PLANT ANATOMY I:
Plant Organs Are Comprised of Tissues
Objectives:
1) Describe the relationship between a tissue and organ.
2) Identify a variety of root, stem, and leaf modifications and explain the adaptive significance
of each.
3) Describe the 3 primary tissue types, give examples of each, and be able to locate the tissues
in a variety of plant organs.
4) Explain primary growth (elongation) and development of plant shoots in terms of meristems,
nodes, internodes, and bud and leaf primordia.
INTRODUCTION
The basic body plan of plants consists of 3 organ systems: roots, stems, and leaves. In this
lab, we will examine a variety of modifications of these organ systems that enable plants to
cope with a range of habitats and environmental conditions. In addition, some of the
modifications you will see enable plants to perform unique functions that other plants are
incapable of doing – e.g., eating animals!
The materials provided in lab represent just some of the organ modifications possible that
enable plants to live in a variety of different environmental conditions and habitat types. In our
examination of roots, stems and leaves, we will emphasize structures and modifications typical
of the group of plants known as “angiosperms” or Phylum Anthophyta (flowering plants).
Angiosperms account for approximately 235,000 of the 260,000 known species of plants and
are considered the most evolutionarily “advanced” of any of the plant groups. In later labs, we
will explore the structural and functional attributes of other kinds of non-angiosperm plants,
such as mosses, ferns, and gymnosperms.
Angiosperms are subdivided into 3 groups: monocots, dicots, and “non-monocot paleoherbs”.
However, this third group constitutes less than 3% of angiosperm species, and since traditional
botany primarily emphasizes the differences between monocots and dicots, they are often
excluded in most discussions of angiosperm structure and classification. Thus, in our
examination of the anatomy of roots, stems, and leaves, we will only emphasize differences
between monocots and dicots without specifically addressing the non-monocot paleoherbs.
When you think of flowering plants, the examples that come to mind will likely be dicots. For
example, roses, oaks, beans, cabbages, and maples are all dicots. Monocots include plants such
as grasses, sedges, rushes, and members of the lily/onion, orchid, and iris families. Monocots
and dicots differ structurally in a number of respects. For example, the names “monocot” and
“dicot” refer to the number of cotyledons, or “seed-leaves” borne on the seedlings of each
group – monocots have 1 cotyledon; dicots have 2. Cotyledons are embryonic leaves that store
starches to support early growth of the seedling. Cotyledons usually fall off after true leaves
become established and start photosynthesizing. Other structural differences are indicated
below:
Number of cotyledons in seed/seedling
Pattern of venation in leaves
Arrangement of vascular bundles in stem
Flower parts in multiples of
Number of pores/furrows in pollen
Monocots
1
parallel
scattered
3
1
Dicots
2
netted
ring
4, 5
3
Some of the differences between monocots and dicots will become apparent to you as you
examine the gross structure and tissue arrangements of the roots, stems, and leaves provided.
The pictures below represent cross sections of monocot and dicot roots and stems. Note the
arrangement of vascular tissues in each.
MONOCOT
ROOTS:
STEMS:
DICOT
PLANTS HAVE 3 ORGAN SYSTEMS: ROOTS, STEMS, LEAVES
Despite the dramatic difference in appearance of a massive oak compared to a single duckweed
plant, the body plan of plants consists of just 3 organ systems – roots, stems, and leaves. Other
anatomical features, including fruits, flowers, and cones are really just modifications of one of
these three organs. The basic structure and function of each is considered below.
1) ROOTS
Roots function primarily to anchor the plant to the substrate and to absorb water and dissolved
minerals and ions. Some roots are further modified for storage of carbohydrates, such as
starch, that can later be used by the plant as food. There are two basic kinds of root systems
in angiosperms: taproot and fibrous root systems.
Tap root system. Tap roots are found in most dicots. They are distinguished by having a single
primary (usually enlarged) root called the "taproot" which derives from the stem. From this
primary root several other secondary, or lateral roots emerge.
Fibrous root system. In a fibrous root system, typical of monocots, there is no primary root.
Instead, all the roots derive from the stem, are approximately equal in size, and tend to occupy
shallower layers of soil. Fibrous root systems are especially well-suited for anchoring the plant
in unstable substrates.
Root Modifications:
 Adventitious and prop roots (e.g., maize): These roots arise higher on the stem to
assist in anchorage and support.
 Pneumatophores (e.g.,mangroves): The uneven particle size of soil naturally creates gaps
and pockets that allow for gas exchange in the roots of most plants. However, plants that
grow in submerged or aquatic habitats must cope with very low or no oxygen at the root
surface. Pneumatophores are roots which grow upwards, above the surface of water for the
purpose of aerating root cells. Why do roots need oxygen?
 Velamen roots (e.g., orchids): Epiphytes (such as orchids and bromeliads) grow upon the
tops of other plants in a forest canopy - their roots never touch the ground. As a result,
water is not taken up from the soil, but must be absorbed from the humid environment and
fiercely conserved. The multiple layers of epidermis in velamen roots reduce water loss and
aid in absorption of water.
 Fleshy roots (e.g., beet, carrot, radish, and sweet potatoes): Fleshy roots are especially
well adapted for the storage of food materials, such as starches.
2) STEMS
Stems function primarily as support for leaves and for the conduction of food and water
between roots and leaves. Stems are divided into nodes (the part of the stem to which leaves
are attached) and internodes (the region between nodes). As the stem elongates from the
apical meristem, leaf and bud primordia arise which define the node. Leaf primordia mature into
leaves, bud primordia develop into buds which give rise to new branches of stem.
Stem Modifications:
 Tendrils (e.g., grape): Tendrils are modifications of shoot (of stem or leaf origin) that
aid in the support of the main stem by wrapping onto and "climbing" existing structures.
 Cladophylls (e.g., Asparagus, Epiphyllum): Although they often resemble leaves,
cladophylls are actually short branches that are modified for photosynthesis.
 Thorns (e.g., Bouganvillia, hawthorn): Modified branches on the stem that serve as
defensive structures.
 Storage stems (e.g., potato tubers, onion bulbs, gladiolus corms, rhizomes in iris,
ginger): These stems are specialized for food storage and typically have high levels of
starches.
 Runners/Stolons (e.g., strawberry, grasses, spiderplant): Stolons and runners are stems
that grow horizontally along the substrate and facilitate vegetative propagation. Clones of
the original parent plant arise at nodes along the runner.
3) LEAVES
The external morphology of leaves is highly variable, but the primary function of leaves is
usually photosynthesis. The leaf generally consists of two portions: the blade (the flattish,
expanded part of the leaf) and the petiole (the stem-like appendage that attaches the leaf to
the stem). Leaves that lack a petiole are referred to as sessile.
Leaves can be described as simple or compound. Blades of compound leaves are sub-divided into
leaflets. Sometimes leaves are so highly modified it is difficult to determine if they are a leaf,
leaflet, or neither! Two good indicators of a leaf are:
1) buds are found in the axis of a leaf, and
2) leaves extend from the stem in various planes while leaflets all lay on the same plane.
Stomata (pores, singular stomate) occur on the upper and lower surfaces of the leaf to
accommodate gas exchange. The size of the opening of a stomate is regulated by guard cells
which contract and expand with changing conditions of humidity and/or solute concentration.
Surfaces of leaves are often covered with fuzz or hairs called trichomes which help reduce
water loss and sometimes play a role in defense.
Leaf Modifications:
 Spines (e.g., cacti): Leaves modified for physical defense and for water conservation.
 Bracts (e.g., Bouganvillea, bird of paradise, poinsettia): Bracts tend to resemble flower
petals and are used primarily for attracting pollinators or dispersers.
 Carnivorous leaves (e.g., sundew, pitcher plant, Venus flytrap): Leaves modified for the
capture and digestion of insects - often found in nitrogen-poor habitats, such as bogs.
 Tendrils (e.g., peas): Tendrils are modifications of shoot (of stem or leaf origin) that
aid in the support of the main stem by wrapping onto and "climbing" existing structures.
 Plantlets (e.g., Kalanchoe): Plantlets are simply small clones of plants that arise from
leaf notches as a means of vegetative propagation.
PLANT ORGANS ARE CONSTRUCTED FROM 3 TYPES OF TISSUES
Although the architecture varies considerably among roots, stems, and leaves, all plant organs
are comprised of 3 basic tissue types: dermal tissue, vascular tissue, and ground tissue. A
tissue is simply an assemblage of cells that work together as a structural or functional unit. In
this lab, we will examine the general arrangements of those tissues among different plant
organs. You will find that the distribution of tissues varies somewhat among different plants as
well as among different organ systems. For example, one of the key differences between
monocots and dicots has to do with the arrangement of vascular tissues in leaves and stems. In
next week’s lab, we will continue our examination of plant tissues by exploring some of the
different cell types that form these three plant tissue sytems.
1) DERMAL TISSUE
The dermal tissue system is derived from the protoderm* and consists of 2 tissue types:
 Periderm. Periderm includes cork and cork-producing tissues and is only found in plants
with secondary growth.
 Epidermis. The epidermis constitutes the outmost layer of cells of roots, stems and
leaves. Epidermal cells are usually covered by a waxy layer, called a cuticle that serves for
waterproofing the cells.
2) VASCULAR TISSUE
Vascular tissues, derived from the procambium*, are the tissues responsible for transporting
substances from one point in the plant body to another. As is the case with animals, the vascular
tissues are responsible for the movement of dissolved gases, sugars, and minerals throughout
the organism.
In plants, there are 2 types of conducting tissues:
 Xylem. Xylem is the tissue specialized for the conduction water and dissolved minerals.
Cells of the xylem are dead at maturity and act as the structural medium for the movement
of water. Individual xylem cells have multiple holes in the walls to allow for the passage of
water between adjacent cells. The bulk of wood consists of xylem tissue.
 Phloem. Phloem conducts dissolved sugars throughout the plant. The main conducting
cells of phloem, called “sieve elements”, are alive at maturity, but are highly modified for
carrying out the function of transport. For example, many of the organelles you might
expect to find in a living plant cell, such as nuclei, ribosomes, Golgi apparati, cytoskeleton,
and vacuoles are absent from the sieve elements in order to make more space for fluid
transport. The small amount of protoplast in sieve elements is distributed in a thin layer
along the periphery of the cell wall. In some cases, you may be able to observe smaller cells
called "companion cells" distributed among the sieve elements. Companion cells contain all
the organelles you would expect to find in a typical plant cell and carry out many of the
necessary life functions for the sieve elements which lack organelles.
3) GROUND TISSUE
Derived from the ground meristem*, ground tissues are distributed between and among the
dermal and vascular tissues of roots, stems, and leaves. Ground tissues include a wide variety
of tissue types that perform a multitude of functions within the plant. Some of the more
familiar ground tissues include the following:
 Cortex – the ground tissue distributed between the epidermis and vascular tissues in
roots and stems. Cortex is usually modified for storage, especially of starches. In
herbaceous green stems, cortex often contains chloroplasts and thus is further modified
for carrying out photosynthesis.
 Pith – like cortex, pith is generally modified for starch storage, but is only found to the
interior of the vascular tissues in stems and roots.
 Mesophyll - the ground tissue of leaves found between the two layers of epidermis.
Leaf mesophyll is the tissue most adapted for photosynthesis and thus is generally rich with
chloroplasts.
In addition to the functions indicated above, ground tissues also have an important role in wound
healing and the regeneration of damaged or dead cells throughout the plant.
PLANT TISSUES ARE DERIVED FROM MERISTEMS
Unlike animals, most plants grow (increase the number and size of their cells) throughout their
lifetime. This ability to grow indeterminately is due to a special form of tissue known as a
“meristem”. A meristem is a region of tissue that is perpetually embryonic – that is, it is
relatively undifferentiated and divides continuously, giving rise to all new cells and tissues that
make up the plant body. Can you think of an analogy to the plant’s meristem in animals/humans?
There are several meristems distributed throughout the plant body which are named for their
location and/or for the tissue systems that are derived from them.
1) APICAL (PRIMARY) MERISTEMS are located in the tips (or, apex) of the growing roots and
shoots of all plants. Primary meristems produce primary tissues and account for all the primary
growth of plants. Primary growth is growth that contributes to elongation of the plant (height
and root lengthening) and for the establishment of its basic organ structure (roots, stems,
leaves).
2) LATERAL (SECONDARY) MERISTEMS are organized as concentric cylinders (think of a can
inside a can inside a can) within the plant body and are responsible for secondary growth.
Secondary growth contributes to the thickening of a plant structure, or increases in girth.
With few exceptions, lateral meristems tend to be present only in woody plants, such as trees,
shrubs, and some vines.
As cells in the apical meristem divide and give rise to new cells, the tips of roots and shoots
extend outward, away from the central plant body. As a consequence of this elongation due to
the addition of new cells, the slightly older meristematic cells become distributed into
different regions within the root and shoot. These slightly more mature cells still retain their
meristematic properties, but are somewhat differentiated from the true apical meristem.
These regions of meristematic tissue derived from the apical meristem are referred to as
“primary meristems” and are named for the tissues that they will give rise to: A) Protoderm gives rise to dermal tissues, B) Ground Meristem - gives rise to ground tissues, and C)
Procambium - gives rise to vascular tissue (aka, "cambial tissue"). The organization of primary
meristems also varies among plant types and organs.
ACTIVITIES:
1. Examine the array of modified roots, leaves and stems that have been provided for you in
lab. Discuss with your group and instructor the environment that each plant occurs in and be
able to answer the following:
*What features of the modified organ make it uniquely adapted for its natural environment?
*Would these modifications be well-suited for a wide variety of habitats?
*Are there any disadvantages associated with these modifications? (think of the primary
functions of stems, roots, and leaves – does the modification interfere with any of the primary
functions?)
*How can you determine if a structure is modification of root, leaf or stem?
2. Roots, Stems, Leaves: a) Examine prepared cross sections of roots, stems and leaves.
Compare the cross sections of a monocot with a dicot for each organ type. How does the
arrangement of tissues compare between dicots and monocots for a given organ (dermal, ground,
vascular)? How does the arrangement of tissues compare between different organs of the same
plant (think of transitions from one part of the plant to another)?
3. Meristems, Shoots: Observe a prepared slide of the growing tip of a shoot and locate the
apical meristem. How does the size of cells vary from the tip of the meristem to regions
further down the stem? Where are the youngest cells located?
Locate the bud and leaf primordia that define the nodal region of a stem. Are the nodes equally
spaced? Why or why not? Can you find the trace of vascular tissue? Can you locate apical
meristems on the live plants in lab? What is the relationship between what you observed under
the microscope to what you observed in the live plant? Can you construct an explanation for how
a plant increases in height based on your observations?
POSTLAB QUESTIONS
1. Choose 3 root, stem, or leaf modifications you see on the plants provided in lab, and answer
the following for each:
a)What features of the modified organ you chose make it uniquely adapted for its natural
environment? i.e., describe the specific structural or anatomical features you see that confer
an advantage to the plant in its native habitat.
b)Would this modification be well-suited to a wide variety of habitats, or is it fairly
restrictive? Explain (i.e., are there disadvantages associated with this modification in other,
non-native habitats?).
2. Using prepared slides or other materials provided, draw cross sections of monocot and dicot
roots, stems, and leaves. Pair the monocot and dicot drawings of a given organ next to each
other in your notebook. Label the 3 basic tissues (dermal, vascular, ground) and any other
structures that you can. To the side of your pair of drawings, note the similarities and
differences in the arrangement of tissues.
3. Explain (or draw/diagram) how a plant grows in height (elongation, or primary growth) based
on what you learned about meristems in today’s lab.