Chapter 25 Plant Structure and Function

I. Tissues A. Dermal Tissue Covers the outside of a plant’s body as protection Forms a “skin” called the epidermis in nonwoody plant parts A waxy cuticle, which prevents water loss, coats the epidermis of the stems and leaves

Extensions on the epidermal cells on root tips help increase water absorption Forms layers of dead cells called cork on woody stems and roots

B. Ground Tissue Makes up most of the inside of most plants Surrounds and supports vascular tissue Specialize in photosynthesis in leaves, as they are packed with chloroplasts Store water, sugar, and starch in stems and roots

C. Vascular Tissue 1. Xylem Thick-walled cells that conduct water and mineral nutrients from a plants roots through its stems to its leaves Must lose their cell membrane, nucleus, and cytoplasm before they can conduct water - All that is left is the cell wall

a. Tracheid Narrow, elongated, and tapered at each end Water flows through pits, or thin areas in between neighboring cell walls

b. Vessels Wider Larger perforations in their ends that allow water to move more quickly

2. Phloem Conduct sugars and other nutrients throughout a plants body through connecting cells that form sieve tubes Cells either lack organelles or have modified organelles

Pores in the walls between neighboring sieve-tube cells connect the cytoplasms and allow substances to pass freely from cell to cell Companion cells lie next to the sieve-tube cells and carry out cellular respiration, protein synthesis, and other metabolic function for the sieve-tube cells

II. Roots Absorb water and mineral nutrients Store organic nutrients such as sugar and starch Dicots, such as carrots and radishes, have a large central root from which much smaller roots branch; a taproot system Monocots, such as grasses, have a highly branched, fibrous root system

Roots contain a central core of vascular tissue that is surrounded by ground tissue, called the cortex A mass of cells called the root cap covers and protects the actively growing root tip

III. Stems House vascular tissue which transports substances between the roots and the leaves Leaves are attached to a stem at points called nodes The space between two nodes is called an internode

A. Nonwoody Stems Also called herbaceous plants Xylem and phloem tissue arranged in vascular bundles which are surrounded by ground tissue Two types of ground tissue 1. Cortex – outside tissue 2. Pith – center of the stem

B. Woody Stems Cork covers woody stems and protects them from physical damage and helps prevent water loss Cork and phloem tissue make up bark Heartwood at the center of the stem contains xylem that no longer conducts water Sapwood, (in between heartwood and bark) contains xylem that can conduct water

IV. Leaves Primary photosynthetic organs of plants Consist of a flat blade that is attached to the stem by a stalk called the petiole

A leaf may be divided into two or more leaflets and is called a compound leaf - Leaflets reduce the surface area of a leaf blade An undivided blade is called a simple leaf

Xylem and phloem are found in the veins of a leaf Veins are extensions of vascular bundles that run from the tips of roots to the edges of leaves The ground tissue in leaves is called mesophyll Mesophyll is packed with chloroplasts which makes the leaves look green

V. Movement of Water Step 1 When the pores of a leaf, the stomata, are open, water vapor diffused out of a leaf This loss of water vapor is called transpiration More than 90 percent of the water taken in by the roots is ultimately lost through transpiration

Step 2 The cohesion of water molecules causes water molecules that are being lost by a plant to pull up on the water molecules still in the xylem that extends from the roots to the edges of leaves

Step 3 Roots take in water from the soil by osmosis and replaces water displaced by cohesion through the xylem caused by transpiration

A. Guard Cells and Transpiration Stoma are surrounded by a pair of guard cells Each guard cell swells in length as they take in water As guard cells take in water, they bend away from each other, opening the stoma and allowing transpiration to proceed

When water leaves the guard cells, they shorten and close the stoma, stopping transpiration and further water loss Homeostasis in action

VI. Movement of Organic Compounds Organic compounds are provided by what botanists call a source Organic compounds are delivered to what botanists call a sink Sugar from a source enters phloem cells by active transport

Step 2 When the sugar concentration in the phloem increases, water enters the cells by osmosis Step 3 Water pressure increases inside the cell and pushes sugar out Step 4 Sugar moves to the sink through active transport