Download I. The Vascular Plant Body Objectives: • Identify the three kinds of

HOLT BIOLOGY
UNIT 6-EXPLORING PLANTS
CHAPTER 25
PLANT STRUCTURE AND FUNCTION
I. The Vascular Plant Body
Objectives:
• Identify the three kinds of tissues in a vascular plant’s body, and state the function of each.
• Compare the structures of different types of roots, stems, and leaves.
• Relate the structures of roots, stems, and leaves to their functions.
A. Tissues
1. In vascular plants, there are three types of tissues—dermal tissue, ground tissue, and vascular tissue.
a. Vascular tissue forms strands that conduct water and nutrients throughout a vascular plant.
b. Dermal tissue forms the protective outer layer of a plant.
c. Ground tissue makes up much of the inside of the nonwoody parts of a plant, including roots,
stems, and leaves.
d. The leaves, stems, and roots of a vascular plant contain all three kinds of plant tissues.
2. Dermal Tissue System
a. In the non-woody parts of a plant, dermal tissue forms a “skin” called the epidermis.
b. The epidermis of most plants is made up of a single layer of flat cells.
c. The dermal tissue on woody stems and roots consists of several layers of dead cells that are
referred to as cork.
3. Ground Tissue System
a. Most ground tissue consists of thin-walled cells that remain alive and keep their nucleus after
they mature.
b. The ground tissue in leaves, which is packed with chloroplasts, is specialized for photosynthesis.
c. The ground tissue in stems and roots functions mainly in the storage of water, sugar, and starch.
4. Vascular Tissue Systems
a. Xylem has thick-walled cells that conduct water and mineral nutrients from a plant’s roots
through its stems to its leaves.
b. The conducting cells in xylem must lose their cell membrane, nucleus, and cytoplasm before
they can conduct water.
c. Gnetophytes and flowering plants also have a second type of xylem cell, which makes up
conducting strands called vessels.
5. Phloem
a. Phloem contains cells that conduct sugars and other nutrients throughout a plant’s body.
b. The conducting cells of phloem have a cell wall, a cell membrane, and cytoplasm.
c. The conducting strands in phloem are called sieve tubes.
d. Pores in the walls between neighboring sieve-tube cells connect the cytoplasms and allow
substances to pass freely from cell to cell.
B. Roots
1. Most plants are anchored to the spot where they grow by roots, which also absorb water and mineral
nutrients.
a. Many dicots, such as carrots and radishes, have a large central root from which much smaller
roots branch.
b. This type of root system is called a taproot system.
c. In contrast, most monocots, such as grasses, have a highly branched, fibrous root system.
2. The ground tissue surrounding the vascular tissue is called the cortex.
a. An epidermis covers the end of a root, or root tip.
b. The epidermal cells just behind a root tip often produce root hairs, which are slender projections
of the cell membrane.
c. A mass of cells called the root cap covers and protects the actively growing root tip.
C. Stems
1. Non-woody Stems
a. A plant with stems that are flexible and usually green is called an herbaceous plant.
b. The stems of herbaceous plants contain bundles of xylem and phloem called vascular bundles.
c. The ground tissue outside the ring of vascular bundles is called the cortex.
d. The ground tissue inside the ring is called the pith.
2. Woody Stems
a. Trees and shrubs, such as pines, oaks, roses, and hollies, have woody stems.
b. Woody stems are stiff and non-green.
3. The wood in the center of a mature stem or tree trunk is called heartwood.
a. The xylem in heartwood, which can no longer conduct water, provides support.
b. Sapwood, which lies outside the heartwood, contains vessel cells that can conduct water.
D. Leafs
1. Most leaves have a flattened portion called the blade that is often attached to a stem by a stalk called
the petiole.
2. A leaf blade may be divided into two or more sections called leaflets.
a. Leaves with an undivided blade are called simple leaves.
b. Leaves with two or more leaflets are called compound leaves.
3. Both xylem and phloem are found in the veins of a leaf.
4. Veins are extensions of vascular bundles that run from the tips of roots to the edges of leaves.
5. In leaves, the ground tissue is called mesophyll.
a. Mesophyll cells are packed with chloroplasts, where photosynthesis occurs.
b. Most plants have leaves with two layers of mesophyll.
II. Transport in Plants
Objectives:
• Relate transpiration to the movement of water up a plant.
• Describe how guard cells regulate the rate of transpiration.
• Recognize several distinguishing features of sugar maple trees.
• Describe the process of translocation in a plant.
A. Movement of Water
1. Transpiration drives the movement of water through a plant:
a. Step 1 When the stomata are open, water vapor diffuses out of a leaf. This loss of water vapor
from a plant is called transpiration.
b. Step 2 The loss of water creates a pull that draws water up through the xylem.
c. Step 3 Water drawn into the roots from the soil by osmosis moves up the stem.
2. Guard Cells and Transpiration
a. A stoma is surrounded by a pair of guard cells that are shaped like two cupped hands.
b. Guard cells that take in water bend away from each other, opening the stoma and allowing
transpiration to proceed.
c. When water leaves the guard cells, they shorten and move closer to each other, closing the stoma
and stopping transpiration.
B. Movement of Organic Compounds
1. Botanists use the term source to refer to a part of a plant that provides organic compounds for other
parts of the plant.
2. Botanists use the term sink to refer to a part of a plant that organic compounds are delivered to.
3. The movement of organic compounds within a plant from a source to a sink is called translocation.
4. The German botanist Ernst Münch proposed a model of translocation called the pressure-flow
model:
a. Step 1 Sugar from a source enters phloem cells by active transport.
b. Step 2 When the sugar concentration in the phloem increases, water enters the sieve tubes in
phloem from xylem by osmosis.
c. Step 3 Pressure builds up inside the sieve-tube cells and pushes sugar through the sieve tubes.
d. Step 4 Sugar moves from phloem cells into a sink by active transport.