Plant Tissue Systems
Plant tissues are classified into three systems. Each tissue includes
a variety of specialized cell types that enable them to carry out
specific functions within the plant body. Some cells are specialized
to perform photosynthesis just below the surface of the leaves,
whereas others are specialized to absorb water. Some specialized
plant cells even act as doors, literally opening and closing to control
the entry and exit of gasses. Just as in animals, these specialized
cells develop from unspecialized cells during the process of cellular
differentiation.
CELLULAR DIFFERENTIATION AND SPECIALIZATION
When a seed starts to grow, the cells divide very quickly. As the
seed develops into an embryo, many of the cells start to differentiate
into specific tissues. The regions of growth are located at the tips of
the plant’s roots and shoots. Plants with woody stems, such as
trees, also have a region of growth just below the surface of their
stems. Plant growth occurs because undifferentiated cells are
actively growing and dividing in these regions. As these new cells
mature, some of them specialize and develop different features
according to their location and future function. For example, cells in
leaves are very different from those in the roots and stem of the
plant.
MERISTEMATIC CELLS
Just as animals contain some unspecialized cells called stem cells,
plants have unspecialized cells that are called meristematic cells.
The meristematic cells in the root tip divide and differentiate into
specialized tissues in the roots. Similarly, the tissues of new stalks,
leaves, and flowers arise from meristem regions in the shoot. As
cells in the meristem regions continue to divide, some of the new
cells always remain in the undifferentiated state.
TISSUE SYSTEMS IN PLANTS
Plants have three major tissue systems: dermal, ground, and
vascular. Each of these three tissue systems contains different
types of specialized cells. The tissues of the three systems are found
in all parts of the body and have a distinct arrangement.
The dermal tissues cover the entire outer surface of the plant but do
not occur within the plant.
The vascular tissues are found within every root, shoot, and leaf of
the plant. The vascular tissue system is continuous, so all plants are
joined together by vascular tissues.
The ground tissues account for all other internal tissues in the root,
stem, and leaves.
DERMAL TISSUE SYSTEM
The dermal tissue system forms the outermost layer of a plant. The
dermal tissue system includes both epidermal and periderm tissues.
Epidermal tissue is a thin layer of cells that covers the surfaces of
leaves, stems, and roots. In woody plants, the epidermal tissue is
replaced by periderm tissue that forms bark on stems and large
roots.
The cells of the dermal tissue system are specialized to perform a
wide variety of unique functions. Some epidermal root cells have
long extensions called root hairs to help absorb water and minerals
from the surrounding soil. Most epidermal leaf cells produce a layer
of wax, called the cuticle, that helps waterproof the leaf’s surface.
Some epidermal leaf cells are adapted for defence: they have
hairlike structures that contain chemical irritants.
VASCULAR TISSUE SYSTEM
A plant must obtain from the soil all the water and minerals that it
needs for growth. These substances are absorbed by the plant’s
roots. The water and nutrients then have to be distributed to all of
the plant’s cells. In addition, plants must be able to distribute the
sugars and other chemicals produced during photosynthesis to all of
its parts.
The plant’s vascular tissue system is the transportation system that
moves water, minerals, and other chemicals around the plant. It is
like a network of tubes that reaches from the roots, up the stalk, and
through the leaves. As the plant grows, new vascular tissues
differentiate in the growing tips of the plant. The new tissues are
lined up with existing vascular tissues, maintaining the tubes’
connections throughout the plant.
There are two types of vascular tissue: xylem and phloem
Xylem is made up of elongated cells. Xylem transports water and
dissolved minerals upward from the roots. This solution travels
along an interconnected network to the stems and leaves. Once
they are fully formed, xylem cells are little more than hollow tubes
with rigid walls. They have no cytoplasm, nucleus, or other
organelles. This allows water to move easily through the tubes.
Mature xylem cells are no longer living tissue.
Phloem is a specialized tissue that transports solutions of sugars
produced by photosynthesis. Phloem also transports other
dissolved nutrients and hormones throughout the plant. Under
certain conditions, food materials are transported downward from
the photosynthesizing leaves to the stem and roots. Under other
conditions, the food materials are carried upward from the root and
stem to the leaves. Phloem is made up of elongated cells that, unlike
xylem cells, are alive when mature and functioning.
The arrangement of vascular tissues within the stems of plants
differs dramatically between woody and non-woody plants. In nonwoody plants, the vascular tissues are arranged in bundles
accompanied by ground tissues.
GROUND TISSUE SYSTEM
Most of the cells of young plants are ground tissue cells. Ground
tissues are the filler between the dermal and vascular tissues.
Ground tissues perform a variety of functions: in the green part of
the plant, they manufacture nutrients by the process of
photosynthesis; in the roots, they store carbohydrates; and in the
stems, they provide storage and support.
HOMEWORK
1. What is the purpose of cell division in plants?
2. What are the main functions of the three plan tissue systems?
3. Explain why the movement of water and minerals in xylem is
always upward.
4. Compare the structure and function of xylem and phloem.
Plant Tissues Working Together
The leaves of most plants are highly specialized structures with one
primary function: performing photosynthesis. The leaf uses light
energy, carbon dioxide, and water to produce glucose and oxygen.
Glucose can then be converted into other carbohydrates, including
complex sugars, in the leaf. These carbohydrates in the form of
starch are used as a source of stored chemical energy.
Alternatively, the plant can use them as the basic building blocks for
all of the complex chemicals and structures that it needs to make.
Sugar is a valuable product needed by all parts of the plant. The
plant therefore has to have some system to transfer sugars from the
leaf to the rest of the plant. Both glucose and oxygen are required in
all plant cells for cellular respiration. However, leaves produce more
oxygen than the plant needs so excess oxygen is released as a waste
product.
ABSORBING LIGHT
Wide, thin leaves have a much greater surface area than thick,
narrow leaves. This enables them to absorb more light and accounts
for why this leaf shape is extremely common. The green colour is
produced by chlorophyll – the pigment that actually absorbs light to
begin the process of photosynthesis.
Chlorophyll is contained in cell organelles called chloroplasts. It is
these organelles that actually conduct the chemical process of
photosynthesis. In a typical leaf the chloroplasts are located mostly
in the palisade layer and the spongy mesophyll. The palisade cells
are located where there is maximum amount of light: just below the
leaf’s upper surface. Spongy mesophyll cells are located throughout
the interior of the leaf. The palisade and spongy mesophyll cells are
part of the ground tissue system.
OBTAINING CARBON DIOXIDE
The entire upper and lower leaf surfaces are covered by a layer of
epidermal tissue. The cells that make up this tissue produce a thin
layer of wax called the cuticle. This waxy layer keeps the leaf from
drying out. This same cuticle also prevents gasses from entering the
leaf by direct diffusion through the surface cells.
Instead, gasses enter and exit through special openings in the leaf
surface called stomata. These openings are surrounded and
controlled by pairs of special epidermal guard cells. The guard cells
can either bend outward, causing stomata to open, or they can
collapse inward, causing stomata to close. The majority of plants
have most or all of their stomata in the lower surface of the leaf. This
location reduces water loss, provides more surface area for
photosynthesis, and reduces the chances of airborne viruses,
bacteria, and fungal spores from entering the leaf. Once carbon
dioxide enters the leaf through the stomata, it can move around
through the air spaces in the spongy mesophyll. This enables
carbon dioxide to more easily reach the photosynthetic cells.
CONTROLLING STOMATA
When stomata are open, carbon dioxide can enter the leaf and
oxygen can escape. This helps the plant photosynthesize. Ideally,
plants would open their stomata whenever it was sunny. However,
when stomata are open, water vapour can also escape. A very thin
leaf could dry out and die very quickly on a sunny or windy day.
Preventing too much water loss is therefore a major concern for
many plants.
Guard cells are an adaptation to help the plant conserve water by
altering their shape in response to water levels in the leaf. If there is
a good supply of water within the leaf, the guard cells expand and
bend apart – opening the stomata. If there is a shortage of water, the
guard cells become soft and collapse – closing the stomata. Guard
cells also have a mechanism that responds to light levels. This lets
them close the stomata at night when carbon dioxide is not needed
because there is no light for photosynthesis.
OBTAINING WATER
Vascular tissues made up of xylem and phloem are arranged as
bundles and run through the plant from root to leaf. Long, thin
epidermal cells on the roots, called root hairs, grow into the
surrounding soil and absorb water. The root hairs greatly increase
the surface available for water absorption, allowing osmosis to
happen very quickly. The water is then transported by xylem from
the roots, up the stem, and through the leaves.
HOMEWORK
1. Explain how each region of the leaf contributes to the task of
photosynthesis.
2. In what ways do the cuticle and guard cells perform the same
function? In what ways do their roles differ?