Download Plant Anatomy 2

PLANT ANATOMY II:
Tissues are Assemblages of Cells
Objectives:
1) Locate, identify, and describe the primary functions and associated structure of key
organelles unique to plants, including:
a) cell wall
b) vacuole
c) plastids
2) Describe the relationship between a cell, tissue, and organ.
3) Describe the 3 basic tissue types, including:
a) a description of where they would be found within the plant,
b) the functional and/or structural significance of the tissue in the plant, and
c) a description of the cell types and/or modifications that would likely be present
within the tissue.
INTRODUCTION
In today's lab, we will examine cells from a variety of plants and focus on the characteristics
that make plant cells unique from the cells of other organisms, such as animals or fungi. When a
group of cells interact with one another to form a structural or functional unit in an organism,
we refer to that group of cells as a tissue. In the previous lab, we learned that the organs of
plants (roots, stems and leaves) are assembled from various arrangements of 3 basic tissue
types (dermal, ground and vascular). In this lab, we will examine some of the different cell
types that are used in order to build those tissues. In addition, you will see examples of cellular
modifications used by plants that enable them to cope with a variety of stresses posed by their
local environment.
PLANTS HAVE 3 TISSUE SYSTEMS: DERMAL, GROUND, VASCULAR
Recall from the Organ/Tissue lab, that plants include 3 tissue systems: the dermal, ground, and
vascular. In that lab, we examined the arrangement of those tissues across different plant
organs and in comparison of monocots vs. dicots. Here, we will examine specific examples of cell
types that are unique within each of the tissue systems.
1) DERMAL TISSUE SYSTEM (EPIDERMIS)
As in animals, the epidermis is the tissue layer that establishes a barrier between internal
tissues and the surrounding external environment. Cells of the epidermis are coated with a
waxy substance, called cuticle, which provides a waterproof seal to protect the cell and internal
tissues from water loss. With the exception of the presence of cuticle, cells of the epidermis
tend to resemble the cells of the interior tissues. However, 2 examples of modifications of
epidermal cells are described below which are designed to serve very specialized functions.
 Guard cells. Guard cells open and close in response to changes in turgor pressure in
surrounding epidermal cells. The opening created by the action of the guard cells is called a
stomate. Guard cells regulate the size of the stomatal opening, thus regulating exchange
of gases between the plant and the atmosphere.
 Trichomes. Trichomes are outgrowths of epidermal cells that often take the form of
hairs, barbs, or scales. In many cases, hairs or fuzz on plants serves to increase surface
area. In fact, nearly all the water uptake by plants is accomplished by absorption through
specialized trichomes on the root surface, called root hairs. Trichomes also have an
important role in defense where they may serve as a mechanical barrier to herbivores or
pathogens. In some cases, the trichomes may be modified as chemical weapons with spikelike projections containing miniature reservoirs of toxins to poison potential predators.
2) GROUND TISSUE SYSTEM
Ground tissues are generally quite variable and can be found throughout roots, stems, and
leaves. Some examples of cells/tissues included in the ground tissue system are described
below.
 Cortex. Cortex is distributed between the epidermis and vascular tissue of roots and
stems. It generally consists of parenchyma cells that are modified for starch storage.
 Pith. Pith consists of undifferentiated parenchyma cells that are often used for
storage. Pith can be found in roots or stems, but is always located within the center of a
cylinder of vascular tissue.
 Leaf Mesophyll. Mesophyll is the tissue distributed between the upper and lower
epidermis of leaves and is modified for photosynthesis.
3) VASCULAR TISSUE SYSTEM
The vascular tissues (xylem and phloem) are the tissues of plants responsible for the transport
of substances throughout the plant body. Cell types that are found in xylem and phloem are
discussed below:
 Xylem. Xylem is the name given to tissue specialized for the conduction water and
dissolved minerals. The dead cells of xylem act as the structural medium for the movement
of water. The efficiency of water conduction varies with the type of xylem cell found in
the plant.
o Tracheids occur in all vascular plants (plants that have vascular tissues). They
are long and slender and are packed tightly against one another in an intact
plant.
o Vessel elements only occur in angiosperms (flowering plants). They tend to be
shorter and wider than tracheids and have large openings (perforations) in their
ends. In intact plants, individual vessel elements are arranged end-to-end
forming long sections of uninterrupted conduit for the rapid movement of water.
 Phloem. Phloem is the living tissue modified for the conduction of dissolved sugars
throughout the plant. Phloem tissue consists of 2 cell types – each modified to perform a
different functional role within the tissue.
o Sieve elements – Like the vessel elements of xylem, the sieve elements of
phloem may be arranged end-to-end for efficient conduction from one cell to
the other. Sieve elements are alive at maturity, but lack many of the organelles
necessary for ordinary cellular function, such as nuclei, ribosomes, vacuoles.
o Companion cells – Companion cells are typically paired 1 to 1 with the sieve
elements. They are much smaller in size than sieve elements, but contain the
organelles that sieve elements lack and carry out many of the functions
necessary to keep sieve elements alive.
The diagrams below illustrate the basic architecture of cells from the phloem (left) and xylem
(right).
vessel element
tracheid
http://www.bbc.co.uk/scotland/education/bitesi
ze/standard/biology/world_of_plants/making_f
ood_rev2.shtml
http://www.emc.maricopa.edu/faculty/farabee/
BIOBK/BioBookPLANTANAT.html
Summary of Tissue/Cell Types:
Tissue
Cell/Tissue Types
Dermal
Epidermis
Periderm
Specific Cell Types and/or Specializations
Guard Cells, Trichomes
Cork, Cork Cambium, Parenchyma
Ground
Cortex
Pith
Leaf Mesophyll
Palisade, Spongy
Xylem
Phloem
Tracheids, Vessel Elements, Fibers, Parenchyma
Sieve Elements, Companion Cells, Parenchyma
Vascular
3 CELL TYPES ARE INVOLVED IN THE CONSTRUCTION OF PLANT
TISSUES: PARENCHYMA, COLLENCHYMA, SCLERENCHYMA
Plant cells originate from undifferentiated meristems, but mature over time and become
specialized to perform different functions within the plant. When groups of cells interact in
such a way as to form a coherent structural or functional unit, they are considered a tissue.
Occasionally, the words “tissues” and “cells” are used interchangeably since an assemblage of
cells of the same type would be referred to as a tissue by the same name (e.g., an assemblage of
parenchyma cells would be referred to as parenchyma tissue). The 3 basic cell types are
described below. Note that these cells/tissues are distributed among the 3 different tissue
systems discussed earlier.
1) Parenchyma. Of the three cell types, parenchyma cells are the most variable – both
structurally and functionally. They tend to be many-sided with relatively thin walls, but the
actual structure of a parenchyma cell will vary depending on the tissue in which it is found.
Parenchyma cells are found throughout the plant body where they are specialized for a variety
of functions, including photosynthesis (leaf mesophyll, cortex of herbaceous stems), storage
(cortex and pith of roots and stems, flesh of fruits), wound healing, and regeneration of
damaged or dead cells.
2) Collenchyma. Collenchyma cells tend to have unevenly thickened cell walls – they are thicker
than the cells walls of parenchyma, but less rigid than sclerenchyma, and serve primarily as
flexible support. Collenchyma cells often form a ring of tissue just beneath the epidermis of
green stems and petioles, and along the veins of some leaves. The “strings” or “ribs” in celery
are a good and familiar example of collenchyma tissue.
3) Sclerenchyma. Sclerenchyma is characterized by thickened cell walls and a very rigid
(sclerified) structure. This rigidity is due to the deposition of a sticky substance called lignin in
the cell walls of sclerenchyma cells. Lignin is a very hard plant polymer that is responsible for
the hardness of wood. Sclerenchyma cells include both fibers and sclereids (described below),
as well as xylem after it has died.

Fibers are long and slender and tend to occur in bundles or strands (e.g., hemp, flax,
wood). You can often find fibers distributed throughout the cortex of stems or among
the cells in xylem and phloem tissues. Fibers are most often associated with structural
support, but sometimes have a role in defense or storage.

Sclereids are sclerenchyma cells that can take on a variety of interesting and beautiful
shapes, but in general, tend to be much shorter than fibers. Sclereids are thought to be
primarily defensive, providing a mechanical nuisance to herbivores. The “grit”
characteristic of pear flesh is due to the presence of a type of sclereid referred to as
a “stone cell”.
PLANT CELLS ARE CHARACTERIZED BY 3 UNIQUE ORGANELLES: CELLULOSE CELL
WALLS, VACUOLES, AND PLASTIDS
Many of the structures and organelles found in plant cells are common to other non-plant,
eukaryotic organisms. For example, plant cells have nuclei, mitochondria, Golgi, ribosomes, and
endoplasmic reticula, much as you would find in the cells of animals, protists, and fungi.
However, in this part of the lab, we will focus on 3 cellular structures that are unique to plants
alone: the cell wall, vacuoles, and plastids.
1) CELLULOSE CELL WALL
The cell wall is a rigid structure that provides a structural barrier between the protoplast
(everything inside the cell wall) of the cell and the surrounding environment. Plant cell walls are
composed primarily of the structural carbohydrate, cellulose, but may also contain other
polysaccharides (such as hemicellulose and pectin, the substance used to gel jellies) and
glycoproteins (sugar-linked proteins).
Communication between adjacent cells is facilitated by tiny holes in the cell wall called
plasmodesmata through which, strands of cytoplasm extend to connect the protoplasts. A
pectin-rich substance, called middle lamella, cements the walls of adjacent cells together. This
prevents individual cells from sliding past one another as well as to help the population of cells
maintain its overall shape within the plant tissue.
plasmodesmata appear as lines traversing
the cell walls of 2 adjacent cells
the middle lamella appears as a slightly
darkened band between the 2 cells
2) VACUOLE
Many plant cells are characterized by the presence of a very large membrane-bound sac, called
a vacuole. The solution inside the vacuole is primarily water, but may also contain dissolved
mineral salts or sugars. The vacuole often serves as a storage site for dissolved compounds
that will be used later by the cell, or as a waste reservoir for compounds the cell needs to
eliminate. In some cases, the vacuole stores toxins or reactive substances that could otherwise
cause damage to other cell constituents.
As an example, some plants store “anthocyanin pigments” in their vacuoles. Anthocyanins are a
class of water-soluble pigments which lend purple/blue to red colors in some plant parts, such as
the fruits of blueberries and grapes. You may be aware of the purported nutritional value of
some plant foods (e.g., blueberries and grapes!) due to their high content of antioxidants.
“Antioxidant” is the term applied to any chemical that is capable of inactivating free radicals
(highly reactive, unstable atoms that cause damage to cells and are thought to contribute to
cellular aging and degeneration). Many plant pigments, including anthocyanins, are among the
much larger class of compounds known as “antioxidants”.
Perhaps the most significant role of the vacuole is in water regulation and maintenance of
turgor pressure in the cell. The vacuole can change size depending on the water and solute
concentration of the surrounding medium. When fully expanded, the vacuole can occupy as much
as 70-90% of the total volume of the protoplast. In the expanded state, the contents of the
vacuole exert pressure outward, pressing the cellular contents outward against the cell wall.
The resultant cell is said to be "turgid", or structurally firm, due to the large amount of water
that fills the entire volume allowed by the cell wall. If the plant becomes dehydrated and loses
water, it will lose turgidity and become "flaccid". The change in the posture of the plant is due
to loss of water out of the vacuole, causing it to shrink within the cytoplasm. The shrunken
vacuole cannot apply pressure to the cell walls and the plant wilts. A flaccid plant can become
turgid again if it is rehydrated before the cells lose their ability to function and die.
3) PLASTIDS
Plastids are a group of organelles that are generally used for storage and synthesis of a variety
of substances, including oils, starches, proteins, and some pigments. Chloroplasts are plastids
modified for carrying out the reactions of photosynthesis.
Most plastids are bound by double membranes and have additional layers of folded membrane
arranged throughout their interiors. The complexity of the internal membrane system varies
considerably among different types of plastids.
 Chloroplasts - contain chlorophylls (a and b) and carotenoid pigments; play a key role in
photosynthesis
 Leucoplasts - non-pigmented plastids modified for storage and synthesis of substances,
such as oils, proteins and starches. Amyloplasts are a type of leucoplast specialized for the
production and storage of starch.

Chromoplasts - storage of carotenoid pigments (yellow to red).
The photo below illustrates some of the major organelles found in a typical plant cell.
mitochondria – these are not usually
visible under the light microscope
chloroplasts
tonoplast
nucleus
the vacuole is quite large and usually
takes up most of the volume of a fully
hydrated cell. if you look carefully, you
can usually make out the faint
membrane that surrounds the vacuole,
called the “tonoplast”
ACTIVITIES:
1. Generalized Plant Cell: Make a wet mount from a leaf of Elodea (pondweed). View and
compare the cells under different magnifications. Once the Elodea slide has warmed a bit, you
should be able to see cytoplasmic streaming (the orderly movement of cytoplasmic contents
within the cell). In your lab notebook, draw and label the cell wall, protoplast, nucleus,
cytoplasm, vacuole, mitochondria (not likely, but maybe), and chloroplasts.
Add a few drops of salt or sugar solution to one side of your slide. Apply a Kimwipe to the
opposite edge of the slide to draw the solution through undereath the coverslip. Re-examine
your Elodea cells. How have they changed? Can you explain the mechanism behind the changes
you've observed?
2. Cell Walls: Obtain a prepared slide of persimmon. Note that in this case, the protoplast is no
longer present because these cells were dead at the time the slide was prepared. All that is left
behind are the thickened cell walls. Locate the cell wall, plasmodesmata, and middle lamella.
3. Trichomes (Epidermis) and Vacuoles: The leaves and stem of Zebrina (wandering jew) are
covered with fine white hairs (trichomes). Scrape a few of these hairs onto a slide and make a
wet mount. Locate the vacuole – does it contain anthocyanin pigments? What color are they?
From what tissue are trichomes derived? Why might a plant have anthocyanin pigments in its
trichomes?
Can you find trichomes (hairs) on other live plants in the lab? Many desert plants (and several
fleshy fruits such as peaches) are covered with white fuzz from trichomes. If trichomes serve
to increase surface area, how could this be adaptive in a hot, dry environment like the desert?
What would be the advantage to a fleshy fruit, like a peach?
4. Plastids: a) Chromoplasts: Twist and tear the petal from a yellow, orange, or red flower.
Make a wet mount of the petal and focus on the torn edge (where the section is thinnest).
Chromoplasts can sometimes also be viewed along the margin of a petal where finger-like cells
of the epidermis stand out. Many red, orange, or yellow fruits (such as peppers or tomatoes) are
colored by carotenoid pigments contained in chromoplasts. If using fruit, smash or make a very
thin section from a small amount of the flesh and wet mount it to observe chromoplasts.
b) Amyloplasts (a type of leucoplast modified for starch storage): Make a wet mount with a
small amount of flesh from a potato tuber. Look for the starch grains in the clear amyloplasts.
After viewing them, remove the slide from the microscope stage and add a few drops of I 2KI
(Potassium iodide) to one side of the coverslip. “Pull” the reagent through by placing a Kimwipe
at the opposite edge. View the slide again. How has the slide changed? What has iodine stain
bonded to?
5. Dermal Tissues: Prepare a wet mount from an epidermal peel of a plant recommended by
your TA. Observe the thickened cuticular layer of the epidermal cells. Try to locate a pair of
guard cells. Is the stomate open or closed? Apply a few drops of pure water or salt solution in
the manner described in #1. Do the guard cells respond to the change in salt concentration?
6. Ground Tissues: Prepare and wet mount a) a thin section of celery (or rhubarb) and look for
collenchyma cells surrounding the "ribs", and b) a smash of pear flesh and look for sclereids
(stone cells). Prepared slides of Nymphaea (water lily) are also good for viewing sclereids. How
do the cell walls of these cells compare to each other?
7. Vascular Tissues: Observe a prepared slides of: a) macerated wood from an angiosperm tree.
Try to distinguish tracheids and vessel elements. You may also find fibers (sclerenchyma –
ground tissue) interspersed among the xylem cells. b) phloem. Can you locate sieve cells and
companion cells?
POSTLAB QUESTIONS
1. Draw your Elodea cell before and after addition of the salt solution. How did your Elodea cell
change with the addition of salt/sugar solution? What organelle is primarily responsible for the
change in shape? Explain what happened.
2. Draw and label a few adjacent persimmon cells noting the cell walls and plasmodesmata.
3. Both types of plant pigments you observed (carotenoids and anthocyanins) are contained in
membrane-bound organelles (chromoplasts and vacuoles, respectively). Why do you suppose this
is the case? In other words, why don’t we find plant pigments distributed throughout the cell
cytoplasm? What are some of the functions of plant pigments?
4. If trichomes serve to increase surface area, what might be an advantage of having fuzzy
trichomes on desert plants? (or fleshy fruits?) in roots?
5. Draw and label guard cells in the open and closed state. What conditions are likely to trigger
changes in the size of the stomatal opening?
6. Use any of the examples provided in lab to draw and compare the basic structure of a
generalized parenchyma, collenchyma, and sclerenchyma cell (hint: note the differences in the
architecture of the cell wall among the 3 cell types). How does the variation in structure relate
to differences in the function of each of the cell types?
7. Draw a tracheid and a vessel element. Which do you think is more efficient at conducting
water?