HL IB-Biology 2011/12
#18 PLANT STRUCTURE
9.1 Plant Structure and Growth
Assessment Statements Addressed:
9.1.1 Draw and label plan diagrams to show the distribution of tissues in the stem and
leaf of a dicotyledonous plant.
9.1.2 Outline three differences between the structures of dicotyledonous and
monocotyledonous plants.
9.1.3 Explain the relationship between the distribution of tissues in the leaf and the
functions of these tissues.
9.1.4 Identify modifications of roots, stems and leaves for different functions: bulb,
stem tubers, storage roots and tendrils.
Part I:
DISTRIBUTION OF TISSUES IN THE STEM AND LEAF OF DICOTYLEDONOUS PLANTS
Text adapted from: Damon et al, Higher Level Biology, Heinemann Baccalaureate 2007
Land plants possess some common characteristics:
• They provide protection for their embryos which has increased over time
• They have multicellular haploid and diploid phases
• They can be compared by the presence or absence of conductive systems
Table 1: LAND PANTS ARE DIVIDED INTO THREE MAJOR GROUPS:
MAJOR FEATURES
PLANT GROUP
NON-VASCULAR LAND PLANTS
SEEDLESS VASCULAR PLANTS
SEED VASCULAR PLANTS
Co conductive tissue
Often grouped together as bryophytes
Usually small and grow close to the ground
Include mosses, liverworts., and hornworts
Well developed vascular tissue
Do not produce seeds
Include horsetail, ferns, club mosses, and whisk ferns
Most living plant species are in this group
Seeds contain an embryo, a supply of nutrients, and a protective outer coat
Have extensive vascular tissue and include some of the world’s largest organisms
The seeded vascular plants are further divided into:
• Gymnosperms - have seeds that do not develop within an enclosed structure
• Angiosperms - have seeds that develop within a protective structure
Labudda, CHS, 18 Plant Structure.pages
We are looking at the angiosperms.
In general, angiosperms have thee basic types of tissue:
1. Dermal tissue - this outer protective covering protects against physical agents and pathogenic
organisms; it prevents water loss and may have specialized structures for various purposes;
2. Ground tissue - consists mostly of thin - walled cells that function in storage, photosynthesis,
support and secretion;
3. Vascular tissue - xylem and phloem carry out long distance conduction of water, minerals and
nutrients within the plant and provide support.
These three tissue types derive from meristematic tissue. Meristematic tissue is composed of
aggregates of small cells that have the same function as stem cells in animals. When these cells
divide, one cell remains meristematic while the other is free to differentiate and become part of the
plant body. By doing this, the population of meristematic cells in continually renewed. The cells that
remain meristematic are referred to as initials, while the cells that begin differentiation are called
derivatives.
Plants have organs just as complex as animals do. The three major organs are roots, stems, and
leaves.
Root tissues
The rootʼs function is to absorb mineral ions and water from the soil. Roots anchor the plant and even
provide food storage in some cases. They have an epidermis that forms an protective outer layer.
Following the epidermis is the cortex which is involved in conducting water from the soil to the interior
vascular tissue of the root. The cortex may also be modified to carry out storage function. Next time
you eat a whole (real) carrot (not a baby carrot), break it in half, and identify the cortex and vascular
tissue. In the middle of the root is the vascular tissue that contains the phloem, which transports
organic nutrients and xylem, which transports water and dissolved minerals. The xylem is located at
the innermost part of the root, surrounded by the phloem. The vascular tissue is surrounded by an
endodermis.
Stem tissues
The stem is the plant region where leaves are attached. The area where a leaf joins the stem is called
the node, and the area between two nodes is called an internode. Leaves are arranged in a number
of different ways of the stem. The arrangement of tissues in the stem is as follows:
The stem is surrounded by an epidermis which mainly serves as a protection, however, it can have
pores (lenticles) that allow gas exchange. Followed by the epidermis is the cortex. The cortex of the
stem resembles that of the root. It supports and may have storage function. The transporting tissues
of the stem include xylem and phloem. They are arranged together as vascular bundles in a circle
with the xylem towards the inside and the phloem towards the outside of the stem. The xylem
transports water and dissolved minerals. In woody plants it also lends support to the plant. The
phloem mainly transports organic nutrients throughout the plant. The tissue that separates the xylem
from the phloem in the stem is called the cambium. The cambium is an area of rapidly dividing cells
that differentiate into xylem and phloem. In the very middle of a stem cross section is the pith region
which functions as storage and support. Turgid, fluid filled cells in the cortex and pith offer support to
the plant.
Leaf tissues
Leaves are involved in photosynthesis. They vary greatly in form but they generally consist of a
flattened portion called the blade and a stalk called petiole that attaches the blade to the stem. The
tissues are distributed as follows:
Many leaves have a layer of wax called the cuticle as their outermost layer. This layer protects
against water loss and insect invasion. If a cuticle is not present, the outermost layer is the epidermis
which protects. In leaves with cuticle the epidermis follows the cuticle and is called the upper
epidermis. Like stems and roots, the leaves have vascular tissue which includes xylem and phloem.
The xylem brings water to the leaves while the phloem carries the products of photosynthesis to the
rest of the plant. The xylem and phloem occur together in veins or vascular bundles. A densely
packed region of cylindrical cells occurs in the upper portion of the leaf. This region is called the
palisade mesophyll. The cells here contain large numbers of chloroplasts to carry out photosynthesis.
The bottom portion of the leaf is composed of the spongy mesophyll. It consists of loosely packed
cells with few chloroplasts. There are many air spaces in this area providing gas exchange surfaces.
Stomata occur on the bottom surface of leaves and they allow oxygen and carbon dioxide exchange.
Specialized cells called guard cells control the opening and closing of stomata.
‣
Find and study drawings showing the distribution of tissues in the stem and leaf of a
dicotyledonous plant. Review the function of each tissue in the stem and the leaf. Note how
the relative location and distribution of the tissues support their function.
NOTES:
Part II: MICROSCOPY LAB
Handling the Microscope:
• Carry using both hands; one at arm the other supporting the base
• Place on table gently
• Do not touch lenses
• Return on low power
• Make sure slide is removed when returning
Focusing:
• Always start on low power, using
COARSE focus ONLY!
• Switch to medium power without
moving the stage
• Refocus using the coarse adjustment
(only minor adjustments are needed)
• Switch to high power without moving
the stage
• Only use FINE focus under high
power!
Measuring with the Microscope:
We will estimate the size of a microscopic structure by comparing it to the known sizes of the
diameter of the field of view and the length and width of the pointer. Refer to the chart in
class for the measurements.
Making to Scale Drawings of Microscopic Structures:
‣ Every drawing needs to provide enough information about the actual size of the structure
shown. This can be done several ways:
• Indicate in your drawing the part you measured and write the actual measurement next
to it.
• You calculate how much bigger your drawing is than the actual size and write the
magnification factor next to your drawing.
• Provide a scale bar at bottom of the drawing
‣ Title each drawing with the structure observed and power used
‣ Each drawing should take up ½ page, do not draw the field of view
‣ All structures shown should be labeled. Draw a line from the structure to the OUTSIDE of
your drawing and add the label
‣ All drawings should be neat!
TASK:
You have available prepared slides of cross-sections of stem and leaf of a dicotyledonous plant.
1. Look at each cross-sections and make detailed (show cellular detail) and labeled drawings
under low, medium, and high power. You might have to make several high power drawings to
show different structures in more detail. Use the information from part I to identify all the
tissues found in each organ.
2. Once you are done with your detailed drawings, make a plan diagram of a cross section of a
stem and leaf to show the position of the different tissues. Remember a plan diagram does
not show the individual cells just the relative position of the different tissues.
3. Add a description of the function next to the label for each tissue shown in the plan diagram
4. Copy table 2 under your plan diagrams and complete the table by explaining for each tissue
how how location supports function.
Table 2: How location of leaf tissue supports its function
Leaf tissue
Palisade mesophyll
Veins
Spongy mesophyll
Stomatal pores
How location supports function:
Part III: MONOCOTS, DICOTS
AND PLANT ORGAN
MODIFICATIONS
Monocots Versus Dicots
Angiospermophytes are organized into monocotyledonous and dicotyledonous plants, based on
their morphological similarities. Morphological similarities between different groups of organism
can imply, that the groups are closely related to each other. However, some structural
similarities can be a result of adaptations to the same selection pressure (bat wing and bird
wing). Evolutionary relatedness can be shown in form of a cladogram. A branching point
represents the last common ancestor of two groups of organisms. Any characteristics the two
groups have, that are due to evolutionary relatedness, the common ancestor should have as well.
If a group has a characteristic the common ancestor or the other group does not share, then it
means that characteristic evolved after the split.
1. Using the internet, research how much the morphological differences between monocots and
dicots represent an evolutionary relationship. In other words, which cladogram more closely
resembles current DNA evidence for “degree of relatedness”? Circle the ‘correct’ cladogram
in figure 1 and provide a justification for your choice below.
Figure 1: Possible Cladograms for the evolutionary Relationship Between Monocots and Dicots.
dicot
Common ancestor
monocots
Common ancestor
dicots
Common ancestor
angiosperm
Justification:
monoc monoc dicot dicot
Common
ancestor
dicot
2. Table 2 compares the the structures common to all monocots and dicots. Describe how each
structure looks like for monocots and dicots and provide a drawing to clarify your
description.
Table 2: MAIN STRUCTURAL DIFFERENCES BETWEEN MONOCOTS AND DICOTS
MONOCOTS
STRUCTURE
DESCRIPTION
VEINS IN LEAVES
FLOWER PARTS
NUMBER OF
COTYLEDONS
(SEED LEAVES)
VASCULAR
BUNDLES IN STEM
ROOT SYSTEM
POLLEN GRAIN
DICOTS
DRAWING
DESCRIPTION
DRAWING
Plant Organ Modifications
Roots, stems, and leaves in plants can be modified to serve a different function, such as food
storage or climbing.
1. Obtain and cut crosswise a whole carrot (not a baby carrot) and a potato. Based on what you
have learned identify the tissues that are visible and decide if a carrot or potato are a
modification of a root, stem, or leaf.
2. Draw and label the cross section of the carrot and potato.
3. Below the drawing state which modification the carrot and potato represent and provide a
justification. (FYI: Repeat with a baby carrot. Was the baby carrot a part of a bigger carrot
or is it a whole carrot but smaller?)
Drawings:
Justification:
4. Tables three to five list the different kinds of modifications for roots, stems and leaves.
Describe each modification and provide an example.
Table 3: MODIFICATION OF ROOTS
ROOT MODIFICATION
DESCRIPTION
EXAMPLE
DESCRIPTION
EXAMPLE
PROP ROOTS
STORAGE ROOTS
PNEUMATOPHORES (AIR
ROOTS)
BUTTRESS ROOTS
Table 4: MODIFICATION OF STEMS
STEM MODIFICATION
BULB
TUBERS
RHIZOMES
STOLONS
Table 5: MODIFICATION OF LEAVES
LEAF MODIFICATION
TENDRILS
REPRODUCTIVE LEAVES
BRACTS OR FLORAL
LEAVES
SPINES
DESCRIPTION
EXAMPLE