18.11: Following fertilization, the ovule develops into a seed.

After fertilization within the ovule, numerous cell divisions occur rapidly. The developing embryo forms a root meristem and a shoot meristem, each of which is a cluster of active, dividing embryonic cells that will generate new tissue. (We discuss the unique qualities of meristems in more detail in Section 18-14.) Also, one or two cotyledons form, structures in the embryo that usually become the embryonic leaves (see Section 17-2). As the embryo matures, the outer cells of the ovule develop into a hard casing. The result is a seed—a sort of “plant in waiting” that consists of the embryo (meristems and cotyledons) and any remaining endosperm, surrounded by the casing (FIGURE 18-22).

At the same time the seed is forming, the ovary wall—surrounding one or more seeds—develops into a fruit, which in many species is fleshy, juicy, and edible, but in other species may be dry or inedible. The fruit both protects the seed inside and, as we’ll see, aids in its dispersal.

Metabolism and oxygen consumption grind to a halt, and the seed dries out, reducing its water content to about 15% of its total weight. Then the waiting begins.

Humans have learned a trick that turns one type of seed into an unusual snack. Each kernel of popcorn comes from a single kernel of corn on a cob and is the equivalent of a fruit: it includes a fertilized egg surrounded by a nutritious, edible outer layer, formed from the ovary wall. (If you plant some popcorn kernels, they will grow!) There is also moisture within the kernel—some oil and some water—and the kernel is surrounded by a moisture-proof coat. Heating the kernel turns the water inside to steam and chemically alters the starch and protein in the seed, making it soft and a bit gelatinous. With continued heating, the pressure from the steam rises and ultimately blasts through the seed coat with a pop. The fluffy white material is the starch and protein that surrounded the seed.

Before a seed can grow into a new plant, it must leave the parent plant. In the next section we look at how ever-immobile plants ensure that their seeds are moved and prepared for growth.