Download Plant Tissue Jigsaw Articles

A Companion to Plant Physiology, Fifth Edition by Lincoln Taiz and Eduardo Zeiger,
http://5e.plantphys.net/article.php?ch=1&id=19
Topic 1.4: Plant Tissue Systems: Dermal, Ground, and Vascular
Dermal Tissue
The epidermis is the dermal tissue of young plants undergoing primary growth (see textbook
Figure 1.3). It is generally composed of specialized, flattened polygonal cells that occur on all
plant surfaces. Shoot surfaces are usually coated with a waxy cuticle to prevent water loss and
are often covered with hairs, or trichomes, which are epidermal cell extensions.
Pairs of specialized epidermal cells, the guard cells, are found surrounding microscopic pores
in all leaves (see textbook Figure 1.3A). The guard cells and pores are called stomata (singular
stoma), and they permit gas exchange (water loss, CO2 uptake, and O2 release or uptake)
between the atmosphere and the interior of the leaf.
The root epidermis is adapted for absorption of water and minerals, and its outer wall surface
typically does not have a waxy cuticle. Extensions from the root epidermal cells, the root hairs ,
increase the surface area over which absorption can take place (see textbook Figure 1.3C).
Ground tissue. Making up the bulk of the plant are cells termed the ground tissue. There are
three types of ground tissue: parenchyma, collenchyma, and sclerenchyma.
• Parenchyma, the most abundant ground tissue, consists of thin-walled, metabolically active
cells that carry out a variety of functions in the plant, including photosynthesis and
storage (see textbook Figure 1.3B)
• Collenchyma tissue is composed of narrow, elongated cells with thick primary walls (see
textbook Figure 1.3C). Collenchyma cells provide structural support to the growing plant
body, particularly shoots, and their thickened walls are nonlignified, so they can stretch
as the organ elongates. Collenchyma cells are typically arranged in bundles or layers
near the periphery of stems or leaf petioles.
• Sclerenchyma consists of two types of cells, sclereids and fibers (see textbook Figure 1.3D)
Both have thick secondary walls and are frequently dead at maturity. Sclereids occur in a
variety of shapes, ranging from roughly spherical to branched, and are widely distributed
throughout the plant. In contrast, fibers are narrow, elongated cells that are commonly
associated with vascular tissues. The main function of sclerenchyma is to provide
mechanical support, particularly to parts of the plant that are no longer elongating.
In the stem, the pith and the cortex make up the ground tissue (see textbook Figure 1.1B). The
pith is located within the cylinder of vascular tissue, where it often exhibits a spongy texture
because of the presence of large intercellular air spaces. If the growth of the pith fails to keep up
with that of the surrounding tissues, the pith may degenerate, producing a hollow stem. In
general, roots lack piths, although there are exceptions to this rule. In contrast, the cortex ,
which is located between the epidermis and the vascular cylinder, is present in both stems and
roots (see textbook Figure 1.1B and C).
At the boundary between the ground tissue and the vascular tissue in roots, and occasionally in
stems, is a specialized layer of co rtex known as the endodermis (see textbook Figure 1.1C).
This single layer of cells originates from cortical tissue at the innermost layer of the root cortex
and forms a cylinder that surrounds the central vascular tissue, or stele. Early in root
development, a narrow band composed of the waxy substance suberin is formed in the cell
walls circumscribing each endodermal cell (see textbook Figure 1.1). These suberin deposits,
called Casparian strips, form a barrier in the endodermal walls to the intercellular movement of
water, ions, and other water-soluble solutes to the vascular cells.
Leaves have two interior layers of ground tissue that are collectively known as the mesophyll
(see textbook Figure 1.1A). The palisade parenchyma consists of closely spaced, columnar
cells located beneath the upper epidermis. There is usually one layer of palisade parenchyma in
the leaf. Palisade parenchyma cells are rich in chloroplasts and are a primary site of
photosynthesis in the leaf. Below the palisade parenchyma are i rregularly shaped, widely
spaced spongy mesophyll cells. The spongy mesophyll cells are also photosynthetic, and the
large spaces between these cells allow diffusion of carbon dioxide. The spongy mesophyll also
contributes to leaf flexibility in the wind, and this flexibility facilitates the movement of gases
within the leaf.
Vascular tissues: xylem and phloem. The vascular tissue is composed of two major
conducting systems: the xylem and the phloem. The xylem transports water and mineral ions
from the root to the rest of the plant. The phloem distributes the products of photosynthesis and
a variety of other solutes throughout the plant (see textbook Figure 1.1B and C).
The tracheids and vessel elements are the conducting cells of the xylem (see textbook Figure
1.3E). Both of these cell types have elaborate secondary-wall thickenings and lose their
cytoplasm at maturity; that is, they are dead when functional. Tracheids overlap each other,
whereas vessel elements have open end walls and are arranged end to end to form a larger unit
called a vessel. Other cell types present in the xylem include parenchyma cells, which are
important for the storage of energy-rich molecules and phenolic compounds, and sclerenchyma
fibers.
The sieve elements and sieve cells are responsible for sugar translocation in the phloem (see
textbook Figure 1.3E). The former are found in angiosperms; the latter perform the same
function in gymnosperms. Like vessel elements, sieve elements are often stacked in vertical
rows, forming larger units called sieve tubes, whereas sieve cells form overlapping arrays. Both
types of conducting cells are living when functional, but they lack nuclei and central vacuoles
and have relatively few cytoplasmic organelles.
Substances are translocated from sieve cell to sieve cell laterally through circular or oval zones
containing enlarged pores, called sieve areas. In contrast, sieve tubes translocate substances
through large pores in the end walls of the sieve elements, called sieve plates. Sugar
movement through sieve tubes is more efficient and rapid than through sieve cells and
represents a more evolutionarily advanced mechanism.
Sieve elements are associated with, and depend on, densely cytoplasmic parenchyma cells
called companion cells. The analogous cells adjacent to the sieve cells of gymnosperms are
called albuminous cells. Companion cells provide proteins and metabolites necessary for the
functions of the sieve tube elements. In addition, the phloem frequently contains storage
parenchyma and fibers that provide mechanical support.
Ground Tissue in Plants: Function, System & Definition, Sarah Fried, http://education­
portal.com/academy/lesson/ground-tissue-in-plants-function-system-definition.html#lesson
How do plants support and feed themselves? Ground tissue is important in plants because it is
responsible for creating food from sunlight, as well helping plants grow longer and stand upright.
Tissue Systems And Ground Tissue Defined
Within a plant, there are three tissue systems, each serving important functions for the plant. If
we start from the outside of the plant, the first tissue system you would encounter is called the
dermal tissue system. This tissue is much like your skin, forming the first line of defense
against physical damage and infection from the outside world.
In the very center of the plant, we find the vascular tissue system. This tissue provides
support, but it also creates a highway of long-distance transport between the roots and other
parts of the plants. Both water and nutrients are transported through the vascular tissue.
The third system is the ground tissue system. This tissue accounts for most of the bulk of
the plant and fills the spaces in between the dermal and vascular tissues. The ground tissue
has a variety of functions depending on what type of ground tissue it is.
Function
There are three types of ground tissue, and each one has a specific function or set of
functions for the plant.
Parenchyma is a very versatile type of ground tissue, and it is responsible for
photosynthesis (how a plant makes food from sunlight) and food storage. Parenchyma cells
are also responsible for healing in the plant - this tissue can go through cell division and
regenerate when needed. You are likely familiar with parenchyma cells, because this is what
the pulp in fruit is comprised of.
Collenchyma is ground tissue that provides structural support to growing parts of the plant.
Like parenchyma cells, collenchyma cells also go through cell division and therefore elongate
parts of the plant such as stems and leaves. Collenchyma cells are long and thin and have
very thick cells walls. This structure makes them very well suited to support the parts of the
plants that are growing and have not become established yet. The thick, rigid structure of
celery is due to collenchyma tissue.
Cross section of collenchyma cells (flip over)
Sclerenchyma is the ground tissue that provides structural support to the parts of the plant
that are no longer growing. There are two types of sclerenchyma cells: fibers and sclereids.
Fibers are long and thin and are responsible for many materials you use on a daily basis, such
as clothing fabrics and rope. Sclereids are compact and dense, and they are what makes up
that tough texture in apple cores.
Summary
The ground tissue system is important because it serves a variety of essential functions for
plants. Each type of ground tissue has its role, such as food creation and storage or support
during and after growth. Filling all the spaces that are not used by the dermal and vascular
tissue systems, ground tissue can be found throughout the plant.
What is Dermal Tissue? http://www.wisegeek.com/what-is-dermal-tissue.htm
Living organisms have an outside "container" that serves to protect the contents of the
organism. In bacteria, the cell wall protects the bacteria's internal structures, such as the
ribosomes and nucleoid, as well as helping keep the bacteria's shape, either spherical, rodshaped or spiral with the exception of mycoplasma bacteria, which do not have cell walls. The
skin, which consists of the outer epidermis and the underlying dermis, helps protect the human
body from being damaged physically, helps protect the body from bacterial and viral infections
and helps protect the body from damage from exposure to ultraviolet rays. Dermal tissue is the
outside layer of a plant, with the exception of woody trees and shrubs, which are covered with
bark for protection.
Plants have two organ systems. These systems are the shoot system, which is the part of the
plant that lives above ground, such as stems and leaves, and the root system. The root system
is the part of the plant that grows below the ground, including roots and tubers. In addition,
plants are divided into three different tissue groups, vascular tissue, ground tissue and the
dermal tissue.
Vascular tissue helps support the plant. In addition, vascular tissue distributes water, minerals,
and food products from photosynthesis throughout the plant. Photosynthesis is the process that
plants use to convert sunlight to carbohydrates for food. The two main components of vascular
tissue are the xylem, which distributes water and minerals throughout the plant from the plant's
roots, and phloem, which transports food through the plant.
Ground tissue is the tissue between the vascular tissue and the dermal tissue. In addition to
being involved in photosynthesis, ground tissue helps provide the plant with support. This tissue,
which consists mainly of parenchyma cells, also has the ability to store food and water.
Dermal tissue is the "outside" or outer part of a plant, which operates to control water and gas
exchanges from the plant to the environment outside of the plant. The outside of the dermal
tissue is called the epidermis. Epidermis produces a waxy layer called the cuticle which helps
keep the plant from losing water. Another part of the epidermis that helps prevent water loss is
the epidermal hair. Root hairs increase water intake into the plant while glandular hairs contain
substances that the plants use to repel harmful insects.
Anatomy Of Plants, http://www.biologyreference.com/A-Ar/Anatomy-of-Plants.html
Dermal Tissue
Dermal tissue makes up the outer layers of the plant and contains epidermal cells that secrete
and are coated with a waxy layer. This waxy coating, the cuticle, prevents excessive water loss
from the plant. While the dermal tissue primarily serves a protective role, it also has a variety of
other specialized functions depending on the particular organ where it is located.
In leaves, dermal tissue contains specialized cells called guard cells that make up structures
called stomata . Stomata facilitate the exchange of gases in the leaf. Carbon dioxide (CO 2 )
diffuses into the leaf through the stomata for use in photosynthesis, and oxygen (O 2 ), the waste
product of photosynthesis, diffuses out of the leaf through stomata. Stomata are also crucial for
water transport through the xylem . Stomatal opening results in the evaporation of water from
the air spaces of the leaf. This creates negative water pressure that pulls on the column of water
in the xylem. The evaporation of water from the stomata is the main driving force for water
transport through the water. In roots, epidermal cells have a specialized structure that facilitates
water and nutrient absorption, the main function of the root. Some of the root epidermal cells
have long membranous extensions called root hairs that increase the absorptive surface area of
the root. Root epidermis also interacts with symbiotic fungi that form mycorrhizae , which
increase nutrient absorption.
Ground Tissue
Many different functions are performed by ground tissue including photosynthesis, storage, and
support. Ground tissue makes up the majority of the plant structure and is composed of three
cell types: parenchyma, collenchyma, and sclerenchyma cells.
Parenchyma cells are the least specialized cells in a plant. These cells are responsible for the
production and storage of nutrients. Photosynthesis occurs in the chloroplasts of parenchyma
cells in leaves. Parenchyma cells in stems, roots, and fruits have structures that store starch.
Most developing plant cells are structurally similar to parenchyma cells. During their
differentiation, they become specialized in form and function and lose the potential to divide.
Mature parenchyma cells do not usually divide, but they retain the ability to divide and
differentiate into different cell and tissue types in the event of an injury to the plant.
Transverse section of tissues of a dicot root. Redrawn from Van de Graaff et al., 1994.
Collenchyma and sclerenchyma cells provide structural support for the plant. Collenchyma cells
have thick, yet pliable, cell walls. These cells give structural support to newly formed portions of
a plant without restricting growth. Collenchyma cells are stacked end on end and are oriented in
strands just beneath the epidermis of the young structure. The relatively soft cell wall allows the
collenchyma cells to elongate as the structure grows.
On the other hand, sclerenchyma cells provide support to mature plant structures. Like
collenchyma cells, they have very thick cell walls. However, the cell walls of sclerenchyma cells
contain lignin , a molecule that makes the cell wall hard. This provides strength to the cell wall,
but restricts the ability of the cells to elongate and grow. Since a sclerenchyma cell functions
solely to provide structural support, many sclerenchyma cells are actually dead at functional
maturity. The cell membrane, protoplasm (cytoplasm) and organelles are gone, leaving only
the rigid cell wall that serves as a scaffolding system for that structure.
Vascular Tissue
Vascular tissues make up the organs that transport water, minerals , and food throughout the
plant. Vascular tissue can be divided into two functional units. Xylem transports water and
minerals from root to shoot. phloem transports nutrients (such as sugar and amino acids ) from
leaves and other production sites to roots, flowers, stems, and other tissues that need them.
The cells that make up vascular tissue are unique in their structure. Their specialized
characteristics allow them to transport material through the plant efficiently while providing
structural support to the plant.
Xylem tissue contains two types of cells: tracheids and vessel elements. Like sclerenchyma,
both of these cell types are dead at functional maturity and therefore lack protoplasm. Tracheids
are long, thin cells that have tapered ends. They overlap on another, and water passes from
tracheid to tracheid via small pores. Vessel elements are shorter and are stacked end to end,
forming more of a tube structure. Water flows in the tube by passing through perforated end
walls between cells.
A photograph of a maidenhair fern, showing its shoot system of stems and
leaves.
Phloem tissue is made up of two different types of cells: sieve tube members and companion
cells. Sieve tube members are the main conducting cells, and are named for the sievelike areas
along their cell walls through which the phloem sap moves from cell to cell. Unlike cells of the
xylem, sieve tube members are alive at functional maturity, but do not have nuclei. For this
reason, companion cells are closely associated with sieve tube members. These cells do have
nuclei and serve to support the sieve tube members. The cytoplasm of sieve tube members
and companion cells is connected through numerous pores called plasmodesmata. These pores
allow the companion cells to regulate the content and activity of the sieve tube member's
cytoplasm. Moreover, the companion cells help to load the sieve tube members with sugar and
the other metabolic products that they transport throughout the plant.
Stomata
http://www.eoearth.org/view/article/156262/
Published: August 3, 2010, 6:53 pm
Updated: September 28, 2012, 7:59 pm
Author: Debbie Swarthout
Contributing Author: C Michael Hogan
Topic Editor: Daniel Robert Taub
Stomata are minute aperture structures on plants found typically on the outer leaf skin layer, also known
as the epidermis. They consist of two specialized cells, called guard cells that surround a tiny pore called
a stoma. The word stomata means mouth in Greek because they allow communication between the
internal and external environments of the plant. Their main function is to allow gases such as carbon
dioxide, water vapor and oxygen to move rapidly into and out of the leaf.
Stomata are found on all above-ground parts of plants, including the petals of flowers, petioles, soft
herbaceous stems and leaves. They are formed during the initial stages of the development of these
various plant organs and therefore reflect the environmental conditions under which they grew.
Figure 1: Stomata of Tall fescue grasses.
Stomatal density, size and shape
Stomatal density refers to the number of stomata per square millimeter. Typical densities can vary from
100 to 1000 depending on the plant species and the environmental conditions during development. More
stomata are made on plant surfaces under higher light, lower atmospheric carbon dioxide concentrations
and moist environments. Grasses typically have lower stomatal densities than deciduous trees. The size
and shape of stomata also vary with different plant species and environmental conditions. For example,
grasses have guard cells that resemble slender dumbbells whereas trees and shrubs have guard cells
that resemble kidney beans.
Physiological function of stomata
Leaves are the main "food manufacturing" organs of plants. They make food from carbon dioxide and
water in the presence of light during a process called photosynthesis. As stomata open in the presence
of sunlight, carbon dioxide will diffuse into the leaf as it is converted to sugars through photosynthesis
inside the leaf. At the same time, water vapor will exit the leaf along a diffusive gradient through the
stomata to the surrounding atmosphere through the process of transpiration. Consequently, plants face
the dilemma of taking up carbon dioxide while losing water vapor through their stomata. If this water loss
remains unchecked, they can deplete their water reserve. This depletion can become catastrophic to the
physiological functioning of the plant given that is the most essential solvent in which biochemical and
growth processes occur. Based on Darwinian principles, it is presumed that selective adaptation has
driven plants to acquire characteristics which enable them to grow more rapidly without diminishing the
probability of survival. If plants have not acquired the characteristics to withstand changes in water
availability in their growth environment, plants may exacerbate their water shortage by not regulating the
size of their stomatal apertures in an optimal manner and may fail to survive when water availability
declines.
Optimal size of stomatal apertures
The Optimisation Theory, first proposed by Ian Cowan and Graham Farquhar (1977) suggests that the
gas exchange of a plant is optimal if the plant is maximizing photosynthesis at a given average rate of
transpiration. This ratio of photosynthesis to transpiration defines the instantaneous water-use efficiency
(WUE) of the plant. The WUE of the leaf, when compared to economic principles, can be considered to
be analogous to the interest rate on an invested resource. The invested resource in this case is water
transpired, while the interest is the carbon gained through photosynthesis. The optimal stomatal aperture
size is one in which the interest rate, WUE, is maximized as the environmental conditions change.
Stomatal apertures will typically vary in response to changes in light intensity, saturation deficit of
ambient water vapor and soil moisture availability. As stomatal aperture size changes, rates of
photosynthesis and transpiration will vary because the pore size will provide a corresponding resistance
to the diffusion of CO2 into and H2O out of the leaf. The inverse of this resistance can be calculated as
the conductance to these two gases across a leaf surface.
Air pollutant uptake
Although the physiological function of stomata addresses the control of gas exchange required for plant
metabolism, the gateway to the atmosphere provided by these plant structures allows the free transport
of unintended gases. Air pollutants such as sulfur dioxide, carbon monoxide and oxides of nitrogen may
invade the leaf through stomatal pathways.(Saxe, 1990) These alien gases have only gained significant
worldwide concentrations since the mid-Holocene (and in fact, chiefly since the Industrial Revolution),
when humans initiated massive emissions of these air pollutants as by-products of manufacturing and
combustion. As a result many plant species experience alteration of metabolic function, often including
reduction in growth rates or outright morphological change. Sometimes the effects are complex to
evaluate since unrelated pathogenic effects or other stressors may be operating simultaneously in a
given ecosystem, especially in locales where humans are exerting a robust presence. (Woodwell, 1989)
Plant biologists typically measure stomatal conductance using a specialized instrument called an IRGA
(Infra Red Gas Analyzer).
Figure 2: Leaf chamber attached to Infra red gas analyzer connected to computer console
Concept 6: Vascular Tissues, BioCoach activity,
http://www.phschool.com/science/biology_place/biocoach/plants/vascular.html
Vascular tissue is composed of xylem and phloem, which function in the transport of water and
dissolved substances.
Plant Cells, Tissues, and Tissue Systems,
http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_cells.htm
Vascular Tissue System:
The vascular tissue system is important in transport. The vascular tissue system is composed of the
xylem (transport of water and dissolved minerals) and phloem (transport of food - usually sucrose and
other sugars-, nitrogen containing compounds, and hormones). The xylem and phloem in the primary
plant body are usually closely associated in the form of vascular bundles. In woody plants the xylem
forms the wood of trunks and branches as well as the central core of the roots. The bark of a tree is a
mixture of old, nonfunctional phloem and the young functional phloem (periderm).
Xylem: There are two types of conducting cells in xylem, tracheids and vessel elements. Both have
thick lignified secondary walls and are dead at maturity. These cells create hollow cylinders that have
high tensile strength. Materials moving within the xylem are under tension. Therefor the high tensile
strength of the xylem cells keeps them from collapsing. Transport in the xylem occurs in one direction =
roots->stems->leaves.
Tracheids: long, slender cells with overlapping, tapered ends. Water moves between tracheid cells via the
bordered pits. Bordered pits are thin areas in the cell walls where only primary cell wall material has been
deposited. Tracheids are the more primitive (less specialized) of the two xylem cells. They are found in most
woody, nonflowering plants.
Vessel Elements: short, wide cells arranged end to end. Their end walls are partially or wholly dissolved
allowing them to form long, hollow tubes up to 3 meters long. The larger diameter and lack of end walls allows
vessel elements to transport water more rapidly. Vessel element are evolutionarily more advanced than tracheids.
are one of the major reasons why angiosperms the dominant land plant.
Xylem Fibers and Xylem Parenchyma: Fibers lend support to the woody tissues (especially in plants
with tracheids) while the parenchyma cells function to store metabolites, or function in secretion (resin
ducts and laticifers).
Phloem: Phloem transports dissolved organic material throughout the plant. Transport within the
phloem is from source to sink. This means that the direction of movement of materials within the phloem
may change over time. This movement depends on the time of year and age of the plant. Phloem cells
are alive at maturity, mainly because movement of materials within the phloem requires energy. Also,
the materials moving within the phloem are under pressure, which means that the cell walls of the
phloem cells do not have to have as great a tensile strength.
Sieve Cells: more primitive phloem conducting cells of ferns and conifers. Sieve cells are long and tapered
with overlapping ends. They have sieve areas, fields of pores scattered over their cell wall surface. These
areas allow direct contact between the protoplasts of adjacent cells. The pores are surrounded by callose, a
complex carbohydrate that can block the pore opening after injury. Associated with the sieve cells are
Albuminous Cells that play a role in aiding the movement of materials within the phloem.
Sieve Tube Members: more advance phloem conducting cells of angiosperms. Sieve tube members are
short and wide, and arranged end to end into sieve tubes. The sieve pores are large and are concentrated
along the end walls of adjacent sieve tube members. These specializations allow solutes to move more
rapidly in sieve tube members and sieve cells. At maturity the nuclei in the sieve tube members disintegrates,
the ribosomes disappear, and the tonoplast (vacuole membrane) breaks down. Mitochondria and plastids are
still present. Sieve Tube Members are always associated with Companion Cells which control the metabolism
of the cells. These two cells are connected by numerous plasmodesmata. The companion cells aid in the
movement of materials into and out of the sieve tube members. Sieve tube members also contain P-protein,
which stands for Phloem-protein. This protein is located along the longitudinal walls of the cells. Some sieve
tube members also contain a glucose polymer called callose. Both P-protein and callose are responsible for
sealing wounds in the sieve tubes.