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.

SEED ANATOMY

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.

SEED MORPHOLOGY

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...