5. Seeds


We have been talking about seeds but haven’t really mentioned what a seed is made of and how it becomes a mature plant.

Seed structure

First of all dicot and monocot seeds are different. Recall that a dicot has two seed leaves in the plant embryo, while a monocot has one seed leaf. These seed leaves are called cotyledons. The cotyledons are the food storage organs and will also serve as the first leaves of the growing plant. If you look at a kidney bean—a dicot—you will notice a small white scar on the inner concave edge of the seed, which is called the hilum. The hilum is where the ovule was attached to the ovary wall—analogous to a belly button in a human. The cotyledons are attached to a tiny embryo plant contains the undeveloped leaves and meristematic tissue at one end. The embryo shoot is called the plumule and the cotyledons are attached just beneath the plumule. Above the cotyledons is the stem portion of the axis, and is called the epicotyl. The portion below the cotyledon attachment is called the hypocotyls. The plant embryo is tiny and it will be difficult to see where the stem ends and the root begins. The embryonic root is called the radicle. In some monocots, the radicle and plumule are enclosed for added protection. The tubular sheathing structures are called the coleoptile for the plumule and the coleorhiza for the radicle. At some point, the embryonic shoot and root will overtake the protective structures, and the sheathing will cease growing.