A plant cannot walk to water, run from a grazing deer or go looking for sunlight. Everything it needs has to be solved by how its body is built. Remarkably, a maple tree 25 m tall and a blade of grass are both constructed from the same four basic tissue types, arranged into just three kinds of organ: roots, stems and leaves. Learn those tissues and organs well and the rest of this unit (transport, growth, reproduction) will make sense, because every plant process happens inside these structures.
Vascular plants in context
| Group | Vascular tissue? | Reproduces by | Canadian example |
|---|---|---|---|
| Mosses (bryophytes) | No true xylem or phloem | Spores | Sphagnum moss in bogs |
| Ferns | Yes | Spores | Ostrich fern (its young shoots are fiddleheads) |
| Gymnosperms (conifers) | Yes | Seeds exposed on cones | Eastern white pine, Pinus strobus |
| Angiosperms (flowering plants) | Yes | Seeds enclosed in fruit, formed in flowers | Sugar maple, Acer saccharum; corn, Zea mays |
The four tissue types
| Tissue | What the cells are like | Job | Where you find it |
|---|---|---|---|
| Meristematic | Small, unspecialized cells with thin walls that keep dividing by mitosis | Makes new cells for growth | Apical meristems at root and shoot tips; lateral meristems (vascular cambium, cork cambium) in woody stems and roots |
| Dermal | Usually one layer of tightly packed cells; leaves and stems are coated with a waxy cuticle | Protection, reducing water loss, gas exchange (stomata), absorption (root hairs) | Epidermis of young organs; bark (periderm) on woody plants |
| Ground | Parenchyma (thin walls, alive), collenchyma (unevenly thickened walls), sclerenchyma (thick, hard walls, often dead) | Photosynthesis, storage, support | Most of the inside of leaves, stems and roots |
| Vascular | Xylem (hollow, dead tubes with lignin-strengthened walls) and phloem (living sieve tube cells with companion cells) | Transport of water, minerals and sugars | Veins of leaves, bundles in stems, central cylinder of roots |
Two ground-tissue variations are worth knowing by name. Mesophyll ("middle leaf") is the photosynthetic parenchyma inside a leaf. Aerenchyma is parenchyma with large air-filled spaces between the cells; it is common in wetland and aquatic plants such as water lilies and cattails. Waterlogged soil holds very little oxygen, so these air channels let oxygen diffuse from the leaves down to the submerged roots, and they also help floating leaves stay buoyant.
Roots, stems and leaves
- Roots anchor the plant, absorb water and minerals, and often store food (a carrot is a swollen taproot). The root cap protects the apical meristem as the tip pushes through soil. Behind it lie the zone of cell division, the zone of elongation and the zone of maturation, where root hairs grow. Each root hair is a thin extension of a single epidermal cell; millions of them enormously increase the surface area for absorption.
- Stems hold leaves up toward light, connect roots to leaves through vascular tissue, and sometimes store food. Leaves and buds attach at nodes; the stretches between are internodes. A potato is a stem, not a root: its "eyes" are buds at nodes.
- Leaves are the main site of photosynthesis. A thin, flat blade gives a large surface to catch light, and a stalk called the petiole attaches it to the stem. The pattern of veins in the blade is called venation.
Inside a typical dicot leaf, the layers are arranged for light capture and gas exchange. A waxy cuticle and a transparent upper epidermis let light through while limiting water loss. Just below, the palisade mesophyll is a layer of tall, column-shaped cells packed with chloroplasts, where most photosynthesis happens. Under that, the spongy mesophyll has loosely arranged cells with air spaces that let carbon dioxide, oxygen and water vapour diffuse. Veins (vascular bundles of xylem and phloem) run through the mesophyll. The lower epidermis usually has most of the stomata, pores that each sit between two bean-shaped guard cells that open and close the pore.
Monocots and dicots
| Feature | Monocots | Dicots |
|---|---|---|
| Cotyledons (seed leaves) in the seed | One | Two |
| Leaf venation | Parallel veins | Net-like (branching) veins |
| Vascular bundles in the stem | Scattered throughout | Arranged in a ring |
| Root system | Usually fibrous (many similar thin roots) | Usually a taproot (one main root with branches) |
| Flower parts | Usually in threes or multiples of three | Usually in fours or fives, or multiples of these |
| Woody growth from a vascular cambium | Rare | Common (e.g. maple, oak) |
| Examples | Grasses, corn, wheat, lilies, onions, orchids | Beans, sunflowers, tomatoes, roses, maples |