The union of gametes triggers the ovule to develop into a seed that can carry the new embryo away from the parent plant. Because seeds develop from fertilized ovules, they contain tissues produced by three generations (Fig. 30.9). On the outside is the protective seed coat, which is formed from tissues that surround the sporangium and thus is a product of the diploid sporophyte. At the center is the embryo, which develops from the zygote and represents the next sporophyte generation. In seed plants, the embryo develops an embryonic root at one end and an embryonic shoot with one to several embryonic leaves at the other. In gymnosperms such as pine trees, the embryo is surrounded by the haploid female gametophyte, which provides the raw materials that support the growth of the embryo. In angiosperms, the origin of this storage tissue differs, but it serves the same role of nourishing the embryo.

Compared with a unicellular spore, a seed is able to store more resources to support the growth and establishment of a new sporophyte generation. Many seeds contain high levels of protein and energy-rich oils (Chapter 2). As a result, seeds are an important food resource for animals and are the cornerstone of agricultural production. Because seeds are larger than spores, however, they are not as easily dispersed by wind. As a result, seed plants have evolved structures to enhance seed dispersal. For example, pine seeds have flattened extensions, referred to as “wings,” that increase the distances over which they can be carried by a breeze. Other seed plants produce brightly colored and/or fleshy structures to attract and reward animals that may inadvertently carry seeds farther away from the parent plant than they could disperse on their own. Many plants dispersed by animals develop seed coats that allow their seeds to pass unharmed through an animal’s digestive system.

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During seed maturation, most seeds lose water. And as water content falls, the seeds’ metabolic activity drops to extremely low levels and the embryo ceases to grow. The combination of low metabolic activity and ample stored resources allows seeds to survive for long periods, in many cases one to several years, and sometimes much longer. Some seeds exhibit dormancy, meaning that they can delay germination even when conditions for growth, notably temperature and moisture, are favorable. Dormancy prevents seeds from germinating at the wrong time, for example on an uncharacteristically warm day at a time of year when the seedlings would be unable to survive. It also prevents seeds from germinating all at once, thus spreading the risk of making the transition from embryo to seedling over a larger time span and range of conditions.

Seeds span over 11 orders of magnitude in size (Fig. 30.10). The 100-nanogram (ng) seeds of orchids are so small (100 ng = 0.0001 mg) that they cannot germinate successfully without obtaining nutrients from a symbiotic fungus. At the other end of the spectrum are the 30-kilogram (kg) seeds of the coco-de-mer, a palm tree found on islands in the Indian Ocean. Large seeds, being better provisioned, are well adapted for the deep shade of a forest understory. However, large seeds typically disperse shorter distances and they often have little or no dormancy. In contrast, for the same investment in resources, a plant can make many small seeds, which can persist in the soil until the right combination of conditions triggers germination. As a result, most weeds produce small seeds.

Quick Check 4 Name three advantages of seeds over spores in terms of the probability that the next sporophyte generation will become successfully established.