The structure of a maize kernel is typical of the grass family (Figure 2). The kernel is a hard, one-seeded fruit called a caryopsis. It consists of:

Pericarp. The pericarp is a protective outer layer, derived from maternal tissue.

Endosperm. The endosperm constitutes the major portion of the kernel (about 82-84 % dry weight), and serves as energy reserve for the growing seedling. The endosperm is about 88 % starch and 8 % protein.

There are two types of endosperm, flinty (hard) and floury (soft). The distribution of flinty and floury endosperm depends on variety and climatic conditions. Dent varieties have flinty endosperm on the sides of the kernel, and a central core of floury endosperm. The floury portions shrink when dry, and the tip of the kernel becomes depressed or dented.

In contrast, flint varieties have flinty endosperm over the tip of the kernel.
Therefore, the crown of the kernel does not shrink as it dries.

The outermost layer of the endosperm is called the aleurone. The aleurone layer produces enzymes that convert starch to sugar. The developing seedling uses the sugar as energy.

Germ (embryo). The germ comprises 10-12 % of total dry weight of the kernel. The germ consists of embryo axis and scutellum (Figure 3). The embryo axis contains both the shoot and root primordial.

The scutellum helps the seedling to digest and absorb starch from the endosperm. The shoot primordial consists of 5 embryonic leaves, the stem and growing point (apical meristem), and a protective sheath for the growing shoot, called the coleoptile.

The embryonic leaves and the stem are collectively called the plumule. The root end of the embryo axis contains the radicle (first root), several lateral root initials, and a protective sheath called the coleorhiza.

The germ contains about 8% starch, 18% protein and 33% oil. The germ contributes most of the oil, sugar, and ash (including minerals and vitamins) in the kernel.

Germination. As soon as the seed absorbs water, certain chemical changes occur. The aleurone layer releases enzymes which convert starch in the endosperm into sugar, thereby providing energy for seedling growth.

The radicle elongates first, emerging from the seed coat. Then the shoot elongates and the coleoptile breaks through the seed coat to complete germination. Elongation of the first internode (mesocotyl; Figure 4) pushes the coleoptile upward towards the soil surface. The seed remains in the soil. Therefore, germination in maize is hypogeal.

The coleoptile emerges from the soil 6-10 days after planting (growth stage VE) and soon stops growing. Sunlight at the soil surface stimulates the plumule to grow and break through the coleoptile.

The coleoptile is the fine green shoot observed when maize is first emerging from the ground. The coleoptile should not be confused with true leaf number 1 which has a rounded tip.

The young seedling develops several lateral roots which, with the radicle, comprise the seminal roots (Figure 4). The initials of these seminal roots are already present in the embryo.

Although the seminal roots make up a small proportion of the total root mass, they play an important role in anchoring the seedling and providing water and nutrients for early growth.

The main root system develops from the crown, which is just below the soil surface. The crown roots develop shortly after the seedling emerges.

Appearance of the collar of the first leaf (stage V1) marks the end of seedling growth. From this stage on, the endosperm becomes less important as a source of food for the young plant. The plant can now photosynthesize on its own.