A.Schevers
Section 23.1
I.
II.
Chapter: 23 Plant Structure and Function
Plant Cells and Tissues
Types of Plant Cells
 Plants cells are different from animal cells because they have a cell wall, a central
vacuole, and can contain chloroplasts.
A. Parenchyma [Fig. 23.2]
 Parenchyma cells are the most abundant kind of plant cell.
 These spherical cells have thin, flexible cell walls, and usually have a large central
vacuole.
 Parenchyma cells have two main functions:
1. Storage
2. Food Production
 The edible portions of many fruits and vegetable are composed mostly of parenchyma
cells.
 Parenchyma cells also contain numerous chloroplasts that produce glucose during
photosynthesis.
B. Collenchyma [Fig 23.2]
 Collenchyma cells are long cells with unevenly thickened cell walls; which allows the cells
to grow.
 The cell walls can stretch as the cells grow while providing strength and support.
 They are arranged in tube like strands or cylinders.
(Ex. Celery)
C. Sclerenchyma [Fig 23.2]
 The cell walls are very thick and rigid.
 At maturity, they often die.
 Their cytoplasm disintegrates, however their thick cell wall still provides the support
needed.
 Two types:
1. Fibers
2. Sclerids
 Fibers are long, thin cells that form strands, which provide support and strength for
the plant. (Used for making linen and rope)
 Sclerids are irregularly shaped and usually found in clusters.
Plant Tissues
 A tissue is a group of cells that function together to perform an activity.
A. Dermal Tissues
 The dermal tissue, or epidermis, is composed of flattened cells that cover all parts of
the plant.
 It functions much like the skin of an animal, covering and protecting the body of a
plant.
 The epidermal cells produce the waxy cuticle that helps prevent water loss.
 Another structure found in the epidermal layer is the stomata, openings in leaf tissue
that control gas exchange.
 Guard cells control the opening and closing of stomata.
 The dermal tissue of roots may have root hairs, which help the roots absorb water and
dissolved minerals.
 Trichomes, found on stems and leaves, are hair like projections that give a stem or a
leaf a “fuzzy” appearance and help to reduce the evaporation of water.
* In some cases, trichomes secrete toxic substances that help protect the plant
from predators.
B. Vascular Tissues
 Food, dissolved minerals, and water are transported throughout the plant by vascular
tissue.
1. Xylem – a plant tissue composed of tubular cells that transports water and
dissolved minerals from the roots to the rest of the plant.
a. Tracheids – tubular cells tapered at each end, and have pits through
which water and dissolved minerals flow.
b. Vessel elements – transport water throughout the plant. These cells
are wider and shorter than tracheids and have openings in their end
walls.
 Vessel elements transport water more efficiently than tracheids because water can
flow more freely through their openings.
2. Phloem – is made up of tubular cells joined end to end.
a. Sieve tube members – alive at maturity; contain cytoplasm, but do not
have a nucleus or ribosomes.
b. Companion cells – located next to each sieve tube cell; nucleated cells
that help with the transport of sugars and other organic compounds.
 Phloem can transport materials from the roots to the leaves.
C. Ground Tissue
 Ground tissue consists mostly of parenchyma cells.
 The functions of ground tissue include: photosynthesis, storage, and support.
 Ground tissue cells in some stems and roots contain large vacuoles that store starch
grains and water.
D. Meristematic Tissue
 A growing plant produces new cells in areas called meristems.
 Meristems are regions of actively dividing cells that contain parenchyma cells with
large nuclei.
1. Apical Meristems – found at on near the tips of roots and stems. They
produce cells that allow the roots and stems to increase in length.
 Lateral meristems are located in the roots and stems and help to increase their
diameters.
1. Vascular cambium – produce new xylem and phloem cells in the stems and
roots.
2. Cork cambium – produce cells with tough cell walls, and cover the surface of
stems and roots. The outer bark is produced by the cork cambium.
Section 23.2 Roots, Stems, and Leaves
I.
Roots
*Roots are plant organs that anchor a plant, usually absorb water and dissolved
minerals, and contain vascular tissues.
A.
B.
II.
Stems
A.
B.
* The type of root can vary based on factors such as soil type, moisture, and
temperate.
* There are two main types of roots.
1. Taproots – are single, think structures with smaller branching roots, which
accumulate and store food. (Ex. Carrots and beets)
2. Fibrous roots – have many small branching roots that grow from a central
point. (Ex. Grasses)
* Other types of roots include prop roots and aerial roots.
The structure of roots
* The epidermal cells are in the epidermis, the outermost layer of the root.
* Root hairs increase the surface area of a root; they absorb water, oxygen, and
dissolved minerals.
* Moving inward from the epidermis, the next layer of cells in the roots is the
cortex.
* The cortex is ground tissue, and helps move water and dissolved minerals into the
vascular tissue.
* The cortex is made up of parenchyma cells that sometimes store food and water.
* The next layer inward from the cortex is the endodermis, which is a layer of
waterproof cells that form a seal around the roots vascular tissue.
* The endodermis controls the flow if water and dissolved material into the root.
* Next to the endodermis is the pericycle.
* Xylem and phloem are located in the center of roots.
Root growth
* There are two areas of rapidly dividing cells in roots.
1. The root apical meristem produces cells that cause a root to increase in length.
2. The vascular cambium develops between the xylem and phloem and contributes
to a roots increase in diameter.
* A protective layer of cells called the root cap covers the tip of each root.
* Most often stems are the above ground part of a plant that supports leaves and
flowers.
* Stems have vascular tissues that move water, dissolved minerals, and sugars to
and from roots and leaves.
* Stems that act as food storage organs are corms, tubers, and rhizomes.
1. Corm – short thickened stem surrounded by leafy scales.
2. Tuber – swollen stem that has buds from which new plants grow.
3. Rhizomes – thick underground stem.
Internal structures
* The vascular tissue inside stems is arranged differently than inside roots.
* Stems have bundles of vascular tissue with parenchyma tissue.
1. Monocots – vascular bundles are scattered throughout the stem.
2. Dicots – vascular bundles form a ring in the cortex.
* As the stems of woody plants grow taller, they also grow in thickness; this added
thickness is called secondary growth.
Stems transport materials.
* The stem transports water, sugars, and other compounds.
* Xylem moves water and dissolved minerals from the roots to the leaves.
* The phloem carries dissolved sugars, which come from the photosynthetic tissues
in the leaves.
* Any part of the plant that stores these sugars is called a sink.
* The movement of sugars in the phloem is called translocation.
Growth of a stem
* Primary growth lengthens a stem, and is due to the production of cells by the
apical meristem at the tip of the stem.
* Meristems located along the stem, called nodes, are where leaves and branches
develop.
C.
III.
Leaves
The primary function of the leaves is photosynthesis.
The flat, broad, green surface of a leaf is called the leaf blade.
A blade of grass is joined directly to the stem, but in other leaves, a stalk joins
the leaf blade to the stem, which is called a petiole.
 A simple leaf is one with a blade that is not divided. (Ex. Maple leaves)
 When a leaf blade is divided into leaflets it is called a compound leaf. (Ex.
Walnut)
A.
Leaf structure
 Between the epidermal layers are two layers of mesophyll – the photosynthetic
tissue.
 The spongy mesophyll cells are surrounded by many air spaces that allow carbon
dioxide, oxygen, and water vapor to flow freely.
B. Transpiration
 The loss of water through the stomata is called transpiration.
 The opening and closing of guard cells regulates transpiration.
C. Leaf modifications
 The pattern of veins in leaves is one way to distinguish difference groups of
plants.
 Many plants have leaves that have functions besides photosynthesis.
- cactus spines are modified leaves
- some leaves can trap insects
- some act as water or food storage
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