Plant Tissue Systems: Meristems, Epidermal, and Ground

Plant tissues are grouped into three tissue systems: the dermal system (the epidermis, which covers the plant), the ground system (which fills most of the plant body) and the vascular system (xylem and phloem, which transport materials), and all of them arise from meristems, regions of dividing cells. In brief: apical meristems at the tips of roots and shoots cause growth in length (primary growth), and lateral meristems (vascular cambium and cork cambium) cause growth in width (secondary growth) in woody plants. The epidermis protects, ground tissue performs photosynthesis, storage and support, and vascular tissue transports.

Plants grow throughout their lives, unlike animals, because they keep populations of unspecialized dividing cells. This guide explains meristems, describes the epidermal tissue and its adaptations, describes the three types of ground tissue, shows the arrangement of the tissues in roots, stems and leaves, compares the tissue types, works through an identification example and corrects common mistakes. Vascular tissue is covered in the next article.

What are the plant tissue systems?

A tissue is a group of cells with a similar structure and function. Plant biologists divide plant tissues into two broad groups: meristematic (dividing) tissues and permanent tissues (cells that have differentiated and do not normally divide). Permanent tissues are arranged into three systems that extend throughout the plant body:

• Dermal tissue system: the outer covering.
• Ground tissue system: the tissue between the dermal and vascular tissue.
• Vascular tissue system: xylem and phloem.

What are meristems?

A meristem is a region of unspecialized cells that retain the ability to divide by mitosis. The cells are small, with thin walls, dense cytoplasm, a large nucleus and small vacuoles. Some daughter cells remain as meristem cells, and others elongate and differentiate into the permanent tissues.

• Apical meristems: found at the tips of roots and shoots (and in buds). They produce primary growth: an increase in length. Root apical meristems are protected by a root cap. Three primary meristems arise from them: protoderm (forms the epidermis), ground meristem (forms ground tissue) and procambium (forms vascular tissue).
• Lateral meristems: cylinders of dividing cells parallel to the sides of stems and roots. The vascular cambium produces secondary xylem (wood) towards the inside and secondary phloem towards the outside. The cork cambium produces cork cells that replace the epidermis with bark. They are responsible for secondary growth, the thickening of stems and roots in trees and shrubs.
• Intercalary meristems: found at the base of leaves and nodes in grasses. They allow grasses to regrow after being grazed or mowed, which explains why lawns can recover.

Many herbaceous plants and monocots, such as grasses, have only primary growth. Growth rings in tree trunks are visible because the vascular cambium makes large, thin-walled xylem cells in the spring and smaller, thick-walled ones in late summer. Counting the rings gives the age of the tree.

What is epidermal tissue?

The epidermis is usually a single layer of tightly packed cells that covers the surface of all young plant parts. Its functions are protection, control of water loss and gas exchange, and absorption.

• Cuticle: a waxy, waterproof layer on the outer surface of the leaf and stem epidermis. It reduces water loss. It is thicker in desert plants.
• Stomata and guard cells: pores in the epidermis of leaves, each controlled by two bean-shaped guard cells. They open for the entry of CO₂ and close to save water. Most stomata are on the lower surface of leaves.
• Trichomes: hair-like outgrowths that reduce water loss, reflect light, deter herbivores or secrete substances (as in stinging nettles).
• Root hairs: extensions of root epidermal cells that increase the surface area for absorption of water and minerals. The root epidermis has no cuticle, which allows absorption.

In woody stems and roots, the epidermis is replaced by the periderm, a protective layer of cork cells, with suberin in the walls to make them waterproof. Lenticels in the bark allow gas exchange.

What is ground tissue?

Ground tissue makes up most of the bulk of a young plant. It includes three cell types, which are distinguished by their cell walls.

• Parenchyma: the most common type, with thin, flexible walls and living cells. It carries out photosynthesis (as mesophyll in leaves, with chloroplasts), stores starch and water (as in potato tubers and fruit), and can divide to heal wounds. Parenchyma cells are totipotent: many can regenerate a whole plant.
• Collenchyma: living cells with walls thickened unevenly with cellulose and pectin. They provide flexible support to growing parts, such as the strings in celery stalks and leaf stalks.
• Sclerenchyma: cells with thick, rigid secondary walls strengthened with lignin, which are usually dead at maturity. They provide strong support. There are two forms: fibers (long, thin cells, as in flax and hemp, used for linen and rope) and sclereids (short and irregular, as in nutshells and the gritty cells of pears).

Plant tissues compared

How are the tissues arranged in organs?

• Dicot stem: epidermis on the outside; cortex (parenchyma and collenchyma) below; vascular bundles arranged in a ring, each with xylem toward the center and phloem outside, and often a layer of cambium between them; pith (parenchyma) in the center.
• Monocot stem: vascular bundles scattered through the ground tissue, with no ring and no cambium.
• Root: epidermis with root hairs, a wide cortex of parenchyma that stores starch, the endodermis (a layer with a waterproof Casparian strip that controls the entry of water into the vascular cylinder), and a central vascular cylinder (stele).
• Leaf: upper and lower epidermis with a cuticle and stomata; the palisade mesophyll and spongy mesophyll (parenchyma with chloroplasts); and vascular bundles in veins.

Worked example: identifying tissues in a stem section

A student examines a thin cross-section of a young sunflower stem under the microscope and notes the following.

1. A single outer layer of small, tightly packed cells with a thin waxy layer on the outside: the epidermis (dermal tissue) with a cuticle.
2. Just inside, a band of cells with thick corners, which stain pink at the corners: collenchyma, which gives flexible support to the growing stem.
3. Large, thin-walled, rounded cells that fill much of the section and the central region: parenchyma (the cortex and pith), which stores starch and water.
4. A ring of vascular bundles, each with large, empty cells of xylem toward the center and small, living cells of phloem toward the outside: vascular tissue. A thin zone of small, dividing cells between them is the vascular cambium.
5. Caps of cells with very thick, deeply stained walls over the phloem: sclerenchyma fibers, which strengthen the stem.

The ring arrangement of the vascular bundles shows that this is a dicot stem. A monocot such as maize would show scattered bundles. If the student looked at a woody stem 10 years old, the cambium would have produced a thick cylinder of secondary xylem, and the epidermis would have been replaced by cork. The identification relies on cell shape, wall thickness and position.

How was plant anatomy studied?

Nehemiah Grew and Marcello Malpighi described plant tissues with early microscopes in the 1670s. Robert Hooke observed cork cells in 1665. The idea of meristems was developed in the nineteenth century by Carl Nägeli, who named them in 1858. Katherine Esau’s textbooks on plant anatomy in the twentieth century shaped the field. Modern research uses genetic tools, such as the identification of genes that maintain stem cells in meristems (the WUSCHEL and CLAVATA genes).

Which mistakes are common?

Mistake 1: Saying that all plant cells can divide

Only meristematic cells normally divide. Most mature cells are differentiated.

Mistake 2: Confusing the three types of ground tissue

Parenchyma has thin walls, collenchyma has uneven thickening and sclerenchyma has thick lignified walls and is dead.

Mistake 3: Describing the epidermis as dead tissue

The epidermis consists of living cells. It is the cork of woody stems that is dead.

Mistake 4: Mixing up primary and secondary growth

Primary growth makes a plant longer, and it happens at apical meristems. Secondary growth makes it thicker, and it happens at lateral meristems.