14.1 Plant Tissues and Organs

Plants are complex organisms built from specialized tissues and organs, each with a distinct role in growth, transport, and reproduction. Understanding how these components work together gives you the foundation for everything else in plant biology.

This section covers the four major tissue types, the three vegetative organs (roots, stems, leaves), and the reproductive structures that allow flowering plants to reproduce.

Plant Tissues

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Meristematic Tissue

Meristematic tissue is where plant growth happens. These cells divide actively and remain unspecialized, with thin cell walls, dense cytoplasm, and prominent nuclei. They're the source of all new cells in the plant.

There are two main locations to know:

• Apical meristems are found at the tips of roots and shoots. They drive primary growth, which increases the plant's length.
• Lateral meristems run along the sides of stems and roots. They drive secondary growth, which increases the plant's girth (thickness). The two lateral meristems you need to know are the vascular cambium (produces new xylem and phloem) and the cork cambium (produces the protective outer bark).

A useful way to remember it: apical = tips = longer; lateral = sides = wider.

Ground Tissue

Ground tissue makes up the bulk of the plant body. It includes three cell types, each with a different structure matched to its function:

• Parenchyma cells are the most common plant cells. They have thin, flexible cell walls and are often loosely packed with air spaces between them, which allows gas exchange. Depending on where they are, parenchyma cells carry out photosynthesis (in leaves), store starch and sugars (in roots and stems), or perform secretion.
• Collenchyma cells provide flexible structural support, especially in young, growing parts of the plant like leaf stalks and green stems. Their cell walls are unevenly thickened, which gives them strength while still allowing the tissue to stretch. Think of the "strings" in a celery stalk.
• Sclerenchyma cells provide rigid support. Unlike collenchyma, their secondary cell walls are thickened with lignin, making them very hard. Most sclerenchyma cells are dead at maturity because the thick walls block nutrient exchange. Two subtypes exist: fibers (long, slender cells found in bundles) and sclereids (shorter, irregular cells, like the gritty bits in a pear).

Dermal and Vascular Tissue

Dermal tissue is the plant's outer covering, controlling what enters and leaves.

• The epidermis is a single cell layer that coats non-woody plant surfaces. It secretes a waxy cuticle that reduces water loss. Scattered across the epidermis (especially on leaves) are stomata, tiny pores flanked by guard cells that open and close to regulate gas exchange and transpiration.
• In woody plants, the epidermis is eventually replaced by the periderm, a tougher outer covering. The periderm consists of the cork cambium, cork cells (dead at maturity, filled with suberin for waterproofing), and the phelloderm (living cells on the inner side).

Vascular tissue is the plant's transport system, moving materials throughout the body.

• Xylem carries water and dissolved minerals upward from roots to leaves. Its conducting cells, tracheids and vessel elements, are dead at maturity and have thick, lignified walls that also provide structural support. Vessel elements are wider and more efficient; tracheids are found in all vascular plants, while vessel elements are mostly in angiosperms.
• Phloem transports sugars and other organic compounds (the products of photosynthesis) from sources (like leaves) to sinks (like roots, fruits, and growing tips). Its main conducting cells are sieve-tube elements, which are alive but lack a nucleus at maturity. Each sieve-tube element depends on an adjacent companion cell to handle its metabolic needs.

Plant Organs

Roots

Roots anchor the plant in the soil, absorb water and dissolved minerals, and store food reserves.

• Taproot systems have a single thick main root with smaller lateral branches (carrots, dandelions). Fibrous root systems have many thin, branching roots of similar size that spread out near the surface (grasses, corn).
• Root hairs are extensions of individual epidermal cells near the root tip. They dramatically increase the surface area available for water and mineral absorption.
• The root cap is a protective layer of cells covering the apical meristem at the very tip. It shields the meristem as the root pushes through soil and also secretes a slimy mucilage that lubricates the root's path.
• Roots undergo both primary growth (elongation at the tip) and secondary growth (thickening). Root vegetables like carrots and beets are familiar examples of roots with extensive secondary growth and food storage.

Stems

Stems connect roots to leaves, support the plant's above-ground structures, transport materials between roots and shoots, and store food.

• Herbaceous stems are soft and flexible, typical of annuals and many perennials. Woody stems are rigid and reinforced with lignin, found in trees and shrubs.
• Nodes are the points where leaves attach to the stem. Internodes are the stretches of stem between nodes.
• The arrangement of vascular bundles inside the stem differs between the two major groups of flowering plants: in monocots, bundles are scattered throughout the stem; in dicots (eudicots), bundles are arranged in a ring around the outer edge.
• Lenticels are small, spongy openings in the bark of woody stems that allow gas exchange between internal tissues and the atmosphere.

Leaves

Leaves are the primary photosynthetic organs. Their broad, flat shape maximizes light capture and gas exchange.

The external structure of a typical leaf includes:

• The blade, the flat, expanded portion where most photosynthesis occurs
• The petiole, the stalk that connects the blade to the stem
• Stipules, small appendages at the base of the petiole (not present in all species)

Leaf arrangement on the stem can be alternate (one leaf per node), opposite (two leaves per node, across from each other), or whorled (three or more leaves per node).

Venation (the pattern of veins) is a reliable way to distinguish monocots from dicots: monocots typically have parallel venation, while dicots have reticulate (net-like) venation.

Internally, a leaf cross-section reveals three tissue layers:

1. Epidermis (upper and lower) with a waxy cuticle and stomata (mostly on the lower surface)
2. Mesophyll, the photosynthetic ground tissue, divided into tightly packed palisade mesophyll (near the upper surface, optimized for light absorption) and loosely arranged spongy mesophyll (near the lower surface, with air spaces for gas circulation)
3. Vascular bundles (leaf veins) containing xylem and phloem that supply water and export sugars

Reproductive Structures

Flowers

Flowers are the reproductive organs of angiosperms (flowering plants). A typical flower has four whorls of modified leaves, arranged from outside to inside:

1. Sepals (collectively the calyx) protect the flower bud before it opens
2. Petals (collectively the corolla) are often colorful and attract pollinators
3. Stamens are the male reproductive structures. Each stamen has a filament (stalk) topped by an anther, which produces pollen grains.
4. Carpels (or pistils) are the female reproductive structures. Each carpel has a stigma (sticky tip that catches pollen), a style (connecting tube), and an ovary (base, containing ovules)

Two classification distinctions to keep straight:

• Complete vs. incomplete: A complete flower has all four whorls (sepals, petals, stamens, carpels). An incomplete flower is missing at least one.
• Perfect vs. imperfect: A perfect flower has both stamens and carpels (it can be incomplete but still perfect if it's only missing sepals or petals). An imperfect flower has only stamens (staminate) or only carpels (carpellate).

Flowers can appear singly (solitary) or grouped in clusters called inflorescences (like the flower heads of sunflowers or the spikes of lavender).

Fruits and Seeds

After fertilization, the ovary develops into a fruit, which protects and helps disperse the seeds inside.

Fruits are classified by how they form:

• Simple fruits develop from a single ovary of one flower (tomato, peach, apple)
• Aggregate fruits develop from multiple ovaries of a single flower (raspberry, strawberry)
• Multiple fruits develop from the fused ovaries of multiple flowers (pineapple, fig)

Common simple fruit types include berries (fleshy throughout; tomatoes, grapes), drupes (fleshy with a hard stone enclosing the seed; peaches, cherries), pomes (fleshy with a core; apples, pears), and nuts (hard, dry shell; acorns, walnuts).

A seed contains three key components: the embryo (the young plant), stored food (endosperm or nutrients within the cotyledons), and a seed coat (protective outer layer).

• Monocot seeds have one cotyledon (seed leaf). Examples: corn, wheat, rice.
• Dicot seeds have two cotyledons. Examples: beans, peas, sunflowers.

Seeds are dispersed by several mechanisms: wind (dandelion, maple), water (coconut), animals (burrs that cling to fur, fleshy fruits eaten by birds), and explosive dehiscence (the fruit splits open forcefully, launching seeds, as in touch-me-nots and witch hazel).