The Plant Body

Core Concepts

Two organ systems, three organs

The shoot system consists of stems, leaves, and (in flowering plants) flowers and fruits. It is usually above ground and specializes in photosynthesis, support, and reproduction. The root system anchors the plant and absorbs water and dissolved minerals from the soil; it also stores carbohydrates. The two systems are physically connected by the vascular tissue that runs continuously through them — there is no "break" between root and shoot plumbing. Leaves are the main photosynthetic organs, stems support and transport, and roots absorb and anchor; each is covered in its own topic.

The three tissue systems

Every plant organ contains all three tissue systems:

• Dermal tissue is the outer covering. In non-woody organs it is the epidermis, a single layer of cells coated with a waxy cuticle that slows water loss; the epidermis bears stomata (pores flanked by guard cells that regulate gas exchange) on leaves and root hairs (epidermal extensions that absorb water) on roots. In woody stems and roots, the dermal layer is replaced by periderm (bark).
• Vascular tissue is the plumbing: xylem carries water and dissolved minerals upward from the roots, and phloem carries sugars from photosynthesis to wherever they are needed — down to roots for storage, up to growing tips and fruits. In stems and roots the vascular tissue is organized into bundles or a central cylinder.
• Ground tissue fills everything between dermis and vascular tissue. It includes parenchyma (thin-walled cells for photosynthesis, storage, and most metabolic work — the most common cell type), collenchyma (cells with unevenly thickened walls that give flexible support to growing stems), and sclerenchyma (cells with thick, often lignified walls that are dead at maturity and provide rigid support — the fibers in celery strings and the wood of trees).

Meristems: where all growth comes from

Meristematic cells stay small, thin-walled, and undifferentiated, and they keep dividing. Apical meristems sit at the tips of roots and shoots; their divisions lengthen the plant — primary growth — and produce all the new leaves, stems, and root tissue a plant will ever have. Because of them, a plant grows from its tips, not from its middle: a nail driven into a young tree trunk stays at the same height for the tree's life. Lateral meristems — the vascular cambium and cork cambium — form cylinders inside woody stems and roots; their divisions thicken the plant — secondary growth — producing wood and bark. Herbaceous (non-woody) plants have only primary growth.

Monocots versus eudicots

Angiosperms split into two great groups, and their body plans differ in consistent ways:

These traits travel together, so a single observation — parallel leaf veins, say — predicts the rest. Knowing the pattern lets you classify an unfamiliar plant from a stem slice or a leaf.

How plant cells differ from animal cells

Three features define plant cells and shape plant physiology: the cell wall (cellulose and other polysaccharides outside the plasma membrane) that supports the plant and resists water loss; the large central vacuole that stores water and solutes and, through turgor pressure, keeps non-woody plants upright; and plasmodesmata, cytoplasmic channels through cell walls that connect neighboring cells into a living continuum. Chloroplasts are present in photosynthetic cells. These are covered in detail in earlier chapters; here they matter because they explain how plant tissues work — walls give wood its strength, turgor makes a wilted plant recover when watered, and plasmodesmata let the plant body behave as one connected organism.

Common Confusions