Seeds are nature’s consummate survivors. The next time you admire a field of waving green grassland or a stunning grove of acacia, stop to consider how it got that wayoften against incredible odds. Seeds can survive freezing temperatures and drought. They can pass through our digestive systems without damage and weather a trip across the ocean, hitching a ride on marine debris. They can even endure complete desiccation, a feat taken to extraordinary lengths by the date palm, a seed from which was recovered from the palace of Herod the Great was germinated after some two thousand years.
The Book of Seeds takes readers through six hundred of the world’s seed species, revealing their extraordinary beauty and rich diversity. Each page pairs a beautifully composed photo of a seedlife-size, and, in some cases, enlarged to display fine detailwith a short description, a map showing distribution, and information on conservation status. The whole spectrum of seeds is covered here. There are prolific species like corn and less widely distributed species, like the brilliant blue seeds of the traveler’s palm or the bird of paradise flower, aptly named for its distinctive orange coiffure. There are tiny seeds and seeds weighing up to forty pounds. And while seeds in all their shapes, sizes, and colors grant us sustenance, there are even some we would be wise to treat with caution, such as the rosary pea, whose seeds are considered more toxic than ricin.
The essential guide to these complex plant creations, The Book of Seeds offers readers a rare, up-close look that will inspire scientists and nature lovers alike.
|Publisher:||University of Chicago Press|
|Edition description:||First Edition|
|Product dimensions:||7.50(w) x 10.90(h) x 1.80(d)|
About the Author
Paul Smith is secretary general of Botanic Gardens Conservation International, a nonprofit organization that promotes plant conservation in botanic gardens. He is a former head of the Millennium Seed Bank at the Royal Botanic Gardens, Kew, the largest and most diverse seed bank in the world.
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WHAT IS A SEED?
Seeds are travelers in space and time — small packages of DNA, protein, and starch that can move over long distances and remain viable for hundreds of years. These packages have everything they need not only to survive, but also to grow into a plant when they encounter the right conditions.
The seed is a reproductive unit that develops from an ovule, usually after fertilization. Ovules are borne by both the angiosperms (flowering plants) and the gymnosperms (conifers and cycads). In the angiosperms, the ovules are totally enclosed within the ovary, while in the gymnosperms the ovules are "naked," typically borne near the base of each scale in a female cone. Since the cone scales remain tightly closed except at the time of pollination and later at seed shed, the term "naked" is a relative one.
All seeds have three basic structures in common: the seed coat (often referred to as the testa), a food source (the endosperm), and the embryo. As the embryo develops, it differentiates into the cotyledon (seed leaf or leaves), the epicotyl (the embryonic axis at the point of attachment of the cotyledon(s)), the plumule (shoot), the hypocotyl (stem), and the radicle (root). Some seeds have appendages such as arils that attract birds and animals (see top right). These are often brightly colored and nutritious, and their consumption doesn't damage the seed.
Seeds have developed a wide range of shapes and sizes in order to maximize their chances of survival, in particular through adaptation for the two most important stages of their development — dispersal and germination.
Wind-dispersed seeds, for example, may be very small and light (such as those of orchids; shown here), or they may develop wings or other appendages that enable them to fly or float on air currents for long distances (such as those of birches (shown here) and Sycamore (Acer pseudoplatanus)). Waterborne seeds, such as the coconut, have a thick, impermeable seed coat enabling them to float on water. Animal- or bird-dispersed seeds have a variety of adaptations that enable them to hitch a ride with their dispersers. These include hooks or grapples on their seed coats that stick to fur or feathers (for instance, those of Uncarina (Uncarina grandidieri)); tasty, often brightly colored seed arils that are attached to the seed and picked up, carried away and eaten, leaving the fertile part of the seed to germinate (as, for example, in the Bird of Paradise (Strelitzia reginae); and a hard, resistant seed coat enclosed in a sweet, juicy fruit that enables the seed to pass through the gut of an animal or bird and emerge intact and ready to germinate (for example, the seeds of the Grape (Vitis vinifera)).
A seed's size, shape, and composition are also critical to a plant's particular germination strategy. As a rule, large seeds (those the size of an acorn or larger) are programmed to germinate rapidly. Their seeds are not designed to last for very long or become dormant. In seed banks, such seeds are referred to as "recalcitrant" because they don't store well. They are generally sensitive to drying and, due to their comparatively high water content, they can't be frozen. Around 20–25 percent of seed-bearing species produce recalcitrant seeds, but the proportion is much higher (more than 50 percent) in wetter habitats such as rainforests, because in those conditions it makes sense for seeds to germinate rapidly and send out a root and shoot as quickly as possible to gain the water, minerals, and light the plant needs to outcompete others around it. To do this, a seed needs to have a comparatively large reservoir of food to draw on before it starts to photosynthesize. For this reason, recalcitrant seeds are larger than their "orthodox" counterparts. Plants that grow in water-limited habitats will die if they germinate immediately and the rain fails to arrive. For these species, it makes more sense to persist in a dormant state until the conditions are right. Here, being small and desiccation-tolerant is an advantage. It is also unnecessary for a seed to have a large food store if light is not a limitation in its habitat, because the shoots it puts up won't be fighting for light with its competitors.
For many plants there are trade-offs between their dispersal and germination strategies. For example, if a species' dispersal strategy is being carried on the wind, then the plant can't produce heavy seeds with large food stores. A particularly extreme example of such a trade-off that has led to the demise of the species is the Coco de Mer (Lodoicea maldivica, and shown opposite), which produces the largest seed in the world. This double coconut, with its enormous food-storage organs, can survive on its seed reserves for months, enabling it to establish in difficult conditions. However, due to its size and weight, this island-bound species doesn't float, severely restricting its ability to disperse, unlike its cousin, the Coconut (Cocos nucifera).
SEED DORMANCY & LONGEVITY
As mentioned above, seeds can be broadly split into two categories: recalcitrant seeds, which are sensitive to desiccation and programmed to germinate rapidly; and orthodox seeds, which survive drying and can persist for long periods on the plant or in the soil before germination. As with most things in nature, seed behavior varies across a spectrum rather than in hard-and-fast categories. Some species, then, are intermediate in their seed behavior. They can survive some drying and can persist for a few months or years, but can't survive complete desiccation or being dry for very long periods.
Seed dormancy can be broadly categorized into three main mechanisms: physical dormancy, physiological dormancy, and morphologicaldormancy, although combinations of these mechanisms are common. Physical dormancy usually takes the form of a hard, impermeable seed coat that prevents the imbibition of water, a necessary step for seed germination. Hardcoated seeds such as those found in the pea family (Fabaceae) display physical dormancy, and it is only the weathering of the seed coat over time or by passing through the gut of an animal that enables water ingress and subsequent germination.
Physiological dormancy refers to seeds that are prevented from germinating until certain chemical changes occur. For example, many temperate plants are thermodormant, requiring vernalization (sometimes called stratification) before they will germinate. Here, the seeds require cold temperatures to break down inhibiting chemicals before they can germinate. This adaptation ensures that seeds do not germinate before the winter once they have been shed, but after the cold weather, in spring. An example of a plant with this kind of dormancy is the Common Bluebell (Hyacinthoides non-scripta). Other species (such as the Blood Amaranth (Amaranthus cruentus) require warm temperatures for germination. Physiological dormancy also encompasses photodormancy, in which a seed responds to day length to trigger germination, again to help ensure that germination is synchronized with the optimal season for establishment and growth. Physiological dormancy can also be broken by external chemical triggers. For example, the seeds of some plant species that live in savanna or fire-dominated habitats will germinate only when they are exposed to smoke, such as King Protea (Protea cynaroides), which flourishes in the fynbos region of South Africa.
Finally, morphological dormancy refers to seeds that are not fully developed when they are dispersed or shed. Here, the embryos are immature or undifferentiated, and further development needs to take place before the seed will germinate. Examples of species that show morphological dormancy are the Cycads.
Seed dormancy enables seeds to survive for a very long time. This property of "longevity" enables farmers, foresters, and conservationists to store seeds for decades under conditions of low moisture and temperature (see here). The oldest documented viable seed is that of the Date Palm (Phoenix dactylifera): a seed discovered in Herod's palace in Israel germinated after 2,000 years. Such longevity is exceptional, but there are other cases of historical seeds retaining their viability after centuries, particularly if they have been stored in a cool, dry place. Understanding seed longevity is essential if you want to keep seed for any length of time, and recent studies in seed banks have shown that even some orthodox seeds are relatively short-lived, losing their viability after a few decades. Seeds with small embryos from species found in temperate ecosystems (among them the Tree Heath, Erica verticillata, and Cowslip, Primula veris) seem to be comparatively short-lived.
HOW DID SEED PLANTS EVOLVE?
The plants we recognize today evolved from cyanobacteria over a period of many millions of years. The first seed-bearing plants (spermatophytes) did not appear until the end of the Devonian period, some 360 million years ago (Mya). The first plants were marine organisms and either small single-celled or branching filaments dating back to the Cambrian period (541–485.4 Mya). However, the process of fossilization makes distinguishing these plants from other soft-bodied life forms difficult. The fossil record shows that the first terrestrial plants appeared about 450 million years ago during the Ordovician period. They were similar to today's liverworts, and produced spores rather than seeds. These early land plants did not have tissues to transport nutrients and water, and so were restricted not only in size but also in habitat. Their inability to transport water meant that they were limited to wet environments.
In order to grow to any size and to survive in drier environments, plants needed to evolve a means to transport both water and nutrients internally. The first evidence of land plants with such a transport system, known as vascular tissue, is found in the Silurian period (444–419 Mya). These plants, called Cooksonia, were small and had branching stems ending in sporangia — flattened knobs filled with spores. It was not until some 410 million years ago, during the Devonian period, that plants started to develop more complex and diverse structures. Stems started to bear scalelike structures that resembled simple leaves, and some fossils have spine-covered stems. As the Devonian period progressed, plants grew taller, reaching up to 60 ft (18 m) tall. However, all of these were spore-bearing species, and it is not until the middle to late Devonian period that seed plants reveal themselves in the fossil record.
The earliest recorded seed-bearing plants had simple, branching stems with seeds located along the length of the branches in loose, cuplike structures called cupules. The cupules are thought to have been formed from fused, reduced leaves. These seeds were primitive and lacked many of the features associated with today's seeds, such as a hard seed coat. The structure that now forms the seed coat, the integument, wrapped around the seed inside the cupule. As seeds evolved, the integument enclosed the seed more tightly, with an opening at one end, called the micropyle, to allow the entry of pollen and sperm to fertilize the egg cell in the preovule. By the end of the Devonian, a number of seed-bearing plants had appeared. Some resembled ferns but had seeds and cupules. During the Carboniferous period (358.9–298.9 Mya), the dominant plants were the horsetails, club mosses, and ferns. In the late Carboniferous and the Permian period that followed, seed-bearing plants began to evolve. These included the gymnosperms in the Pinophyta, and the Ginkgophyta and Gnetophyta. The Cycadophyta appear in the fossil record at the beginning of the Mesozoic era some 250 million years ago.
The Magnoliophyta or flowering plants (shown here), first appear in the fossil record some 125–130 Mya, during the Triassic period, when they diverged from the gymnosperms. How this happened, and from which gymnosperms, is still not clear; it is possible that the gymnosperm ancestors of today's flowering plants are now extinct. The flowering plants diversified significantly during the Cretaceous period, replacing the gymnosperms as the dominant tree species 100–60 Mya. Today, these are the dominant plants — an estimated 350,000 species of flowering plants have been described, compared to around 1,000 gymnosperm species.
SEEDS & HUMANS
Early humans supplemented their meat-based diets through gathering fruits, roots, and seeds, and the importance of seeds in the diet of hunter-gatherers is still apparent today. For example, the Manketti Tree (Schinziophyton rautanenii) is still an essential part of the bushman's diet in the Kalahari Desert, producing highly nutritious almondlike seeds in profusion. However, hunting and gathering is a precarious way to survive, and large areas of land are needed to support comparatively few people.
The adaptive leap that humans made from collecting grains and seeds to planting and harvesting them seems to have occurred in parallel in several different places. In around 9500 BCE, Wheat (Triticum aestivum), Barley (Hordeum vulgare), Pea (Pisum sativum), and Lentil (Lens culinaris) were domesticated in the Fertile Crescent — in what is now Iran and Iraq. At around the same time, Rice (Oryza sativa) was first cultivated in China, followed by Soybean (Glycine max). In the Andes, the Potato (Solanum tuberosum) was domesticated around 8000 BCE, together with beans. In New Guinea, Sugarcane (Saccharum officinarum) and the Yam (Dioscorea alata) appear in the archeological record about 7000 BCE. In Africa, Sorghum (Sorghum bicolor) was domesticated in about 5000 BCE, and in Central America, Maize (Zea mays) was first cultivated around 4000 BCE. Domestication of livestock occurred over a similar period of time. The transformation of wild plants into crops through artificial selection and breeding enabled human communities to establish themselves in villages, towns, and cities, and to flourish. Furthermore, the ample food that agriculture provided meant that not everyone needed to be occupied gathering food, enabling complex societies to evolve. It is no exaggeration to say that the seed is the basis of human civilization. To this day, 50 percent of our calorific intake comes from just three grains — Wheat, Rice, and Maize.
Since the advent of agriculture there have been various technological advances in crop production. Early farmers selectively bred higher-yielding crops and practiced primitive forms of crop rotation based on trial and error. Irrigation followed, then the use of oxen or other draft animals for plowing. By the advent of the Industrial Revolution, farming was a specialized and sophisticated endeavor. Between the sixteenth and nineteenth centuries, the Agricultural Revolution in Europe saw yields double due to technological leaps in crop selection, rotation, and mechanization. In the mid-nineteenth century, scientists began to gain an understanding of fertilization and the first artificial fertilizer factories were established. The invention of what is termed the Haber–Bosch process in the early twentieth century enabled mass production of ammonium nitrate. Despite these technological advances, famines were still a common experience in the twentieth century. The end of World War Two heralded a new era for global agriculture, which has since come to be known as the Green Revolution.
Led by an American crop scientist, Norman Borlaug, the Green Revolution took place between the 1940s and the 1970s, and comprised a combination of research, development, and technology transfer. Major breakthroughs were made in crop breeding, particularly with the development of high-yielding hybrids. In addition, the expansion of irrigation, the scaling up of land under agriculture, the introduction of modern management techniques, and the widespread use of fertilizers and pesticides all led to huge increases in food production.
While these technological advances have undoubtedly saved many lives — Borlaug is credited with saving more than a billion people from starvation — the costs to the environment have been high. Fertilizers and pesticides have polluted water and soil, and in the case of pesticides, have entered the food chain, causing health problems for people and animals. Furthermore, the industrialization of agriculture has created large-scale farms at the expense of natural habitats and wildlife. It is estimated that humans have transformed 40 percent of the terrestrial landscape for both crop and livestock production. Another consequence of the industrialization of agriculture is that the most widely used seed varieties are produced by a handful of large, multinational companies, leading to the loss of many traditional crop varieties and landraces.
Excerpted from "The Book Of Seeds"
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Table of Contents
Introduction What is a seed? How did seed plants evolve? Seeds & humans Seed conservation Plant diversity & why it matters Seed-bearing plants CYCADOPHYTA GINKOPHYTA & GNETOPHYTA PINOPHYTA MAGNOLIOPHYTA Appendices Glossary Resources Index of common names Index of Latin names Acknowledgments