Chapter 37 will describe the events of plant reproduction and development that lead to the formation of seeds. Here we begin with the seed, the structure that contains the early embryo. Unlike most animal embryos, plant seeds may be held in “suspended animation,” with the development of the embryo halted, for long periods. If development stops even when external conditions (such as water supply) are adequate for development, the seed is said to be dormant.

DORMANCY Seed dormancy may last for weeks, months, or years. Plants use several mechanisms to maintain dormancy:

Dormancy can be broken by conditions that overcome these mechanisms. For example, the seed coat may be damaged by passage through an animal’s digestive system, or heavy rains may wash away chemical inhibitors. There are some unusual methods to overcome dormancy. One example is the breaking of dormancy by components of smoke. Emmenanthe penduliflora is a common plant in dry chaparral of the southwestern United States, an area that is prone to wildfires.

767

These plants germinate rapidly after a fire. John Keeley of Occidental College in Los Angeles found that dormancy in seeds of this plant is broken not by heat but by smoke—in particular, by the nitrogen oxides found in smoke. Other molecules in smoke have been identified that regulate seed germination.

Plant biologists distinguish between seed dormancy, which prevents germination under conditions that are suitable for plant growth, and seed quiescence, which occurs when a seed fails to germinate because conditions are unfavorable for growth. Some seeds may remain quiescent, yet viable, for centuries; botanists have germinated a 1,300-year-old lotus seed recovered from a dry lake bed in China.

Seed dormancy and quiescence are common, so they must provide selective advantages for plants. Dormancy ensures that the seed will germinate at a time suitable for the plant to complete its life cycle. For example, some seeds require exposure to a long cold period (winter) before they germinate in the spring; this ensures that the plant has the entire growing season to mature and set new seeds. Dormancy and quiescence also help seeds survive droughts or long-distance dispersal, allowing plants to colonize new territory.

GERMINATION Seeds begin to germinate, or sprout, when dormancy is broken and environmental conditions are satisfactory. The first step in germination is the uptake of water, called imbibition (from imbibe, “to drink in”). Before germination, a seed contains very little water: only 5–15 percent of its weight is water, compared with 80–95 percent in most other plant parts. Seeds also contain polar macromolecules, such as cellulose and starch, which attract and bind water molecules. Consequently a seed has a very negative water potential (see Chapter 34) and will take up water if the seed coat is permeable to water. The force exerted by imbibing seeds, which expand several-fold in volume, demonstrates the magnitude of their water potential; for example, imbibing cocklebur seeds can exert a pressure of up to 1,000 atmospheres.

As a seed takes up water, it undergoes metabolic changes: enzymes are activated upon hydration, RNA and then proteins are synthesized, the rate of cellular respiration increases, and other metabolic pathways are activated. In many seeds, cell division is not initiated during the early stages of germination. Instead, growth results solely from the expansion of small, preformed cells.

As germination proceeds, starch, proteins, and lipids that are stored in the seed are hydrolyzed to provide metabolic energy and chemical building blocks—carbohydrates, amino acids, and lipid monomers—for the growing embryo. These reserves are stored in the cotyledons (the first leaf or leaves of the embryo) or in the endosperm (the non-embryonic storage tissue of the seed). Germination is completed when the radicle (embryonic root) emerges from the seed coat. The plant is then called a seedling.

If the seed germinates underground, the new seedling must elongate rapidly (in the right direction!) and cope with a period of life in darkness or dim light. Photoreceptors that sense light and specialized cells that sense gravity direct this stage of development and prepare the seedling for growth in the light.

The pattern of early shoot development varies among the flowering plants. Figure 36.1 shows the shoot development patterns of monocots and eudicots. In monocots, the growing shoot is protected by a sheath of cells called the coleoptile as it pushes its way through the soil. In eudicots, the shoot is protected by the cotyledons.