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4.6.3: Mature Embryos and Seed Structure

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    32043

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    Learning Objectives
    • Locate the major seed structures and identify the function of each.
    • Compare eudicot and monocot seeds.

    Eudicot Seeds

    The seed is protected by a seed coat that is formed from the integument of the ovule (Figure \(\PageIndex{1}\)). The seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen. The hilum is a scar on the outside of the seed where it was attached to the endocarp (inner layer of the fruit wall). The micropyle is a small round structure next to the hilum where the pollen tube entered.

    A magnified bean seed showing the seed coat, hilum, and micropyle
    Figure \(\PageIndex{1}\): The external structures of a bean seed, an example of a eudicot (7X). The seed coat surrounds the seed. There is a round micropyle, where the pollen tube originally entered the ovule. The oval hilum is a scar from where the ovule was attached to the ovary. Image by Melissa Ha (CC-BY).

    The embryonic axis (root-shoot axis) runs the length of the embryo. On end of the embryonic axis is the plumule, the young shoot apex, which includes the shoot apical meristem and developing leaves (leaf primordia). At the other end of the embryonic axis is the radicle (embryonic root). In some species, the radicle is not apparent in the embryo (in which case the distal end of the root is simply the root tip). The embryonic axis does not include the cotyledons. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The portion of the embryonic axis between the cotyledon attachment and the shoot tip is the epicotyl (epicotyl means "above the cotyledons; Figures \(\PageIndex{2-3}\))). Some embryos lack a visible epicotyl because the cotyledons are attached to the embryonic axis at the shoot tip.

    A bean seed cut away to reveal the cotyledons and other embryonic structures
    Figure \(\PageIndex{2}\): Diagram of a mature embryo from a bean seed, an example of a eudicot. The seed coat is a protective layer surrounding the seed. The two cotyledons form the bulk of the embryo. The stem-like axis below the cotyledons in the hypocotyl. At the tip of the hypocotyl is the radicle (the embryonic root). The hilum is an oval scar on the outside of the seed, and the micropyle is the small round structure adjacent to it. Image by LadyofHats (public domain).
    The lower half of a corn kernel is the scutellum, and the upper half is endosperm. The two bean cotyledons fill most of it.
    Figure \(\PageIndex{3}\): The structures of bean (eudicot) and corn (monocot) seeds. Eudicots (left) have two cotyledons. Monocots (right) have one cotyledon, called the scutellum, which channels nutrition to the growing embryo. Both monocot and eudicot embryos have a plumule (which contains developing leaves and the shoot apical meristem), a hypocotyl (embryonic stem below the cotyledons), and a radicle that forms the root. The embryonic axis comprises everything between the plumule and the radicle, not including the cotyledon(s). The corn "seed" is technically a caryopsis, a one-seeded fruit in which the pericarp (fruit wall) is fused with the seed coat.

    The two cotyledons in the eudicot seed are connected to the rest of the embryo via vascular tissue (xylem and phloem). In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea; Figure \(\PageIndex{4}\)) or a bean (Phaseolus; Figures \(\PageIndex{2-3}\)) and the split peas (Pisum sativum; Figure \(\PageIndex{5}\)) of split pea soup are individual cotyledons loaded with food reserves.

    Peanuts with key structures labeled
    Figure \(\PageIndex{4}\): Each half of a peanut represents a cotyledon, which is rich with nutrients. The plumule, which contains tiny embryonic leaves, is found between the cotyledons. Image labeled from Jesusorizales (CC-BY-SA).
    A dissected pea embryo stained with iodine
    Figure \(\PageIndex{5}\): Like the peanut, each half of a pea represents a nutrient-filled cotyledon. The black coloration in the cotyledon is due to iodine staining the starch in the cotyledons. Most of the pea seed is filled with two cotyledons. The tiny embryonic axis consists of the plumule at the top and the descending hypocotyl. The radicle is at the tip of the hypocotyl. The seed coat surrounds the entire seed. Image by Melissa Ha (CC-BY).

    Because seeds have food reserves to fuel germination, they also are a nutritious food source for people. Studying the nutrient content of seed crops such as beans can be used to increase the nutritive value of the plant using biotechnology. Maria Elena Zavala (Figure \(\PageIndex{6}\)) is working to combat world hunger by manipulating plants to improve their nutritional qualities. For example, her research with beans is looking at how genetic engineering can be used to make the bean proteins more digestible and nutritious.

    Portrait of Maria Elena Zavala
    Figure \(\PageIndex{6}\): Maria Elena Zavala was the first Mexican-American woman to earn a PhD in botany in the United States. Her research includes manipulating plant genes as a way to improve plant productivity and address world hunger. Image by Lee Choo (CC BY-SA 4.0 )

    Monocot Seeds

    The seeds of the most complex monocot family, Poaceae (the grass family), which includes corn and wheat, are highly specialized. The testa and tegmen of the seed coat are fused. The fruit is a caryopsis (grain), a one-seeded fruit in which the fruit wall (pericarp) is fused to the seed coat. Thus, not only are the two layers of the seed coat fused, but the seed coat is fused to the pericarp.

    The single cotyledon is called a scutellum; the scutellum also has vascular connections to the rest of the embryo. The large inner layer of the endosperm that stores nutrients is called the starchy endosperm. The thin outer layer of the endosperm, which is a single layer of cells, is called the aleurone. Upon germination, enzymes are secreted by the aleurone. The enzymes degrade the stored carbohydrates, proteins and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.

    The root tip is protected by a sheath-like structure called the coleorhiza. Similarly, the coleoptile ensheaths the plumule at the shoot tip (Figure \(\PageIndex{7-10}\)).

    Labeled section of a corn grain revealing the embryo and endosperm. The most prominent feature of the embryo is the cotyledon (scutellum).
    Figure \(\PageIndex{7}\): Diagram of a corn kernel, an example of a "complex" monocot. The single cotyledon is called the scutellum. The outer layer is the pericarp (fruit wall) fused with the seed coat. The endosperm fills much of the seed and stores starch. It actually consists of a thin outer layer (aleurone) and a thick inner layer (starchy endosperm). The young shoot (plumule) consists of the shoot apical meristem surrounded by young leaves (first foliage leaves). It is surrounded by a sheath called the coleoptile. The young root (radicle) is surrounded by a sheath called the coleorhiza (not labeled). Image by Sarah Greenwood (CC-BY-SA).
    Microscope view of a corn grain, showing the plumule, coleoptile, scutellum, and endosperm
    Figure \(\PageIndex{8}\): Longitudinal section of a the upper portion of a corn embryo, a monocot (40X). The coleoptile is the sheath that surrounds the plumule, which consists of the shoot apical meristem and layers of developing leaves. The scutellum is the single cotyledon. A small portion of the endosperm is visible in this view. Image by Melissa Ha (CC-BY).
    Microscope view of the lower portion of a corn embryo
    Figure \(\PageIndex{9}\): Longitudinal section of a the lower portion of a corn embryo, a monocot (40X). The outer layer is the pericarp fused with the seed coat. The coleorhiza surrounds the radicle, which is the embryonic root. Image by Melissa Ha (CC-BY).
    A grass seed with each part of the embryo labeled with numbers
    Figure \(\PageIndex{10}\): A grass seed (monocot). The scutellum (1) is the modified cotyledon of complex monocots such as grass. The coleoptile (2) is the sheath covering the young shoot tip (plumule, 4). The coleorhiza (3) is the sheath covering the young root (radicle, 5). The endosperm (6) fills much of the seed and nourishes the seedling upon germination. The seed coat (7) surrounds the seed and protects it.

    Other Variations

    Seeds are diverse. Pine (Pinus, a gymnosperm and thus neither a monocot nor eudicot) has multiple (five or more) cotyledons. Some plants like orchids (Orchidaceae, a monocot) do not have developed embryo and even endosperm in seeds; their germination depends on a presence of symbiotic (mycorrhizal) fungus.

    Attributions

    Curated and authored by Melissa Ha using the following sources:

    This page titled 4.6.3: Mature Embryos and Seed Structure is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .