(C) Seeds of Flowering Plants

Although some are quite large, most flowering plant seeds are tiny. You will examine seeds of two dicotyledons and one monocotyledon. Some variation exists in shape and structure, but in general, seeds of all flowering plant species are similar. In some species seeds contain an endosperm while in others nutrients have been stored in the cotyledons.

(1) Beans and Peas

Study the external structure of the soaked bean and pea seeds. Note the conspicuous scar or hilum on one side. To what is this scar due? With a dissecting microscope examine the seed coat at either end of the hilum. Locate at one end of the hilum the micropyle. This is the structure through which the pollen tube entered the ovule, and in the seed functions as the structure through which water will enter the seed prior to germination.

Remove the seed coat noting the relative position of the micropyle and the radicle of the

embryo. This relation is true for all seeds. Is there an endosperm? The fleshy halves of the cotyledons are attached by slender connectives to the axis of the embryo. What is the function of these cotyledons? Are beans and peas monocotyledons or dicotyledons?

Examine the small embryonic axis and locate the epicotyl with its minute foliage leaves (sometimes called plumules). The radicle can be found at the other end. Can you observe the tip of the hypocotyl? From what did the seed coat develop?

(2) Corn

Examine a grain of corn (a grass) that has been soaked overnight in water. A corn grain is actually a fruit containing a single seed. What does this imply about the outer structure of the corn grain? From what parts of a flower did it develop? Note the yellower portion (endosperm) and the lighter shield‑shaped area (embryo) on one side. Neither hilum nor micropyle is apparent (why not?). Cut the seed longitudinally through the area containing the embryo. Examine the grain under a dissecting microscope. Stain the grain with iodine. Which area turns blue: embryo or endosperm? What is the significance of this reaction? You should be able to find, closest to the center of the grain, one large cotyledon, connected at its midpoint to the embryonic axis. The epicotyl of corn also contains a plumule enclosed in a coleoptile. Find and identify these areas. Below the epicotyl is the radicle, the hypocotyl along with the root cap. The radicle will also be enclosed in a sheath of cells called the coleorhiza. Coleoptiles and coleorhizae are found only in grasses.