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Plant Biology. PLANT ORGANS. 1. THE BASIC PLANT ORGANS Plants draw resources from two very different environments: below-ground and above-ground. Plants must absorb water and minerals from below the ground and carbon dioxide and light from above the ground.
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PLANT ORGANS • 1. THE BASIC PLANT ORGANS • Plants draw resources from two very different environments: below-ground and above-ground. Plants must absorb water and minerals from below the ground and carbon dioxide and light from above the ground. • Therefore, they have three basic organs: roots, stems, and leaves.
Roots are not photosynthetic and would starve without the organic nutrients imported from the stems and leaves. • Conversely, the stems and leaves depend on the water and minerals that roots absorb from the soil.
ROOTS • The root is an organ that anchors a vascular plant, usually to the soil. It absorbs minerals and water, and often stores organic nutrients. • A taproot system consists of one main vertical root which gives rise to lateral roots. The taproot often stores organic nutrients that the plant consumes during flowering and fruit production. For this reason, root crops such as carrots, turnips, and sugar beets are harvested before they flower. • Taproot systems generally penetrate deeply into the ground.
ROOTS • In seedless vascular plants and grasses, many small roots grow from the stem in what is called a fibrous root system. No roots stand out as the main one. • These roots are said to be adventitious. • A fibrous root system is usually shallower than a taproot system. • Grass roots are fibrous root systems that hold the top soil in place, preventing erosion.
ROOTS • The entire root system helps anchor a plant, but the absorption of water and minerals occurs primarily near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously. • A root hair is an extension of a root at the dermal cell. Absorption is often enhanced by symbiotic relationships between plant roots and fungi and bacteria.
STEMS • A stem is an organ system consisting of nodes (the points at which leaves are attached), and internodes (the stem segments between nodes).
STEMS • In the angle formed by each leaf and the stem is an axillary bud, a structure that has the potential to form a lateral shoot, commonly called a branch.
STEMS • Most axillary buds of a young shoot are dormant. • Thus, elongation of a young shoot is usually concentrated near the shoot apex (tip), which consists of a terminal bud with developing leaves.
STEMS • The resources of a plant are concentrated at the apex for elongation growth to increase the plant's exposure to light. But what if an animal eats the end of the shoot? Or what if light is obstructed there? • Under such conditions, axillary buds began growing. A growing axillary bud gives rise to a lateral shoot with its own terminal bud, leaves, and axillary buds. • Removing the terminal bud usually stimulates the growth of axillary buds resulting in more lateral shoots. • That is why pruning trees and shrubs and pinching back houseplants will make them bushier.
Removal of the growing tip will produce a short and bushy plant.
STEMS • Modified stems with different functions have evolved in many plants as an adaptation to the environment. • These modified stems, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots.
A stolon is a horizontal stem that grows along the surface of the soil. These runners enable a plant to reproduce asexually, as plantlets form at nodes along each runner. An example is found in the strawberry plant.
A rhizome is a horizontal stem that grows just below the surface of the soil. An example is the edible base of a ginger plant.
Atuber is an enlarged end of a rhizome that has become specialized for storing food. An example is a potato. The eyes of a potato are clusters of axillary buds that mark nodes.
A bulb is a vertical, underground shoot consisting mostly of the enlarged bases of leaves that store food. An example is an onion.
LEAVES • The leaf is the main photosynthetic organ of most plants, although green stems also perform photosynthesis. • Leaves generally consist of a flattened blade and a stalk (the petiole), which joins the leaf to a node of the stem.
Plants differ in the arrangement of veins, which are the vascular tissue of leaves.
LEAVES • Most monocot leaves (like grass) have parallel major veins that run the length of the leaf blade.
LEAVES • In contrast, eudicot leaves (like trees and most other plants) generally have a multi-branched network of major veins.
Plants are sometimes classified according to the shape of the leaves and the pattern of the veins.
Plants are sometimes classified according to the shape of the leaves and the pattern of the veins.
LEAVES • Most leaves are specialized for photosynthesis. However, some plant species have leaves that have become adapted for other functions, such as support, protection, storage, or reproduction. • Tendrils • Spines • Storage leaves • Bracts • Reproduction
LEAVES • Tendrilsare modified leaves which allow a pea plant to cling for support.
LEAVES • The spines of a cactus are modified leaves which serve as protection.
LEAVES • Succulent plants, such as the ice plant, have storage leaves for storing water.
LEAVES • The red parts of a poinsettia plant are often mistaken for petals but are actually modified leaves called bracts that attract pollinators.
LEAVES • Some leaves are modified for reproduction, such as those which produce tiny plantlets, which fall off the leaf and take root in the soil.
PLANT TISSUES • Each plant organ (root, stem, or leaf) has dermal, vascular, and ground tissues. • A tissue system consists of one or more tissues organized into a functional unit connecting the organs of a plant.
DERMAL TISSUE SYSTEM • The dermal tissue system is the outer protective covering of a plant. • Like our skin, it forms the first line of defense against physical damage and pathogenic (disease causing) organisms. • In non-woody plants, the dermal tissue usually consists of a single layer of tightly packed cells called the epidermis.
In woody plants, protective tissues known as periderm replace the epidermis in older regions of the stems and roots.
DERMAL TISSUE SYSTEM • In addition to protecting the plant from water loss and disease, the epidermis has special characteristics in each organ. • For example, at the tip of roots, the epidermis has extensions called root hairs which absorb water and minerals. • In the epidermis of leaves and most stems, a waxy coating called the cuticle prevents water loss.
VASCULAR TISSUE SYSTEM • The vascular tissue system carries out long distance transport of materials between roots and shoots. • The two vascular tissues are xylem and phloem. • Xylemconveys water and dissolved minerals upward from roots in to be shoots. • Phloemtransports nutrients such as sugars from where they are made (usually the leaves) to where they are needed (usually the roots, developing leaves, and fruits). • The vascular tissue of a root or stem is collectively called the stele.
GROUND TISSUE SYSTEM • Tissues that are neither dermal nor vascular are part of the ground tissues system. • Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that is external to the vascular tissue is called cortex. • The ground tissues system includes various cells specialized for functions such as storage, photosynthesis, and support.
TYPES OF GROWTH • Unlike most animals, plant growth occurs throughout the life of the plant. • Except for periods of dormancy, most plants grow continuously. • Eventually of course, plants die. • Based on the length of their lifecycle, flowering plants can be categorized as annuals, biennials, or perennials.
Annuals • Annuals complete their lifecycle (from germination to flowering to seed production to death) in a single year or less. • Many wildflowers are annuals, as are the most important food crops, including the cereal grains and legumes.
Biennials • Biennials generally live two years, often including a cold period (winter) between vegetative growth (first spring/summer) and flowering (second spring/summer). • Beets and carrots are biennials but are rarely left in the ground long enough to flower.
Perennials • Perennials live many years and include trees, shrubs, and some grasses.
Perennials • Some buffalo grass of the North American plains is believed to have been growing for 10,000 years from seeds that sprouted at the close of the last ice age. When a perennial dies, it is usually not from old age, but from an infection or some environmental trauma, such as fire or severe drought.
