The three plant tissues · Chapter 10

Plants sort their tissues differently from animals. Instead of four types, most plants have three: one that covers, one that fills, and one that carries water and food.

Simple and complex tissues

Plant biologists first sort tissues by how many cell types they contain.

A simple tissue is made of one cell type, or possibly two.
A complex tissue is made of two or more cell types, but without the organization that would make it an organ.

The three general plant tissues are dermal, ground, and vascular tissue, each with a structure suited to its jobs. Dermal and vascular tissue are complex tissues, while ground tissue is made up of three simple tissues.

A whole plant with leaves, stem, and roots, with cutaway views of a leaf, stem, and root. In each cutaway, an outer covering layer is labeled dermal tissue, the bulk of the interior is labeled ground tissue, and strands running through the middle are labeled vascular tissue — Figure 10.8 All three tissues run through leaves, stems, and roots: dermal on the outside, vascular in strands through the middle, and ground filling the rest.

Dermal tissue covers the plant

Dermal tissue is a complex tissue that forms a covering over the plant. Newly made dermal tissue on the surfaces of leaves, stems, and roots is called the epidermis. Epidermal cells have thick primary walls and are tightly interlocked by their middle lamellae, so they hold together closely, much like the epithelial sheets of animals.

The epidermis is specialized differently in leaves and roots:

Feature	Where	What it does
Waxy cuticle	Leaf epidermis	A waterproof coating, secreted by epidermal cells, that prevents water loss
Trichomes	Leaf epidermis	Hairs made of specialized epidermal cells; can secrete oils and protect the leaf
Guard cells and stomata	Leaf epidermis	Pairs of guard cells form pores (stomata; singular, stoma) that allow gas exchange
No cuticle	Root epidermis	The root absorbs water and nutrients, and wax would block that

Ground tissue fills the plant

Most of a plant’s body is ground tissue. Its jobs include photosynthesis, storing carbohydrates, and support. It comes in three simple tissues that differ mainly in their cell walls.

Ground tissue	Main role	Cell walls
Parenchyma	Metabolically active: photosynthesis (the mesophyll, the leaf’s photosynthetic middle layer, is parenchyma) and carbohydrate storage	Usually no secondary wall
Collenchyma	Flexible support in growing regions, such as the outer parts of stems and leaves	Thick secondary walls with little lignin, so they can bend and stretch
Sclerenchyma	Rigid support in parts that have stopped growing, such as woody stems	Secondary walls full of lignin; cells are often dead at maturity, but their walls keep supporting the plant

Notice the pattern in the cell walls: the more lignin, the stiffer the support. Parenchyma usually skips the secondary wall, collenchyma’s secondary wall has little lignin, and sclerenchyma’s is packed with it.

Vascular tissue carries water and food

Some plants, such as certain mosses, are nonvascular: they have no conducting vessels, so they stay small and live in damp, shady places. Most plants today, including ferns and seed plants, are vascular plants.

In vascular plants, vascular tissue is a complex tissue of interconnected cells that form tubes for moving water and nutrients. It has two types:

Xylem carries water and mineral ions up from the roots to the rest of the plant.
Phloem distributes the sugars made in photosynthesis, plus other nutrients, throughout the plant.

Comparing plant and animal tissues

Plant and animal tissues line up in some ways but not others.

Dermal ≈ epithelial. Both cover the organism, and both hold their cells tightly together (middle lamellae in plants, tight junctions in animals), limiting what passes between cells.
Ground ≈ connective. Both provide structural support, and both contain a lot of material outside the cells: thick cell walls in plants, abundant ECM in animals.
Vascular ≠ nervous. Vascular tissue moves water and nutrients; it doesn’t carry electrical signals the way nervous tissue does. If anything, it’s closer to an animal’s blood vessels.

Check yourselfWhich plant tissue is the closest counterpart to animal connective tissue, and why?

Ground tissue. Like connective tissue, it gives the body structural support, and it has a large amount of material outside the cells, in its case thick cell walls (especially in collenchyma and sclerenchyma).

Recap

Plant tissues are simple (one or two cell types) or complex (two or more types, not organized into an organ); the three general types are dermal, ground, and vascular.
Dermal tissue covers the plant; leaf epidermis has a waxy cuticle, trichomes, and guard cells forming stomata, while root epidermis lacks a cuticle so it can absorb water.
Ground tissue makes up most of the plant: parenchyma (photosynthesis and storage), collenchyma (flexible support, little lignin), and sclerenchyma (rigid, lignin-rich support, often dead at maturity).
Vascular tissue forms conducting vessels: xylem carries water and minerals up from the roots, and phloem distributes the products of photosynthesis.
Dermal tissue parallels epithelial tissue and ground tissue parallels connective tissue; vascular tissue does not do the job of nervous tissue.