Maize morphology and anatomy

Introduction

The purpose of this page is to introduce the basic structure of the maize plant. Generally speaking, maize plants are organized like other grasses in tribe Andropogoneae (the Bluestem or Sorghum Tribe), except that they have been modified by the process of domestication. The domestication of maize altered it from its wild form by reducing the amount of stem branching and by radically modifying its female reproductive structures, called ears. Maize ears are unusually large with many rows of soft-walled kernels (fruits) that stay attached to the ear at maturity.

For background on the general structure of the grass plant, visit the following page:

For background on the structure of grasses in Tribe Andropogoneae, visit the following page:

Stems

The stem of a maize plant is a long central stalk that supports every part of the plant. The only branches on the stem are those that support the reproductive parts. The strength of the stem allows maize to grow very tall; maize plants can reach as much as 12 feet (4 meters) in height, depending on the variety. Only one leaf grows per node, and stalks can have 20 or more nodes.

Unlike in many other grasses, the interior of a maize stem is solid in both nodal and internodal regions. Vascular bundles are scattered throughout the stem when it is viewed in cross section. 

Roots

Maize plants have three types of roots: the radicle or primary root, the seminal roots, and the nodal roots.

The radicle (called the primary root once it emerges from the kernel) makes up the lower part of the maize embryo. Seminal roots ("seed roots") are roots that first start to form on the embryo above the radicle. The primary root is the first root to emerge from the corn kernel when the seedling starts to grow. Three to four seminal roots appear after the radicle has emerged. The primary root and seminal roots are responsible for absorbing water and nutrients in order to establish the young seedling.

The radicle and the seminal roots will eventually die, and their function will be taken over by nodal roots. Nodal roots are adventitious roots, or roots that grows from the stem. Nodal roots arise from the stem at the nodes, places where leaves and their associated lateral buds develop. On a healthy maize plant, nodal roots can grow up to six feet (two meters) deep underground. 

Special nodal roots, called brace roots or prop roots, form at nodes above the soil surface. These roots function primarily to support the aboveground parts of the plant once it grows tall. When brace roots reach the soil, they can also perform regular root functions such as water and nutrient uptake. Over the course of a maize plant’s life, as many as 70 nodal roots will arise from the stem, both above and below the ground.

Leaves

Leaf morphology

As in other grasses, maize leaves alternate along the maize stem. This arrangement spreads the leaves out to maximize the amount of light they can capture. 

Maize leaves are organized like the leaves of other grasses. Each leaf consists of a sheath that wraps around the stem and an elongated blade that sticks out from the side of the stem and intercepts sunlight. The place where the sheath and the blade come together is the collar. The major leaf veins are parallel.

Each new leaf begins to grow from a node on the inside of the older (lower) adjacent leaf’s sheath. The last foliage leaf on the stem (the highest leaf) is called the flag leaf. The flag leaf is the leaf directly beneath the tassel.

Leaf anatomy

Leaf epidermis

The epidermis of the maize leaf is similar to that of other grasses. It has elongated long cells and shorter short cells that may contain silica (silica cells) or suberin (cork cells). The long cells have wavy side walls. The silica cells have crossed-shaped silica bodies (phytoliths).

Stomata (pores) occur between some of the long cells. The stomata are opened and closed by dumbbell-shaped guard cells, which are flanked on either side by triangular subsidiary cells.

Leaf vascular bundles and mesophyll

Like other members of Tribe Andropogoneae, maize is a C₄ plant. Each vascular bundle has a bundle sheath made up of one layer of cells with large chloroplasts near their outer walls. As is common in C₄ plants, mesophyll cells are arranged concentrically around the bundle sheath.

Reproductive structures

Maize plants are monoecious, meaning that flowers are unisexual (male or female), but both male (staminate, or pollen-producing) and female (pistillate, or ovule-producing) flowers grow on the same plant.

Male (staminate) structures

The male flowers are grouped in an inflorescence called a tassel at the tip of the stem. Maize tassels consist of a central stem bearing branches (also called rames) on its sides. Each branch bears many pairs of spikelets. Each spikelet pair includes one pedicellate (stalked) spikelet and one sessile (stalkless) spikelet.

Each spikelet has two male florets (tiny flowers). Each male floret has three stamens consisting of a stalk-like filament and an anther, or pollen-producing structure. Depending on the variety of maize, the anthers can vary in color between purple, red, and yellow. 

Female (pistillate or carpellate) structures

Female maize flowers are arranged in ears (also called rames or spikes), which occur at the ends of short, thick branches on the sides of the main stem. A series of modified leaves surround each ear, forming a structure called a husk. The thick central stem of the ear is called a cob. The female spikelets are in pairs like the male spikelets. Each cob may have as many as 24 rows of spikelets. 

Each spikelet has one fertile floret and one sterile floret. The sterile floret is made up of just a lemma and a palea. The fertile floret includes a lemma, a palea, and a pistil. Each fertile floret has one ovary and a short style with two long, fused stigmas, which are called a silk. The two glumes at the base of each female spikelet cover much of the ovary.

Fruits

Following pollination and fertilization, the fertile female florets mature into fruits. Maize fruits are often called kernels. Unlike in most andropogonoid grasses, ears of corn do not disarticulate (break apart) when they are mature. Ears that do not break apart when mature are one of the common features of domesticated grasses that are grown for their grains.

A kernel consists of a maize embryo and endosperm, which serves as food for the embryo once it begins growing into a seedling. The endosperm is divided into two types: the harder horny endosperm and the softer floury endosperm. The outer layer of the endosperm is the aleurone layer. The embryo and endosperm are surrounded by a thin pericarp, or fruit wall. The glumes do not cover the fruit when it is mature.

The maize embryo is complex. The upper end of the embryo is the plumule (young stem with young leaves), which is covered by a coleoptile, or protective sheath. The radicle (embryonic root) at the opposite end of the embryo, is also protected by a sheath, the coleorhiza. A cotyledon (seed leaf) called a scutellum is attached to the embryo between the plumule and the radicle. The scutellum shuttles nutrition from the endosperm to the embryo to provide energy during seed germination. 

The region of the embryo between the radicle and the attachment point of the scutellum is the hypocotyl, and the region between the plumule and the attachment point of the scutellum is the mesocotyl. Seminal roots, some of the first roots of the young maize plant, may develop on the hypocotyl. The mesocotyl elongates during germination, helping to push the shoot of the developing seedling through the soil.