and allows water vapor to escape from the seed; the fissure, however, immediately closes in damp air, so the seeds can continue to dry without gaining any water.
Some seeds bear outgrowths of the hilum. One such structure is called the strophiole, and its function is to restrict water from entering or leaving a seed. Another type of structure is called an aril. Arils are appendages that vary in shape and, depending on the kind of seed, form ridges, bands, knobs, and even cup-shaped outgrowths of the testa. Some contain various chemicals; others may be brightly colored. Both attributes help to attract animals that aid in seed distribution. The aril of the nutmeg (Myristica fragrans) is the source of the spice called mace, whereas the seed coat itself is ground up to produce its namesake.
Seed coats often contain mucilaginous cells that break open upon contact with water and provide a barrier around the seed. The seed of the common plantain (Plantago major) becomes slimy the minute it hits water. And according to present medical authorities, such mucilage may lower cholesterol levels.
The seed embryo of a dicotyledon is made up of the embryonic axis epicotyl (or the main body of the embryo) that bears two very large seed leaves or cotyledons. The axis itself is divided into the embryonic root (or radicle) at the bottom end, the central hypocotyl (or eventual stem), followed by the two cotyledons. Above the cotyledons is the hypocotyl, called the epicotyl, the part of the seedling that will eventually develop into the shoots. The very top of the epicotyl is known as the plumule and consists of several very tiny, immature leaves. It's all very straightforward, and all these parts are easily seen by examining a growing bean seedling.
But when it comes to monocotyledons, identification gets a bit more difficult. Unlike dicots, the monocot embryo stores its food in the surrounding endosperm, and the single cotyledon is modified and becomes the scutellum. A special structure called the coleoptile protects the young stems and leaves as they push up through the soil. The same job is done by the coleorhiza for the emerging root tip.
Just as there are twins in the animal world, a few species exhibit polyembryony, or more than one embryo in a seed. Twins occur for a number of complicated, developmental reasons, including a fertilized egg cell dividing into a number of parts. Opuntia, many citrus species, and the grass Poa alpina are examples of polyembryony.
The endosperm is the food source for the embryo plant, and basically, it's also the food source for humanity. Remember, about 70 percent of all the food that people eat comes from seeds.
Seeds are divided into two categories: those with endosperms and those without. Some seed endosperms have a great deal of endosperm, like the cereal grains and the date palm, whereas others like lettuce have an endosperm that is only a few cell layers thick. Monocot seeds usually have a large endosperm compared with the embryo. The coconut has the most unusual endosperm, because although white and solid in a mature nut, it's in liquid form when immature.
If an endosperm fails to develop normally, the embryo will suffer. Such embryos can abort or permanently remain in an imperfect stage of development.
Food reserves in a seed are mostly carbohydrates and starch. Other forms of carbohydrates occur as cellulose, pectins, and mucilages.
Starch is found in bodies called starch grains, and these grains have an individual aspect that is unique to a certain species. Such grains are elliptical in runner beans, angular in corn, and round in barley. In some seeds, the carbohydrates are stored as hemicellulose that appear as a very strong cell wall and make the endosperm extremely hard, like a coffee bean.
Chemically, the fats and oils of seeds are known as triacylglycerols (once called triglycerides), usually in a liquid form above 68°C. In addition, some seeds will contain phospholipids, glycolipids, and sterols. The fatty acid found in corn and sunflower seeds is used to manufacture several cooking oils and margarine. The advantage, of course, to using these so-called vegetable oils is their higher unsaturated fatty-acid content.
Among the many kinds of proteins worldwide, after cereal seeds, legumes are the most important source of proteins. Unfortunately, although cereal proteins are sufficient to provide adequate nutrition to human beings (if there are also enough calories), they are not enough for farm animals. And the amino acids found in both legume and cereal protein are not the best for either human or animal diets. But given their drawbacks, they are still amazing foods.
The different proteins have names like cysteine and methionine. There are holoproteins, acidic polypeptides, and a range of amino acids known as glycine, lysine, proline, serine, tyrosine, and valine, plus many, many more. Seed storage proteins are usually stored as special cellular organelles called protein bodies.
Seed embryos need minerals too. Phytin is a mix of magnesium, potassium, and calcium. Some phytins also contain manganese, iron, copper, and rarely, sodium. Phytin is considered nutritionally undesirable because it can chemically bind essential dietary minerals like zinc, iron, and calcium, so they cannot be absorbed by human cells. How much of a problem this is in the Western world, where food processing removes most of the phytins, is not completely understood. In third world countries, where food is rarely refined, the presence of phytin may be a greater problem.
Seeds are made up of many chemicals. Alkaloids are organic, nitrogenous, basic substances derived from plants. They include morphine, codeine, strychnine, and quinine. There is theobromine in the cacao bean, caffeine in both coffee and cocoa. The soybean contains sitosterols and stimasterols, which are pharmaceutically important because they can be converted to the animal steroid progesterone. Presumably, nature prevents insects and animals from using seeds as food by including many of these substances.
Today, castor oil from castor beans (Ricinus communus) is rarely used for children's medicines. It is, however, a valuable ingredient in lubricants and waxes, not to mention a source of sebacic acid, used to make synthetic fibers and lubricants for jet engines.
Soybean oils are used in both plastics and resins. Copra comes from the oil-bearing flesh of the coconut. Other liquid fats from the oilseeds are used to make soap and glycerin for explosives. In tropical countries, solid fats are used to make candles. Several soft drinks contain extracts from kola nuts, the seed of the kola tree of the West Indies and South America. Zein, the alcohol-soluble protein from corn gluten, is used as an adhesive in printing inks. And for people who take a fiber supplement in their diets, a plant gum from psyllium seed (Plantago psyllium), once regarded as a nuisance plant, is used to promote regularity.