Chapter 25Plant Structure and Function

I. TissuesA. 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 Tissue1. 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 tissue2. 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 WaterStep 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 osmosisStep 3• Water pressure increases inside the cell and

pushes sugar outStep 4• Sugar moves to the sink through active

transport