A seed is a plant embryo surrounded by a protective coat called the seed coat. The structure of seeds differs among the major groups of seed plants—angiosperms, which include monocots and dicots, and gymnosperms. To understand some of the differences, examine the seeds shown in Figure 30-12.
Look at the bean seed in Figure 30-12a, which has been opened to reveal the structures inside. Most of the interior of a bean seed is filled by two large, fleshy cotyledons (seed leaves), which are part of the embryo. Recall that dicots have two cotyledons in their embryos. Therefore, beans are dicots. A mature bean seed has no endosperm. The endosperm was absorbed by the fleshy cotyledons.
Between the two cotyledons of a bean seed are the parts that make up the rest of the embryo. The shoot tip, along with any embryonic leaves, is called the plumule (PLOO-myool). The epicotyl (EP-I-KAHT-uhl) extends from the plumule to the attachment point of the cotyledons. The hypocotyl (HIE-poh-KAHT-uhl) extends from the attachment point of the cotyledons to the radicle. The radicle is the embryonic root. Along the concave edge of the seed, beneath the radicle, is the hilum (HIE-luhm), which is a scar that marks where the seed was attached to the ovary wall.
Now examine the corn kernel in Figure 30-12b. Technically, a corn kernel is a fruit, but the seed occupies almost the entire kernel. The wall of the fruit is very thin and is fused to the seed coat. A single umbrella-shaped cotyledon is pressed close to the endosperm. The cotyledon of a monocot seed does not store nutrients, as bean cotyledons do. Instead, it absorbs nutrients from the endosperm and transfers them to the embryo.
Finally, look at the pine seed in Figure 30-12c. A pine seed contains a sporophyte embryo that has needle-like cotyledons. The embryo is surrounded by tissue of the female gametophyte. Like the triploid endosperm of angiosperm seeds, the haploid tissue of the female gametophyte functions as a source of nourishment for the embryo.
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