Roots Stems and Leaves Chapter 23 p 578

Roots, Stems and Leaves Chapter 23 p. 578

Structure of Seed Plants • The cells of seed plants are organized into different tissues and organs. • The three principal organs of seed plants are: –Roots –Stems –Leaves

Roots • Absorb water and dissolved nutrients from moist soil. • Anchor Plants in the ground • Hold plants upright and prevents them from being knocked over by wind and rain. • They grow and develop complex branching networks that penetrate the soil and grow between soil particles.

Stems • Supports the plant body. • Transports nutrients among different parts of the plant. • Hold leaves up to the sun. • To support the weight of tall plants, stems have modified vascular tissue. • Provides the passageway for xylem and phloem to transport water and nutrients throughout the plant.

Leaves • Principle organs for photosynthesis. • Broad, flat surfaces help increase the amount of sunlight absorbed by plants. • Stomata are adjustable pores that help conserve water while allowing oxygen and carbon dioxide to enter and leave the leaf.

Types of Plant Tissues • Plants have specialized tissue systems. • Plants consist of four tissue systems: – Meristematic Tissue – Dermal Tissue – Vascular Tissue – Ground Tissue

Meristematic Tissue • Found ONLY in the tips of shoots and roots. • The ONLY plant tissue that produces new cells by mitosis. • Site of rapid cell division. • The Apical Meristem is a group of undifferentiated cells that divide to produce increased length of stems and roots.

Lateral Meristems • Lateral Meristem: cylinders of dividing cells along the length of roots and stems and are responsible for secondary growth (increase in width/girth).

Dermal Tissue • The outer covering of a plants. • Typically consists of a single layer of Epidermal Cells. • Cuticle – thick, waxy layer that protects against water loss and injury. • Some have Trichomes – protect the leaf from water loss or secrete toxic substances and make it look fuzzy. • Also includes root hairs, guard cells, and stomata.

Vascular Tissue • Transport water and nutrients throughout the plant. • Consists of two major types: – Xylem – Phloem

Xylem • Made of Tracheids – long, narrow cells with walls that are impermeable to water. • Angiosperms have a xylem called a Vessel Element. – Much wider than tracheids and allow for greater movement.

Phloem • The main phloem cells are Sieve Tube Elements. – Are arranged end to form sieve tubes. • Materials can move through holes in the tubes from one adjacent cell to another. • Companion Cells are phloem cells that surround sieve tube elements. • Support the phloem cells and aid in the movement of substances in and out of the phloem stream.

Ground Tissue contains 3 Main Plant cell types: • The cells that lie between dermal and vascular tissue. • Parenchyma – thin cell wall and large central vacuoles, site of most photosynthesis. • Collenchyma – Strong, flexible cell walls, used for support (thick ridges in celery). • Sclerenchyma – Extremely thick, rigid cell walls, make ground tissue tough and strong.

Dermal Tissue

Tissue Systems

Review Questions… • 1. List the three tissue systems of plants. • 2. Describe how each tissue is distributed in stems, tissues and leaves. • 3. What is the function of meristematic tissue? • 4. What two cell types make up the xylem? • 5. What two cell types make up the phloem? • 6. In a stem that needs to support heave leaves, what type of ground tissue might you expect to find? • 7. The vascular system of a plant has been compared to the circulatory system of a human. How are they similar? Different?

Types of Roots • The two main types of roots are Taproots and Fibrous Roots. • Taproots are found mainly in dicots. – Have a central primary root and generally grow deep into the soil. • Fibrous Roots are found mainly in monocots. – Branch to such an extent that no single root grows larger than the rest. – Typically help topsoil from being washed away.

Types of Roots

Root Structure and Growth • A mature root has an outside layer, the epidermis, and a central cylinder of vascular tissue. • Between these two lies a large area of ground tissue. • Root Hairs – produce a large surface area through which water can enter the plant. • Cortex – spongy layer of ground tissue. • Endodermis – completely encloses vascular tissue in a central region called the Vascular Cylinder. • Root Cap – protects cells in the apical meristem while they grow.

Root Structure

Root Functions • Roots anchor a plant in the ground absorb water and dissolved nutrients from the soil. • Plants uptake necessary nutrients. • Active transport of minerals. • Movement into vascular tissue. • Root pressure.

Review Questions • 1. Compare a taproot and a fibrous root. • 2. How are tissues distributed in plant roots? • 3. Describe the two main functions of roots. • 4. How is osmosis involved in the absorption of water and nutrients? • 5. Why is it important that the root endodermis permit only a one-way passage of materials?

23 -3: STEMS • In general, stems have three important functions: – They produce leaves, branches and flowers. – They hold leaves up in the sunlight. – They transport substances between roots and leaves. • Most plants also contain nodes where leaves branch from. • Buds contain undeveloped tissue that can produce new stems and leaves.

Herbaceous Stems: • Soft, fleshy stems that produce little or no secondary growth. • Partially supported by the water that fills their cells. The water in each cell causes the cell to be pressed against the cell wall. This creates a force called turgor pressure, which makes the cell rigid.

Woody Stems: • Many perennial dicots and conifers produce thick, woody stems that support tall upright growth. • The outer portion of woody stems develops bark, which is composed of phloem and cork cambium cells, and helps prevent damage to the stem. • Lenticels: tiny openings in the bark which permit air to pass through the epidermis to green -colored cells.

Stems

Monocot and Dicot Stems • In monocots, vascular bundles are scattered throughout the stem. • In dicots and most gymnosperms, vascular bundles are arranged in a cylinder.

Monocot Stems • The cross section of young monocot stem shows all three tissue systems clearly. • Has a distinct epidermis that encloses vascular bundles, containing xylem and phloem. • These bundles are scattered throughout the ground tissue.

Dicot Stems • Young dicot stems have vascular bundles, but they are generally arranged in an organized, ring-like pattern. • The parenchyma cells are know as pith. • These tissue patterns become more complex as the plant grows larger and the stem increases in diameter.

Growth of Stems • Growth at the end of the plant is primary growth. • Primary growth of stems is produced by cell divisions in the apical meristem (in all seed plants). • When stems increase in width it is secondary growth. • In conifers and dicots, secondary growth takes place in lateral meristematic tissues called the vascular cambium and cork cambium. – This tissue forms between the xylem and phloem of the individual vascular bundles. – Once the tissue forms it divides to produce xylem cells toward the center of the stem and phloem cells toward the outside. – These form the bark and wood of a mature stem.

Growth of Stems

Formation of Wood • Wood is actually layers of xylem. • These can build up year after year, layer on layer. • The older xylem becomes Heartwood. • Heartwood is surrounded by Sapwood. • The alternation of dark wood and light wood forms tree rings. • These rings correspond to the age of the tree.

Formation of Bark • Bark includes all of the tissues outside the vascular cambium. • These tissues include phloem, the cork cambium, and cork. • Phloem grows to accommodate the larger size of the tree. • Tissue is forced outward. • Cork cambium prevents the stem from splitting and breaking. • The outermost cork cells are usually dead which is why bark cracks and breaks.

Adapted Stems • Tuber – stems grows underground for food storage. – Ex: potatoes • Bulb – made up of a central stem surrounded by short thick leaves. – Ex: Amaryllis • Rhizome – A horizontal, underground stem. • Ex: Ginger • Corm – A thickened stem that stores food. – Ex: Gladiolus

Adapted Stems

Review Questions… • 1. How do the functions of a stem relate to the roots and leaves of a plant? • 2. Describe how the arrangement of vascular bundles differs between monocot and dicot stems. • 3. Define primary and secondary growth. • 4. Which type of growth involves division of the apical meristem? • 5. How do heartwood and sapwood differ?

23 -4: Leaves

Leaf Structure • The structure of a leaf is optimized for absorbing light and carrying out photosynthesis. • Leaves can differ in shape but have the same structural features. • Blades are thin flattened sections. • Petiole – thin stalk that attaches the blade to the stem. • The vascular tissue of leaves are connected directly to the vascular tissue of the stems. • Xylem and Phloem tissues are gathered together into bundles that run from the stem into the petiole.

Leaf Structure

Leaf Function • Photosynthesis – occurs in specialized ground tissue known as MESOPHYLL. • Palisade Mesophyll – closely packed cells that absorb most of the sunlight. • Spongy Mesophyll – Loose tissue that connects to the exterior through Stomata. • Stomata – pore-like openings for gas exchange. • Guard cells – control the opening and closing of stomata. • The top surface of the leaf is known as the upper epidermis and the bottom is the lower epidermis. Secretes a layer of cutin (forms the waxy cuticle) which slows evaporation from the leaf blade.

Transpiration • Transpiration is the loss of water from a plant through its leaves. • This water is replaced by water drawn into the leaf through xylem vessels in the vascular tissue.

Gas Exchange • Plants keep their stomata open just enough to allow photosynthesis to take place, but not so much that they lose an excessive amount of water.

Review Questions… • 1. Describe how the structure of a leaf is optimized for light absorption. • 2. What factors regulate the opening and closing of guard cells? • 3. Are stomata more likely to be open or closed on a hot day? Explain your answer. • 4. Describe the cell types found within a typical leaf.

Transport in Plants • The combination of root pressure, capillary action, and transpiration provides enough force to move water through the xylem tissue of even the largest plant. • Capillary Action – the tendency of water to rise in a thin tube. • Adhesion – tendency of water to form hydrogen bonds. • Transpiration – pulls water upwards. – Too much transpiration cause excess water loss and wilting.

Transport of Water

Nutrient Transport… • The phloem is responsible for transporting nutrients throughout the plant. • When nutrients are pumped into or removed from the phloem system, the change in concentration causes a movement of fluid in that same direction. • As a result, phloem is able to move nutrients in either direction to meet the nutritional needs of the plant.

Review Questions… • 1. What three processes work together to cause water to flow upward through a plant? • 2. Why is capillary action insufficient to move water through a plant?