Chapter 28 Plant Structure and Growth

Chapter 28

Plant Structure and Growth

The Three Basic Plant Organs: Roots, Stems, & Leaves

● As in animals, plants have tissues and organs.

● Plants draw water and minerals from the ground with their root system and acquire CO

and light from their shoot system.

● Vegetative growth is production of

non-reproductive leaves, stems, and roots.

● Plants also undergo growth related to sexual reproduction as in the flowers of

angiosperms.

The Three Basic Plant Organs: Roots

● A root is an organ that anchors and absorbs water and minerals.

● A taproot is the main vertical root of the plant in gymnosperms and eudicots.

● Lateral roots or branch roots grow from the taproot in monocots.

● Fibrous roots are mats of thin roots that spread out below the surface, great for holding soil together ie grasses.

● Root hairs are thin tubular extensions of the root epidermal cells that increase surface area for absorption.

● Besides anchorage and absorption, storage of

carbohydrates is another function of root systems.

The Three Basic Plant Organs: Stems

● A stem is an organ that separates or raises leaves and reproductive structures.

● Stems consists of nodes (point of leaf

attachment) and internodes (stem segments between nodes).

● Axillary buds are structures that form lateral shoots.

● Apical or terminal buds are structures where most shoot growth occurs.

● Apical buds can inhibit the growth from axillary buds called apical dominance.

● Other functions of stems are food storage

and asexual reproduction.

The Three Basic Plant Organs: Leaves

● The leaf is the main photosynthetic organ of the plant.

● Leaves consist of a blade and stalk or petiole which joins leaf to stem at the node.

● Veins are vascular tissue that transports water and sugars through the plant leaf.

● Other functions of plant leaves include:

support, protection, storage, or

reproduction.

The Different Types of Plant Tissues

● A tissue system is a functional unit connecting all of the plant’s organs.

● Dermal tissue is the outer protective covering of the plant..

● The epidermis, a layer of dermal tissue in nonwoody plants, is a layer of tightly packed cells.

● Periderm replaces the epidermis in stems and roots of woody plants.

● The cuticle is a waxy coating on the epidermal surface of leaves and most stems.

● Vascular tissue system is responsible for long distance transport of materials between roots and shoots.

○ xylem conducts water and phloem conducts sugars.

● Tissues of the ground tissue system are neither vascular

and dermal called pith and cortex.

Common Plant Cell Types

● Parenchyma cells - perform most of the metabolic functions of the plant.

● Collenchyma cells - generally elongated cells that form strands to support young parts of the plant shoot.

● Sclerenchyma cells - are rigid cells that are specialized to support, most at maturity are dead, made of two cell types:

○ Sclereids are cells that give hardness to nut shells

○ Fibers are specialized for support.

● Xylem, made of dead two cell types:

○ Tracheids are long thin cells with tapered ends.

○ Vessel elements are wider, shorter, and thinner with less tapering.

● Phloem is made of chains of cells called sieve-tube elements that are alive but lack most of the common cell machinery.

○ Companion cells lie along side of sieve-tube elements and

help to load sugars into the sieve-tube elements.

Meristems and Plant Growth

● Plants exhibit indeterminate growth throughout much of its life span.

● Meristems are undifferentiated tissues that lead to growth under the right conditions:

○ apical meristems are located at axillary buds where primary growth occurs.

○ lateral meristems aid in secondary growth.

● Vascular cambium adds secondary xylem and secondary phloem.

● Cork cambium produces periderm in woody plants.

● Plants lifespan includes:

○ annuals - complete their life cycle in 1 year

○ biennials - complete their life cycle in 2 years

○ perennials - live for many years ie. some

buffalo grasess in the prairies are over 10,000

years old.

Primary Growth in Roots and Shoots

● The root cap protects the apical meristem by secreting a polysaccharide that loosens the soil around the root,

● Growth in the root tip occurs in 3 zones:

○ zone of cell division- produces new root cells

○ zone of elongation - pushes root tip down

○ zone of differentiation- cells become specialized here

● Leaves develop from leaf of primordia at the

shoot apical meristem.

Tissue Organization in Roots, Stems, & Leaves

● Roots are characterized by:

○ an outer epidermis

○ ground tissue that fills the cortex with an innermost layer called the endodermis

○ vascular tissue in vascular cylinder

● Stems are characterized by:

○ an outer epidermis

○ ground tissue, in the pith and cortex

○ vascular tissue arranged in bundles

● Leaves are characterized by:

○ a lower epidermis with stomata and guard cells and upper epidermis with a cuticle.

○ The mesophyll is the ground tissue of a leaf made primarily of palisade and spongy.

○ Vascular tissue is continuous with the stem in veins.

Secondary Growth in Woody Plants

● Secondary growth occurs mainly in roots and leaves.

● Vascular cambium adds secondary xylem to its interior and secondary phloem to its

exterior.

○ each layer adds more diameter to the previous

● Over the years layers of secondary xylem accumulate with heavily lignified xylem cells that gives hardness to the wood.

● Yearly growth appears in rings, thick rings

represent fast growth.