Lesson Overview 24.2 Fruits and Seeds

THINK ABOUT IT What are fruits, and what purpose do they serve for the plants that produce them? You, and all the animals that enjoy eating fruits, are being used. Plants may be smarter than you think.

Seed and Fruit Development How do fruits form?

Seed and Fruit Development How do fruits form? As angiosperm seeds mature, ovary walls thicken to form a fruit that encloses the developing seeds.

Seed and Fruit Development Once fertilization of an angiosperm is complete, nutrients flow into the flower tissue and support the development of the growing embryo within the seed. A fruit is a matured angiosperm ovary, usually containing seeds. Fruits vary in their structure.

Seed and Fruit Development The term fruit applies to the sweet things we usually think of as fruits, such as apples and strawberries. However, foods such as peas, corn, rice, and tomatoes, which we commonly call vegetables, are also fruits.

Seed and Fruit Development The ovary wall surrounding a fruit may be fleshy, as it is in grapes and tomatoes, or tough and dry, like the shell that surrounds peanuts. (The peanuts themselves are the seeds.)

Seed Dispersal How are seeds dispersed?

Seed Dispersal How are seeds dispersed? Seeds contained in fleshy, nutritious fruits are usually dispersed by animals. Seeds dispersed by wind or water are typically contained in lightweight fruits that allow them to be carried in the air or in buoyant fruits that allow them to float on the surface of the water.

Dispersal by Animals The seeds of many plants, especially those encased in sweet, fleshy fruits, are often eaten by animals. The seeds are covered with tough coatings, allowing them to pass through an animal’s digestive system unharmed.

Dispersal by Animals The seeds then sprout in the feces eliminated from the animal. These fruits provide nutrition for the animal and also help the plant disperse its seeds—often to areas where there is less competition with the parent plants.

Dispersal by Animals Animals also disperse many dry fruits, but not necessarily by eating them. Dry fruits sometimes have burrs or hooks that catch in an animal’s fur, enabling them to be carried many miles from the parent plant.

Dispersal by Wind and Water Seeds dispersed by wind or water are typically contained in lightweight fruits that allow them to be carried in the air or in buoyant fruits that allow them to float on the surface of the water. A dandelion seed, for example, is attached to a dry fruit that has a parachute-like structure, allowing the seed to glide considerable distances away from the parent plant.

Dispersal by Wind and Water Some seeds, like a coconut, are dispersed by water. Coconut fruits are buoyant enough to float in seawater for many weeks, enabling the seeds to reach and colonize even remote islands.

Seed Dormancy and Germination What factors influence the dormancy and germination of seeds?

Seed Dormancy and Germination What factors influence the dormancy and germination of seeds? Environmental factors such as temperature and moisture can cause a seed to end dormancy and germinate.

Seed Dormancy and Germination Many seeds will not grow when they first mature. Instead, these seeds enter a period of dormancy, during which the embryo is alive but not growing. Germination is the resumption of growth of the plant embryo.

Seed Dormancy and Germination Environmental factors such as temperature and moisture can cause a seed to end dormancy and germinate. The effect of temperature on the germination of Arisaema seeds is shown in the graph.

How Seeds Germinate Before germinating, seeds absorb water, which causes food-storing tissues to swell and crack open the seed coat. Through the cracked seed coat, the young root emerges and begins to grow. The shoot—the part of the plant that will emerge above ground—emerges next, as seen in the germinating corn seed.

The Role of Cotyledons Cotyledons are a flowering plant’s first leaves. They store nutrients and then transfer them to the growing embryo as the seed germinates.

The Role of Cotyledons Monocots have a single cotyledon, which usually remains underground while it passes nutrients to the young plant. The growing monocot shoot emerges from the soil protected by a sheath.

The Role of Cotyledons In dicots, which have two cotyledons, there is no sheath to protect the tip of the young plant. Instead, the upper end of the shoot bends to form a hook that forces its way through the soil. This protects the delicate tip of the plant, which straightens as it emerges into the sunlight.

Advantages of Dormancy Seed dormancy can allow for long-distance dispersal, and for seeds to germinate under ideal growth conditions. For some species, a period of cold temperatures during which the seeds are dormant is required before growth can begin. The period of cold that is required is long enough that seeds will not germinate until the dangerous winter season has passed.

Advantages of Dormancy Sometimes, only extreme environmental conditions can end seed dormancy. Some pine trees, for example, produce seeds in cones that remain sealed until the high temperatures generated by forest fires cause the cones to open. The high temperature both activates and releases the seeds, allowing the plants to reclaim the forest quickly after a fire.