31
Plant Structure, Growth, and Differentiation
Lecture Outline
I.
Plant structure and life span
A. General features
1. Almost all plants have the same body plan, consisting of root, stem, and
leaves
2. Plants are either herbaceous or woody
3. Plants can be annuals, biennials, or perennials
4. Woody perennials can be deciduous or evergreen
B. Plants have different life history strategies
1. Woody plants often live for hundreds of years; herbaceous plants may live
only a few months
2. Favorable environments favor plants with long life spans
3. Unfavorable environments favor short-lived plants
4. Each species thus has its own characteristic life history
II.
The plant body
A. A plant is typically organized into a root system and a shoot system
1. The root system is typically below-ground, and the shoot system is the
above-ground tissue, consisting of the stem, leaves, flowers, and fruits
2. The root and shoot system are exposed to very different environmental
conditions, but exist because plants require materials from both
environments
B. The plant body consists of cells and tissues
1. Tissues of plants, like animal tissues, are composed of cells that form a
structural unit
a) Some tissues are simple (composed of only one type of cell), others are
complex (composed of more than one cell type)
b) The three tissue systems of plants are the ground tissue system, the
vascular tissue system (functions in conduction), and the dermal tissue
system (functions in covering the body)
c) Roots, stems, leaves, floral parts, and fruits are called organs as each is
composed of different tissue types
d) The tissue systems of the root and shoot systems are connected
C. The ground tissue system is composed of three simple tissues
1. The ground tissue system makes up the bulk of herbaceous plants
a) Parenchyma, collenchyma, and sclerenchyma are differentiated based
on their cell wall structure
b) The primary cell is laid down first and it grows with the cell
c)
A secondary cell wall may be laid down internal to the primary cell
wall
2.
Parenchyma cells have thin primary walls
a) Parenchyma is a simple tissue composed of parenchyma cells
b) The functions of parenchyma include storage, photosynthesis, and
secretion
c) Storage of starch, oil, water, and salts occurs in parenchyma
d) Parenchyma cells are alive at maturity
3. Collenchyma cells have unevenly thickened primary walls
a) Collenchyma is also a simple tissue made of collenchyma cells
b) Collenchyma provides support in soft plant organs
c) Collenchyma are alive at maturity and have uneven thickenings of the
primary cell walls, particularly in the corners
4. Sclerenchyma cells have both primary walls and thick secondary walls
a) Sclerenchyma is a simple plant tissue composed of sclerenchyma cells
specialized for support
b) At maturity, sclerenchyma cells are often dead
c) Sclerids are short, cubical cells found in nut shells and the pits of stone
fruits
d) Fibers are long cells found in clumps, common in the wood and bark
of many angiosperms
5. Cells of the three simple tissues vary in their cell wall chemistry
a) Cellulose accounts for 40-60% of the dry weight of cell walls
(1) Cellulose is a polysaccharide of glucose
(2) Cellulose molecules are wound into microfibrils
b) Cellulose microfibrils are cemented together by a mixture of
hemicelluloses and pectins
(1) Hemicelluloses are a group of polysaccharides that often include
xyloglucans
(2) Pectins are polymers of galacturonic acid
c) Secondary plant cell walls contain lignin
(1) Lignin is a polymer of monomers derived from certain amino
acids
(2) Lignin is extremely complex, and its structure has not been
completely determined
d) Parenchyma, collenchyma, and sclerenchyma differ in the composition
of their cell walls
D. The vascular tissue system consists of two complex tissues
1. The vascular tissue system is composed of xylem and phloem, which are
continuous in the root and shoot systems
2. The conducting cells in xylem are tracheids and vessel elements
a) Xylem conducts water and dissolved materials from the root to the
shoot system and provides structural support
b) In flowering plants, four types of cells make up the xylem
c)
Tracheids and vessel elements conduct water
(1) These cells are dead at maturity, and are hollow tubes
(2) Tracheids are the major components of xylem in gymnosperms
(a) Water passes from one tracheid to the next through pits in
the cell walls
(3) Flowering plants have tracheids and vessel elements
(a) Vessel elements have a larger diameter and may be
connected directly, end to end
(b) Unlike tracheids, the end walls of vessel elements have
holes called perforations
(c) A stack of vessel elements is called a vessel
(d) Water may also pass laterally between vessel elements via
pits
d) Xylem also has fibers and parenchyma cells that are not conducting
cells
3. Sieve tube elements are the conducting cells of phloem
a) Phloem conducts dissolved sugars throughout the plant and provides
support
b) Phloem is a complex tissue composed of two types of conducting cells,
and fibers and parenchyma
c) Dissolved sugars are conducted in sieve tube elements
(1) Sieve tube elements are stacked like vessel elements in sieve tubes
(2) Sieve tube elements have sieve plates
(3) Sieve tube elements are alive at maturity, but most organelles
have degenerated, including the nucleus
d) Companion cells are adjacent to sieve tube elements and direct the
activities of both cells via cytoplasmic connections through
plasmodesmata
E. The dermal tissue system consists of two complex tissues
1. The dermal tissue system is a single layer in herbaceous plants, and can be
very thick in woody plants
2. Epidermis is the outermost layer of cells of a herbaceous plant
a) The epidermis is a complex tissue made of parenchyma cells with a
small number of guard cells; may contain outgrowths called trichomes
b) The epidermis is typically one cell thick; the epidermal cells have
thickened cell walls on the outer margin
c) Epidermal cells are typically nonphotosynthetic and are relatively
transparent, allowing light to penetrate deeper tissues
d) Epidermal tissue retards water loss with a waxy cuticle
e) Stomata, surrounded by guard cells, allow diffusion of gases into and
out of the leaf
f)
Trichomes are hair-like projections which function in increasing the
effective surface area of the root (root hairs), salt excretion in
halophytes, and may be possibly protection in plants like nettles
3.
III.
Epidermis is replaced by periderm in woody plants
a) As a woody plant grows, the epidermis is lost and replaced by
periderm
b) Periderm is a complex tissue made of cork cells and cork parenchyma
cells
c) Cork cells are dead at maturity and function in waterproofing
d) Cork parenchyma cells function in storage
Plants exhibit localized growth at meristems
A. Growth includes cell division, cell elongation, and cell differentiation
1. Cell elongation occurs as the central vacuole fills with water
2. Cell differentiation involves specialization into the different cell types
3. Unlike animals, plants grow primarily in specialized areas known as
meristems
a) Cells in the meristems do not differentiate, and they retain the ability
to divide
4. Primary growth involves an increase in the length of a plant, and all plants
grow this way
5. Secondary growth involves an increase in the diameter of a plant, and
typically only gymnosperms and woody plants exhibit secondary growth
B. Primary growth takes place at apical meristems
1. In the root of a plant, an apical meristem occurs just behind the root cap
a) Behind the meristematic area, the area of cell elongation is marked by
cells that are growing in size and begin differentiation
2. In the buds on the shoot system, a dome of meristematic cells forms an
apical meristem
a) Life primordia and bud primordia arise from this meristem
C. Secondary growth takes place at lateral meristems
1. Lateral meristems extend the length of stems and roots and have two
meristematic areas: the vascular cambium and the cork cambium
2. The vascular cambium is a cylinder of meristematic cells between the wood
and bark of a woody plant
a) Cells of this cambium add to the wood (secondary xylem) and inner
bark (secondary phloem)
3. The cork cambium is a meristematic cylinder in the outer bark and cells
produced here form the cork cells and cork parenchyma cells of the
periderm
4. Bark, therefore, can be defined as a living inner bark made of secondary
phloem and a dead outer bark made of periderm
Research and Discussion Topics

Describe the various life strategies seen in herbaceous annuals, biennials, and
perennials. Which types tend to be "weeds"? What types of environments tend to
foster growth of each type of plant?

What type of cell makes up the "strings" of celery? What cell type makes pears gritty
in texture? What about plants we use for fabric or rope?