Seed Germination

Many plants are easily grown from seeds. Although its embryo is alive, a seed will not germinate, or sprout, until it is exposed to certain environmental conditions, The delay of germination often assures the survival of a plant. For example, if seeds that mature in the fall were to sprout immediately, the young plants could be killed by cold weather. Similarly, if a plant's seeds were to sprout all at once and all of the new plants died before producing seeds, the species could become extinct. Many seeds will not germinate even when exposed to conditions ideal for germination. Such seeds exhibit dormancy, which is a state of reduced metabolism in which growth and development do not occur. The longevity of dormant seeds is often remarkable. A botanist once germinated lotus seeds that were almost 1,000 years old.

Conditions Needed for Germination

Environmental factors, such as water, oxygen, and temperature, can trigger seed germination. Most mature seeds are very dry and must absorb water to germinate. Water softens the seed coat and activates enzymes that convert starch in the cotyledons or endosperm into simple sugars, which provide energy for the embryo to grow. As the embryo begins to grow, the softened seed coat cracks open. This enables the oxygen needed for cellular respiration to reach the embryo. Many small seeds need light for germination. This adaptation prevents the seeds from sprouting if they are buried too deeply in the soil. In addition, some seeds germinate only if exposed to temperatures within a certain range.

Some seeds germinate only after being exposed to extreme conditions. For example, animals often swallow the seeds of fruits they eat, as shown in Figure 30-13. Acids in the digestive system wear away the hard seed coat. The seeds may germinate after passing through the digestive systems of these animals. As an added bonus, the seed is deposited with a bit of natural fertilizer.

Seeds Through Animals

figure 30-13

Word Roots and Origins dormancy from the Latin dormire, meaning "to sleep" Topic: Seeds Keyword: HM61370 Topic: Seeds Keyword: HM61370

Maintained by the

Maintained by the ru rur National Science l!\ J\ J. Teachers Association figure 30-13

Many animals eat apples or other fruits. The seeds are swallowed and are exposed to acids as they pass through the animal's digestive system. The acids wear away the seed coat. Once the seed is on the ground, water and oxygen can enter the seed and enable the growing embryo to break out.

Bean Seed Germination Logo Png

figure 30-14

(a) During the germination of a bean seed, the cotyledons and seed coat emerge, and the plumule is protected by a hook in the hypocotyl. (b) During the germination of a corn seed, the cotyledon and seed coat remain below ground and the plumule is protected by a sheath.

Plumule Cotyledon Radicle


First leaf

First leaf

Germinating Maize Seed

Endosperm (b) CORN SEED GERMINATION Topic: Seed Germination Keyword: HM61366

Maintained by the

Apple Endosperm Topic: Seed Germination Keyword: HM61366

Maintained by the r. |S | r - Hj National Science ll\ !\ J. Tsachen Association

Other seeds, including apple seeds, must be exposed to near-freezing temperatures for several weeks before they sprout. This temperature requirement prevents the seeds from germinating in the fall and thus ensures that the seedlings will not be killed by the cold temperatures of winter. The cold temperatures cause chemical changes within the seed. These changes enable the embryo to grow.

Process of Germination

Figure 30-14 compares germination in corn and bean seeds. The first visible sign of seed germination is the emergence of the radicle. In beans, the entire root system develops from the radicle. In corn, most of the root system develops from the lower part of the radicle. Soon after the radicle breaks the seed coat, the shoot begins to grow.

In some seeds, such as bean seeds (Figure 30-14a), the hypocotyl curves and becomes hook-shaped. Once the hook breaks through the soil, the hypocotyl straightens. This straightening pulls the cotyledons and the embryonic leaves into the air. The embryonic leaves unfold, synthesize chlorophyll, and begin photosynthesis. After their stored nutrients are used up, the shrunken bean cotyledons fall off.

In contrast, the cotyledon of the corn seed (Figure 30-14b) remains underground and transfers nutrients from the endosperm to the growing embryo. Unlike the bean shoot, the corn cotyledon remains below ground. The corn plumule is protected by a sheath that pushes through the soil. Once the sheath has broken through the soil surface, the plumule grows up through the sheath and the first leaf unfolds.

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