Corn is a coarse annual grass, ranging in height from 2 feet (60 cm) to more than 20 feet (6 m). The stem is pithy, not hollow as in most grasses, and is jointed like the bamboo. A long, pointed leaf springs from each joint. There may be as few as 7 leaves or as many as 45 on a single plant. The root system is fibrous. Most corn plants develop prop roots extra roots that form above ground and eventually penetrate the soil. These help to anchor the heavy plant.

The corn plant has two kinds of flowers. The male, or staminate, flowers bloom on a branching tassel at the top of the plant. They produce pollen. The female, or pistillate, flowers grow out of a spike that springs from the place where a leaf joins the stem. A single plant may have one or more spikes.

Tiny female flowers grow in rows along each spike. Each female flower consists of a few small petals and a single pistil. Each pistil consists of an ovary from which extends a long style. Together, all the styles on a spike are called the silk. The silk and spike are encased in leaves, but the tip of each style extends out beyond the leaves.

When a grain of pollen from a tassel reaches a style, the pollen germinates, forming a pollen tube that grows down the style until it reaches the single ovule in the ovary. Sperm nuclei from the pollen tube are discharged into the ovule, where one of them fertilizes the egg nucleus in the ovule. The ovary then matures into a kernel or grain and the spike develops into a pithy structure called a cob. Only those ovaries that receive pollen will produce kernels.

The kernel has three main parts. The hull is the tough outer covering. The embryo corn plant, or germ, is found in the bottom of the kernel. The rest of the kernel consists of the endosperm, a material composed largely of starch with some sugar. The endosperm serves as food for the developing germ.

The cob with its kernels is known as the ear. The ear is enclosed by long, strap-shaped leaves, called the husk. The size of the ear varies from less than six inches (15 cm) longas in some kinds of popcornto about 15 inches (38 cm) in varieties of dent corn. One extraordinary variety of Mexican corn has ears nearly three feet (90 cm) long. The grains may be white, yellow, red, brown, black, blue, or alternate colors. Most varieties of corn have yellow or white grains.

Some small varieties mature in 60 to 70 days. Larger types of corn require nearly 150 days. Giant tropical varieties take nearly 11 months to ripen.