What is plant anatomy? Tissues first
Anatomy means the inside structure of a body. In plants we study it by cutting very thin slices (transverse sections) and looking at them under a microscope. A tissue is a group of cells with the same origin that do one job together.
Meristematic tissues (dividing tissues)
- Apical meristem: at the tips of root and shoot; makes the plant longer (primary growth).
- Intercalary meristem: between mature tissues, at the base of leaves or internodes of grasses; helps regrow after grazing.
- Lateral meristem: on the sides (vascular cambium, cork cambium); makes the plant thicker (secondary growth).
Permanent tissues (cells that have stopped dividing)
- Simple (one kind of cell): parenchyma (thin-walled living cells that store food and do photosynthesis), collenchyma (living cells thickened at the corners with pectin; give flexible support to young stems and petioles), sclerenchyma (dead cells with thick lignin walls: long fibres and short sclereids, like the grit in a pear or the hard shell of a walnut).
- Complex (many kinds of cells): xylem (carries water and minerals; tracheids, vessels, xylem fibres, xylem parenchyma) and phloem (carries food; sieve tube elements, companion cells, phloem parenchyma, phloem fibres). Gymnosperms lack vessels in xylem and have albuminous cells instead of companion cells.
First-formed xylem is protoxylem; later xylem is metaxylem. In stems protoxylem lies towards the centre (endarch); in roots it lies towards the outside (exarch).
The three tissue systems and their roles
1. Epidermal tissue system (the skin)
It is the outer cover. It has the epidermis (one layer of tight cells, often coated with a waxy cuticle that stops water loss; roots have no cuticle), stomata and hairs. A stoma is a tiny pore surrounded by two guard cells: bean-shaped in dicots, dumb-bell-shaped in grasses. Nearby special cells are subsidiary cells. Stomata control gas exchange and water loss (transpiration). Root hairs are single-celled and absorb water; trichomes on the stem are often many-celled and cut down water loss.
2. Ground tissue system (the filling)
All tissues except the epidermis and the vascular bundles: parenchyma, collenchyma and sclerenchyma of the cortex, pericycle, pith and medullary rays. In leaves, the ground tissue is the mesophyll (thin-walled cells with chloroplasts). Roles: storage, support and photosynthesis.
3. Vascular tissue system (the pipes)
Xylem and phloem together form vascular bundles. Roles: xylem carries water and minerals up; phloem carries food to all parts.
- Open bundle: has cambium between xylem and phloem, so it can make new tissue (dicot stems).
- Closed bundle: no cambium (monocots).
- Radial: xylem and phloem on different radii, alternating (roots).
- Conjoint: xylem and phloem on the same radius (stems, leaves). Usually collateral with phloem outside and xylem inside.
Dicot root and monocot root
Dicot root (like sunflower)
From outside in: epiblema (outer layer with root hairs) → cortex (many layers of loose parenchyma) → endodermis (one layer of barrel-shaped cells with waxy, water-proof Casparian strips of suberin on their side walls) → pericycle (a few thick-walled layers; gives lateral roots and, later, part of the vascular cambium) → radial vascular bundles with 2 to 4 xylem groups (diarch to tetrarch) → small or no pith. The parenchyma between xylem and phloem is conjunctive tissue. All tissues inside the endodermis (pericycle, bundles, pith) together form the stele. Dicot roots later show secondary growth.
Monocot root (like maize)
Same basic layers, but it has more than 6 xylem groups (polyarch), a large well-developed pith, and no secondary growth.
Dicot stem and monocot stem
Dicot stem (like sunflower)
Epidermis with cuticle and hairs, a few stomata → hypodermis of collenchyma (gives strength to the young stem) → cortex of parenchyma → innermost cortex layer rich in starch, called the starch sheath (endodermis) → pericycle as half-moon patches of sclerenchyma above each bundle → vascular bundles in a ring, each conjoint, collateral, open with endarch protoxylem → medullary rays of parenchyma between bundles → a large central pith.
Monocot stem (like maize)
A sclerenchymatous hypodermis, then ground tissue of parenchyma with many scattered bundles. Each bundle is conjoint, collateral, closed and surrounded by a sclerenchyma bundle sheath. Peripheral bundles are smaller than central ones. Phloem parenchyma is absent, and there are water-containing cavities in the bundles. There is no clear cortex, endodermis, pericycle or pith.
Dorsiventral (dicot) leaf and isobilateral (monocot) leaf
Dorsiventral leaf (dicot)
Three parts in a vertical section: epidermis on the upper (adaxial) and lower (abaxial) side, with a cuticle; the lower side usually has more stomata (the upper may have none). Mesophyll between them: palisade parenchyma (tall, tight cells under the upper epidermis) and spongy parenchyma (round or oval, loose cells with big air spaces, below). Vascular bundles in the veins and midrib, each surrounded by a layer of thick-walled bundle sheath cells. The size of the bundles depends on the size of the veins.
Isobilateral leaf (monocot)
Stomata on both surfaces, and the mesophyll is not split into palisade and spongy. In grasses, some upper epidermal cells along the veins become large, empty, colourless bulliform cells. When they are full of water the leaf is flat; when they lose water (in dry weather) the leaf rolls inward to cut water loss. Veins are parallel, so the bundles in a cross-section are of almost the same size (except the main ones).
Dicot vs monocot: one table
| Feature | Dicot | Monocot |
|---|---|---|
| Root xylem groups | 2–4 | More than 6 |
| Root pith | Small or absent | Large |
| Stem bundles | In a ring, open | Scattered, closed, with sheath |
| Stem hypodermis | Collenchyma | Sclerenchyma |
| Leaf | Dorsiventral; palisade + spongy | Isobilateral; no split |
| Stomata | More on lower side | About equal on both sides |
| Secondary growth | Yes (cambium) | Usually no |
Key formulas and definitions
- Tissue systems: epidermal (skin) + ground (filling) + vascular (xylem + phloem)
- Open bundle = has cambium; closed bundle = no cambium
- Radial = xylem and phloem on different radii (roots); conjoint = same radius (stems, leaves)
- Stem xylem: endarch (protoxylem inside); root xylem: exarch (protoxylem outside)
- Dicot root 2–4 xylem (diarch–tetrarch); monocot root > 6 (polyarch)
- Stele = pericycle + vascular bundles + pith (everything inside the endodermis)
Worked examples
1. A section shows radial bundles with 4 xylem groups, a thin pith and an endodermis with Casparian strips. Identify it.
Radial bundles → it is a root. 4 xylem groups (tetrarch) and a small pith → dicot. Answer: dicot root (e.g. sunflower or gram).
2. A section shows many scattered bundles, each with a sclerenchyma sheath and a water cavity, and no cambium. Identify it and name the bundle type.
Scattered bundles with sheath and water cavities, no cambium → monocot stem (e.g. maize). Bundles are conjoint, collateral and closed.
3. A leaf section shows stomata on both surfaces and large empty cells in the upper epidermis. What are those cells and what kind of leaf is this?
The large empty cells are bulliform cells, which roll the leaf in dry weather. Stomata on both sides and bulliform cells → isobilateral monocot (grass) leaf.
4. Why can a dicot stem grow thicker but a monocot stem usually cannot?
Dicot stem bundles are open: they have cambium between xylem and phloem, which divides to make new xylem and phloem (secondary growth). Monocot bundles are closed (no cambium), so they cannot add rings of new tissue.
5. Water in a root moves from the cortex into the xylem. Why must it pass through living endodermis cells rather than between them?
The side walls of endodermis cells have Casparian strips of suberin, which are water-proof. They block the path along the cell walls, so water has to cross the cell membranes. This lets the root control which minerals enter the xylem.
6. Count and compare: in the 3D, the dicot root shows 4 red xylem groups and the monocot root shows 8. Name each using the '-arch' words.
4 xylem groups = tetrarch (dicot root, allowed range 2–4: diarch, triarch, tetrarch). 8 groups = polyarch (monocot root, more than 6).
Common mistakes
- Saying monocot stems have a ring of bundles. Only dicot stems have a ring; monocot bundles are scattered.
- Mixing endarch and exarch. Stem = endarch (protoxylem towards the centre); root = exarch (protoxylem towards the outside).
- Calling root bundles conjoint. Roots have radial bundles; stems and leaves have conjoint bundles.
- Thinking a dicot leaf has equal stomata on both sides. It usually has more on the lower surface; the isobilateral monocot leaf has about equal numbers.