| Themes > Science > Botanical Sciences > Plant Reproduction and Development > Plant Propagation Methods > Seed Structure |
A seed consists of three parts: a Dormant Embryo, a Storage Tissue, and a Seed Coat. Not every seed that has evolved on this planet has precisely the same structure. In some seeds, the endosperm is retained as the storage tissue. In other seeds, the endosperm is more or less used up to put storage chemicals into the embryo itself (commonly in the cotyledons). Below is a diagram of two hypthetical seeds. The upper seed shows a dicot that lacks endosperm; its storage material (blue) is held in the cotyledons, the lower seed shows a monocot that has a well-developed endosperm (also blue). Both of these examples qualify as true seeds because they possess all three parts needed to make a true seed. |
Below is a photomicrograph of a longitudinal section of a Capsella seed. The seed coat is the three-layered outer covering. The outermost and innermost layers of the seed coat pick up a red dye used in the preparation of this sample. That dye is preferentially held by parts of the specimen that have waxy or brittle biomolecules. Cutin, suberin, lignin are three components of plant cells that pick up the red dye. This leads us to the idea that the integument has a waterproofing (cutin/suberin) function and also possibly a mechanical strengthening (lignin) function that could impede herbivory.
The embryo has an axis (on the left) with a downward-pointing radicle including a root apex. The radicle ultimately penetrates the seed coat in seed germination, branches profusely, and becomes the primary root system. Toward the top of the embryonic axis, the radicle becomes hypocotyl...transitioning from root-type anatomy into stem-type anatomy as we shall see later. In seeds with hypogeous germination, this hypocotyl is too short and does not grow above the soil. In species with epigeous germination, the hypocotyl elongates rapidly, lifting the cotyledon(s) out of the soil and into the air. The axis has two appendages, the cotyledons; as there are two cotyledons, Capsella is a dicotyledonous plant. There are two classes of flowering plants, the dicots and the monocots, and they are distinguished (in part) on the basis of the number of cotyledons found on their dormant embryos. The part of the axis where the cotyledons attach is called a node because this zone of the axis does not elongate. At the extreme top of the axis is a shoot apex; after germination this shoot apex will elongate, and make appendages to produce the collection of stems and leaves that constitute the plant shoot. |
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Looking
toward the inside of the seed coat, there is a bent embryo (dark
looking) surrounded by some loose and light colored cells of the remaining
endosperm. Indeed the paucity of endosperm leads us to the
conclusion that much of the storage material has been moved to the embryo
itself...perhaps much of that in the cotyledons. However, this seed is
clearly somewhere between the two extremes diagrammed above.