Root structure

The first root produced by a young plant is called the primary root. The primary root develops into one of two types of root systems. In many dicots, such as carrots and oak trees, the primary root matures into a large, thick root, called a taproot that reaches deep into the soil. As a taproot grows, it produces smaller roots called secondary roots. In most monocots, such as grasses, the primary root shrivels and dies as the plant matures. It is replaced by secondary roots that grow from the base of the stem. These roots grow out over a wide area, forming a fibrous root system. A fibrous root system often extends father underground than the visible parts of the plant extend above ground.

Both taproot systems and fibrous root systems consist of individual roots with a similar internal structure. The roots of most angiosperms and gymnosperms contain three concentric layers of tissue: the epidermis, the cortex and the vascular cylinder.

Epidermis

The root epidermis forms a protective outer layer around internal root tissues. The epidermis also produces tiny outgrowth called root hairs, which absorb water and minerals from the soil.

Water enters the root hairs by osmosis (movement of water across a membrane into a region of lower water concentration). Because root hairs contain higher concentrations of solutes than the soil does, water moves into the hairs. Tiny pores in the root hairs allow water and minerals to pass into the cell. However, these pores prevent the escape of the larger molecules of sugar and starch that are stored in root cells.

Root hairs greatly increase the surface area of the root system and so increase the plant’s capacity to absorb water.

Root hairs are very numerous. One rye plant, for example, may have l4 billion root hairs. These hairs obtain minerals, dissolved in water, from the soil.

Cortex

In a young plant, the cortex forms the bulk of root tissue. The root cortex primarily consists of loosely packed cells. Water moves easily around and through these cells. The innermost layer of cells in the root cortex, however, is tightly packed. This single layer of cells is called the endodermis. Water must flow through the endodermis cells. Each endodermis cell has a waxy ring that prevents water from flowing around it. The endodermis produces root pressure that helps to force the water up the xylem. When the endodermal cells absorb minerals and create a low concentration of water inside the endodermal cells, water rushes in creating root pressure.

Vascular cylinder

The endodermis forms a sheath around the tissues in the centre of the root. These central tissues, which form the vascular cylinder, consist of xylem, phloem, and special layer of meristematic cells. One of the layers of meristematic cells, called the pericycle, forms the outer layer of the vascular cylinder. The pericycle produces secondary roots. The other layer of meristematic cells develops between the xylem and phloem. This layer, called the vascular cambium, produces new xylem and phloem cells.

Stems

A plant’s roots need food from the leaves to carry out their functions. At the same time, the leaves need water and minerals absorbed by the roots to carry out photosynthesis. Stems contain vessels through which these materials flow. Stems transport food, water, and minerals between the roots and leaves, and support plant growth above the ground. All kinds of stems perform these tasks.

Structure of stems

Like young root cells, stem cells differentiate into the epidermis, vascular tissues, and ground tissues. The tubes of xylem and phloem that begin in the root continue up through the stem. In some stems, the vascular tissues are arranged in groupings called vascular bundles. The pattern of vascular bundles in a monocot differs from the pattern in a dicot. In monocots, bundles are scattered through the ground tissues. In dicots, bundles are arranged in a ring that separates the inner core of pith from the outer ring of cortex cells.

Herbaceous stems

Most herbaceous monocots and dicots have soft, fleshy stems that produce little or no secondary growth. For this reason, the structure of a young herbaceous stem does non change significally as the plant matures, though some of the stem tissues may become slightly lignified.

Herbaceous stems are partially supported by the water that fills their cells. The water in each cell presses against the cell wall. This creates a force called turgor pressure, which makes the cell rigid.

Woody stems

Many dicots and most gymnosperms produce woody stems. As a woody plant matures and produces secondary growth, the structure of its stem changes. A young dicot stem contains vascular bundles arranged in a ring. As the stem matures, the vascular cambium produces xylem toward the center of the stem and phloem toward the outside. Each year the plant adds another ring of xylem. This increasingly wider column of wood supports the plant’s vertical growth.

In time, the conducting xylem cells, called sapwood, surround a core of older xylem cells that stop conducting water. This nonfunctioning xylem, called heartwood, becomes plugged with substances that make the wood hard and dry. Heartwood forms the bulk of a mature woody stem.

At the same time, the increasing diameter of the stem splits the epidermis and triggers cell division in the cork cambium. The new cork cells produced by this cell division gradually replace the cortex. Together with phloem, cork forms a layer of bark around the stem. Bark protects stems from physical injury and insects. The hollow, air-filled cork cells insulate the stem from extremes in temperature. They also contain a waxy substance that helps retrain water in the stem. These traits are important to the survival of trees and other woody plants that must endure harsh winters. Bark is not airtight, however. Tiny openings in the bark, called lenticels, permit air to pass through the epidermis to green-colored cells. You can see bumps on the outside of small twigs. They appear where lenticels were on twigs when they were green and herbaceous.

Buds

Like roots, stems continue to produce primary growth at the tip, or apical meristem. During the growing season, the tips of stems do not need a protective covering, such as a root cap, because they grow into open air. However, if they remained exposed during the winter, the sensitive meristematic cells would freeze and dry out. Woody plants protect their apical meristems and other meristematic cells on the stem in small structures called buds.

Buds usually develop during the fall. They consist of three basic parts: meristematic cells; embryonic, or partially developed, plant structures; and bud scales. Bud scales are modified leaves that wrap tightly around the meristem and become hard, sealing in moisture and keeping out cold and protecting the apical meristem. In the spring, the embryonic structures inside the bud absorb moisture, swell and force the scales to open. Terminal buds will lengthen the stem in the spring. Buds along the side of the stem called lateral buds, may produce leaves, flowers or a new branch. The place on the stem where new structure arises is called a node. The gap between each node is called an internode. Wherever bud scales drop off to make way for new growth, they leave a bud scale scar.

Leaves

Young, growing stems often carry out a small amount of photosynthesis. In most plants, however, food production takes place mainly in the leaves. The broad part of the leaf, called the blade, contains most of the plant’s photosynthetic cells. The petiole, or leaf stalk, supports the blade. The petiole is attached to the stem at the leaf base. A leaf that has a single, undivided blade is called a simple leaf. If the blade is divided into several separate parts that are attached to an extension of the petiole, the leaf is called a compound leaf. These traits, as well as others like leaf arrangement are used to classify leaves.

Structure of leaves

Leaves use sunlight, water, and carbon dioxide to carry out photosynthesis. They also transport the food they produce to the rest of the plant in a process called translocation. In addition, leaves exchange gases and water vapor with the atmosphere. The structure of the leaf is well suited to performing these and other functions.

Epidermis

The epidermis in most leaves is a single, transparent layer of cells. Sunlight passes directly through these cells to the photosynthetic cells below. The epidermis secretes a layer of cutin which slows evaporation from the leaf blade.

Carbon dioxide, oxygen, and water vapor enter and exit the leaf through openings in the epidermis called stomata. Each stoma is flanked by two kidney-shaped guard cells. Guard cells change shape to allow the entry and exit of gases. For example, when plant needs to retrain water, the guard cells close. Most guard cells are located on the underside of the leaf where the surface is shaded and somewhat protected from dust hat might clog the stomata.

Mesophyll

The epidermis encloses the middle portion of the leaf, called the mesophyll. The parenchyma cells in the Mesophyll contain chlorophyll and other pigments. Cells in the upper layer of the Mesophyll are arranged in close-fitting column that expose a maximum number of chloroplasts to the sun. This layer is called the palisade layer. The layer below, called the spongy layer, consists of loosely bunched, irregularly shaped cells surrounded by air spaces. Gases and water vapor accumulate in these spaces.

Vascular bundles extend through the spongy layer. Vascular bundles are seen in the blade of the leaves as veins. The main vein extends through the petiole and is called the midrib. The veins transport water and minerals into the leaf and carry organic materials from the leaf.

Date: 2014-12-22; view: 1605