Grass morphology and anatomy

Introduction

The purpose of this page is to introduce you to the basic structure of the grass plant. Generally speaking, grass plants are organized like other flowering plants. They have three types of vegetative (non-reproductive) organs. These are the stem, which supports the leaves and in grasses may also help the plant spread horizontally; the root, the anchoring and absorptive organ that takes up water and some nutrients; and the leaf, the major photosynthetic organ. Branching in grasses occurs by means of buds that form on the sides of the stems, typically in the site of a leaf axil (the angle between the stem and the upper, or adaxial, side of the leaf). Grasses are typical of monocots in having parallel leaf venation and fibrous root systems. Like other monocots, they cannot produce true wood.

The reproductive unit of a grass is a floret (a tiny flower), and florets are typically arranged in groups called spikelets, which are themselves grouped into larger structures called inflorescences. Grass flowers are very different than what we may think of when we picture a flower. They have no petals and are not very eye-catching. Nevertheless, they produce pollen and ovules (immature seeds), structures that carry out sexual reproduction in the plant.

Stems

Stem morphology (form)

The stems of grasses, called culms or stalks, are generally cylindrical (circular in cross section). They do not commonly branch above the ground. Daughter shoots, called tillers, may sprout from the base of the stalk. They may also sprout from stolons, stems that creep along the ground for a short distance, or from rhizomes, stems that grow horizontally below the ground. The base of the plant at ground level is called the crown. Most grasses can send up new tillers from the crown if the original stem is cut.

The stem of a grass is punctuated by nodes. A node is a place where a leaf attaches to a stem. In grasses, nodes are often swollen; because of their swollen nodes, grass stems are sometimes described as having jointed stems. Intervals of stem between nodes are known as internodes.

Note: Be careful in identifying grass nodes! Grasses have sheathing leaves, meaning that the base of the leaf forms a sheath that clasps the stem, whereas the leaf blade is free. The node is found at the base of the sheath. For more discussion, see the section on leaves below.

Stem anatomy (internal structure)

Grass stems are frequently hollow in the internodes and solid at the nodes; however some grasses, like maize (Zea), have a solid core in both the nodal and internodal regions. The outside of the stem is covered by a layer of cells known as the epidermis. The inside of the stem consists of vascular bundles (strands of vascular tissue, meaning water-conducting xylem and food-conducting phloem) that are surrounded by ground tissue (tissue within the epidermis and surrounding the vascular tissue) that is often made up of thin-walled cells known as parenchyma cells. The vascular bundles are either scattered randomly or arranged in two rings within the ground tissue. Each vascular bundle typically has a bundle cap made up of thick-walled, elongated cells called fibers.

The vascular bundles of grasses cannot transition to secondary growth, or the type of growth that produces wood. Thus, grasses are always non-woody plants. (Yes, even bamboo!)

Roots

Root morphology

Grasses have a fibrous root system. This root system does not develop from the radicle, or the root of the grass embryo. Instead, it is made up of adventitious roots, or roots that grow from the stem. The fibrous root systems of grasses differ in size and depth depending on environmental and genetic factors as well as the amount of grazing or mowing that occurs during the plant’s lifetime. Unlike stems, roots are not organized into nodes and internodes.

Root anatomy

The grass root is covered by an outer epidermis. Beneath the epidermis is a region of ground tissue known as the cortex. Ground tissue may also occur in the center of the root, forming a pith. The vascular tissues may be organized in a ring between the cortex and pith, although the vascular anatomy of grass roots varies.

Leaves

Leaf morphology

Grass leaves are alternate and distichous in arrangement. This means that one leaf is attached at each node, and the leaves form two vertical rows on the stem. There are two main parts of a grass leaf: the blade and the sheath. The sheath, extending from the node toward the tip of the stem, surrounds the stem like a tube. Farther up the leaf is the blade, which is the part of the leaf that does not surround the stem but instead extends outward to intercept sunlight. The blade usually tapers to a pointed or blunt tip.

The ligule is a structure at the junction of the sheath and the blade. It is a membranous flap of tissue or a ring of tiny hairs on the upper side (adaxial side) of the leaf. The ligule clasps the stem, preventing dirt and water from entering the sheath. The outside (also known as the abaxial side, or the side facing away from the stem) of the leaf at the junction between the sheath and blade is called the collar. The collar is often distinctive and can be used to identify different grass species. Some grasses also have auricles, small projections at the base of the blade that may help to prevent the blade from being torn away from the stem.

Grass leaves have long, parallel veins. Veins are strands of vascular tissue. The large parallel veins are connected by minute transverse veins. The parallel veins converge where the leaf blade tapers at its tip. The midvein is the central vein of the leaf.

Leaf anatomy

A grass leaf in cross section is typically thin, with an upper and lower epidermis surrounding a series of roughly circular vascular bundles (veins) embedded in ground tissue called mesophyll. The cells of the mesophyll contain chloroplasts (membrane-bound structures containing green pigment) that carry out photosynthesis.

Leaf epidermis

The grass leaf epidermis is made up of rows of cells that parallel the length (the long axis) of the leaf. These include elongated long cells, short cells that are about the same size in length and width, large bulliform cells, and the cells of the stomatal apparatuses that surround the stomata (pores) in the epidermis.

Much of the epidermis is made up of long cells. The walls on the long sides of the long cells may be sinuous, giving them a puzzle-piece like appearance. Short cells occur singly or in short rows, with the long cells between them. The short cells include silica cells that contain silica bodies and cork cells that have suberin (a fatty compound) in their walls. In some grasses, large bulliform cells may occur in rows in the upper epidermis. These specialized cells can cause the leaf to fold or curl up in hot, dry conditions, helping to prevent water loss. 

The epidermis is punctured by stomata that exchange gases. Each stoma is surrounded by a stomatal apparatus made up of guard cells and subsidiary cells. The guard cells open and close the stoma. While in many plants the guard cells of the stomata are kidney-shaped, in grasses they are dumbbell-shaped. Each guard cell is flanked by a subsidiary cell. Subsidiary cells in grasses may be triangular, dome-shaped, or parallel to the guard cells. Long cells with concave end walls occur at either end of each stomatal apparatus; these are sometimes called interstomatal cells.

Leaf vascular bundles (veins)

In cross section, the vascular bundles in a leaf are arranged more or less in a line. Xylem (water-conducting tissue) occurs toward the upper side of each vascular bundle, and phloem (food-conducting tissue) toward the lower side. Thick-walled cells may extend from the vascular bundles toward the upper and lower sides of the leaf to provide support. 

The vascular tissue of each vascular bundle is surrounded by one or two rings of cells that make up a bundle sheath. Most grasses have a two-layered bundle sheath; the outer ring of cells is called the parenchymatous sheath and the inner ring of cells is the mestome sheath. The cells of the mesotome sheath may have thick walls impregnated with a fatty material called suberin, which is waterproof. Some C₄ grasses have a single-layered bundle sheath that lacks a mestome sheath.

In C₄ grasses, the cells of the parenchymatous sheath or bundle sheath (if only a single layer of cells is present) have chloroplasts. The chloroplasts are needed in the bundle sheath cells of C₄ grasses because carbon is fixed (incorporated into sugars) in these cells, a process that takes place in the chloroplasts. The bundle sheath cells of C₃ grasses have few chloroplasts, because the entire process of photosynthesis can take place in the mesophyll cells in these plants.

Mesophyll

The arrangement of cells in the mesophyll, a tissue made up of thin-walled parenchyma cells, varies depending on the type of grass. In C₃ grasses, the mesophyll that looks similar throughout the leaf. In C₄ grasses, the mesophyll cells are arranged in rings around the vascular bundles.

The anatomical differences between C₃ and C₄ grasses have to do with differences in the process of photosynthesis between these two types of plants. In C₃ plants, photosynthesis takes place in the mesophyll cells. In C₄ plants, photosynthetic reactions begin in the mesophyll cells and end in the bundle sheath cells.

Reproductive structures

Inflorescences, spikelets, and florets

An inflorescence (group of flowers) in grasses may have a stalk, called a peduncle, to which branches are attached. It may also have a central axis, the rachis. The peduncle or rachis often bears branches on which reproductive units called spikelets are arranged.

Each spikelet bears one, several, or many florets. The florets may or may not be attached to a rachilla (central axis or stem of a spikelet). Typically, two bracts (small modified leaves) called glumes occur at the base of each spikelet, below the florets. The glumes protect the florets in the spikelet before they are mature. Spikelets may be sessile (lack a stalk) or pedicellate (have a pedicel, or stalk).

The florets are usually attached to the rachilla in an alternate and distichous arrangement; in other words, the florets are attached one at a time to the rachilla and are arranged in two vertical rows. Often, the rachilla has a zig-zag course, changing direction at the attachment point of each spikelet.

Each floret consists of bracts and reproductive structures. Two types of bracts are attached at the base of each floret. The lower bract is the lemma, and the upper bract is the palea. The lemma and palea may be large enough to mostly or entirely cover the reproductive parts, although in some cases they are very small. 

The reproductive parts of the floret are attached above the lemma and palea. The reproductive parts may consist of stamens (pollen-producing male organs), a pistil (ovule-producing female organ), or both, with the stamens attached below the pistil. Florets that produce only pollen or ovules are unisexual; pollen-producing florets are called staminate or male florets and ovule-producing florets are called carpellate, pistillate, or female florets. Florets with both stamens and pistils are bisexual. Sometimes, florets may be sterile, which means that they do not have functional reproductive parts.

Florets that produce pollen (male and bisexual florets) most commonly have three stamens. Each stamen consists of a filament (stalk) and an anther (pollen-producing structure, usually with four fused pollen sacs).

Florets that produce ovules (female and bisexual florets) have one pistil, consisting of an ovary (ovule-containing structure) with two or three feathery stigmas (structures that receive pollen). The stigmas may be elevated on stalks known as styles. The ovary contains a single ovule, which is an immature seed.

Lodicules (small bracts, which may look like swellings or bumps) are typically found below the reproductive structures. The lodicules enlarge to help the floret open.

Fruits

After pollination and fertilization, female and bisexual flowers will mature to produce fruits. The grass fruit is the caryopsis or grain. A grain is fruit that contains one seed. The thin fruit wall is fused to the thin seed coat. The seed contains a tiny grass embryo and endosperm, which is stored food for the young grass.