Embryos develop within seeds

Shortly after fertilization, highly coordinated growth and development of embryo, endosperm, integuments, and carpel ensues. The integuments—protective tissue layers immediately surrounding the megasporangium—develop into the seed coat, and the carpel ultimately becomes the wall of the fruit that encloses the seed.

The first step in the formation of the embryo is a mitotic division of the zygote that gives rise to two daughter cells. These two cells face different fates. An asymmetrical (uneven) distribution of cytoplasm within the zygote causes one daughter cell to produce the embryo proper and the other daughter cell to produce a supporting structure, the suspensor (Figure 39.6). The suspensor pushes the embryo against or into the endosperm and provides one route by which nutrients pass from the endosperm into the embryo.

With the asymmetrical division of the zygote, polarity has been established, as has the longitudinal axis of the new plant. A long, thin suspensor and a more spherical or globular embryo are distinguishable after just four mitotic divisions. The suspensor soon ceases to elongate. However, cell divisions continue, the primary meristems form, and the first organs begin to form within the embryo.

In eudicots (monocots are somewhat different), the initially globular embryo takes on a characteristic heart stage form as the cotyledons ("seed leaves") start to grow. Further elongation of the cotyledons and of the main axis of the embryo gives rise to what is called the torpedo stage, during which some of

Torpedo-stage

Torpedo-stage

Embryo r

Shoot Apex And Cotyledons

Endosperm

Cotyledons

Shoot - apex

Hypocotyl

Root apex

Suspensor

Seed coat the internal tissues begin to differentiate (see Figure 39.6). Between the cotyledons is the shoot apex; at the other end is the root apex. Between the shoot and root apices is the hypocotyl. Each of the apical regions contains an apical meristem whose dividing cells will give rise to the organs of the mature plant.

During seed formation, large amounts of nutrients are moved in from other parts of the plant, and the endosperm accumulates starch, lipids, and proteins. In many species, the cotyledons absorb the nutrient reserves from the surrounding endosperm and grow very large in relation to the rest of the embryo (Figure 39.7a). In others, the cotyledons remain thin (Figure 39.7b); they draw on the reserves in the endosperm as needed when the seed germinates.

In some eudicots, the cotyledons absorb much of the endosperm and fill most of the seed.

In other eudicots, the endosperm remains separate and the cotyledons remain thin.

In some eudicots, the cotyledons absorb much of the endosperm and fill most of the seed.

Seed coat Cotyledon Shoot apex

Root apex-Cotyledon Endosperm

(a) Kidney bean

Seed coat Cotyledon Shoot apex

Root apex-Cotyledon Endosperm

In other eudicots, the endosperm remains separate and the cotyledons remain thin.

Endosperm Kidney Bean

(b) Castor bean

39.7 Variety in Angiosperm Seeds In some seeds,such as kidney beans (a), the nutrient reserves of the endosperm are absorbed by the cotyledons. In others, such as castor beans (b) and corn (c), the reserves in the endosperm will be drawn upon after germination.

Seed coat Cotyledon Shoot apex

Root apex Endosperm

Seed coat Cotyledon Shoot apex

Root apex Endosperm

Kidney Bean Germination

(a) Kidney bean

(b) Castor bean

39.7 Variety in Angiosperm Seeds In some seeds,such as kidney beans (a), the nutrient reserves of the endosperm are absorbed by the cotyledons. In others, such as castor beans (b) and corn (c), the reserves in the endosperm will be drawn upon after germination.

In the late stages of embryonic development, the seed loses water—sometimes as much as 95 percent of its original water content. In this desiccated state, the embryo is incapable of further development; it remains quiescent until internal and external conditions are right for germination. (Recall from Chapter 38 that a necessary early step in seed germination is the massive imbibition of water.) In addition to embryo and endosperm development, the structures of the ovary are also undergoing developmental changes to form a seed and fruit.

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Responses

  • Gandolfo
    What is the torpedo stage in seeds?
    8 years ago
  • Susanne
    What are the steps in seed and embryo development?
    8 years ago
  • jorma
    What is the difference between endosperm and cotyledon?
    8 years ago
  • ines
    Does a kidney bean seed have an endosperm?
    8 years ago

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