24-2 Seed Development and Germination Photo Credit: ©Carl R. Sams II/Peter Arnold, Inc. Copyright Pearson Prentice Hall

Seed and Fruit Development Once fertilization is complete, nutrients flow into the flower tissue and support the development of the growing embryo within the seed. As angiosperm seeds mature, the ovary walls thicken to form a fruit that encloses the developing seeds. A fruit is a ripened ovary that contains angiosperm seeds. Biologically, a fruit is any seed that is enclosed within its embryo wall. This includes apples and grapes, as well as peas, corn, beans, rice, cucumbers, and tomatoes, which we commonly call vegetables. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Dispersal Seed Dispersal Seeds are dispersed by animals, wind, and water. Seeds of many plants are eaten by animals. These seeds are covered with tough coatings that protect them from digestive chemicals, allowing them to pass through a digestive system unharmed. The seeds then sprout in the feces eliminated from the animal. Seeds dispersed by animals are typically contained in fleshy, nutritious fruits. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Dispersal Seeds dispersed by wind or water are typically lightweight, allowing them to be carried in the air or to float on the surface of the water. Some seeds are encased in winglike structures that spin and twirl, helping to glide a considerable distance. A coconut is a seed that contains a liquid endosperm (“milk”) and is buoyant enough to float in seawater within its protective coating for many weeks. Tumbleweed plants break off at their roots and scatter their seeds as they are blown by the wind. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Dormancy Seed Dormancy Many seeds will not grow when they first mature. These seeds enter a period of dormancy, during which the embryo is alive but not growing. The length of dormancy varies in different plant species. Environmental factors such as temperature and moisture can cause a seed to end dormancy and germinate. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Dormancy Seed dormancy can be adaptive in several ways: allows for long-distance dispersal allows seeds to germinate under ideal growth conditions Other environmental conditions can end dormancy: Some pine seeds produced in sealed cones remain dormant until high temperatures generated by forest fires cause the cones to open. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Germination Seed Germination Seed germination is the early growth stage of the plant embryo. When seeds germinate, they absorb water which causes food-storing tissues to swell and crack open the seed coat. The young root grows through the cracked seed coat. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Germination In most monocots, the cotyledon remains underground. The growing shoot emerges while protected by a sheath. The corn seedling is a monocot in which the shoot grows directly upward, protected by its sheath. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Germination In dicots, germination takes place in one of two ways. In some species, the cotyledons emerge above ground, protecting the stem and first foliage leaves. The cotyledons may then wither and drop off the plant or become photosynthetic. In other species, the cotyledons stay underground and provide a food source for the growing seedling. The young stem grows longer and forms an arch that protects the delicate shoot tip. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Seed Germination Foliage leaves Young shoot Cotyledons Cotyledons Seed coat The garden bean is a dicot in which the cotyledons emerge aboveground. Primary root Germinating seed Bean (dicot) Copyright Pearson Prentice Hall