3.3 Vascular Tissue
3.4 Ground Tissue & Cell Types
3_Plant-Tissues-and-Cell-Types
Plant Tissues and Cell Types
Overview
Introduction
Learning Objectives
- List three types of tissues in plants.
- Describe the identifying features of dermal tissue.
- List the most common modifications of dermal tissue.
- List two types of vascular tissues.
- Explain the structure of xylem tracheids and vessels.
- Explain the structure of phloem sieve tube members and companion cells.
- Differentiate between xylem and phloem.
- List the three types of plant cells.
- List the identifying features of parenchyma, collenchyma and sclerenchyma and their modifications.
Key Terms
adventitious root - an above ground root that arises from a plant part other than the radicle of the plant embryo
apical bud - bud formed at the tip of the shoot
apical meristem - meristematic tissue located at the tips of stems and roots; enables a plant to extend in length
axillary bud - bud located in the axil of a leaf, area of the stem where the petiole connects to the stem
bark - the tough, waterproof, outer epidermal layer of cork cells
bulb - modified underground stem that consists of a large bud surrounded by numerous leaf scales
Casparian strip - waxy coating that forces water to cross endodermal plasma membranes before entering the vascular cylinder, instead of moving between endodermal cells
collenchyma cell - elongated plant cell with unevenly thickened walls; provides structural support to the stem and leaves
companion cell - phloem cell that is connected to sieve-tube cells; has large amounts of ribosomes and mitochondria
compound leaf - a leaf in which the leaf blade is subdivided to form leaflets, all attached to the midrib
corm - rounded, fleshy underground stem that contains stored food
cortex - ground tissue found between the vascular tissue and the epidermis in a stem or root
cuticle - waxy covering on the outside of the leaf and stem that prevents the loss of water
dermal tissue - a protective plant tissue covering the outermost part of the plant; controls the gas exchange
endodermis - a layer of cells in the root that forms a selective barrier between the ground tissue and the vascular tissue, allowing water and minerals to enter the root while excluding toxins and pathogens
epidermis - a single layer of cells found in plant dermal tissue; covers and protects underlying tissue
fibrous root system - type of root system in which the roots arise from the base of the stem in a cluster, forming a dense network of roots; found in monocots
ground tissue - plant tissue involved in photosynthesis; provides support, and stores water and sugars
guard cells - paired cells on either side of a stoma that control the stomatal opening and thereby regulate the movement of gases and water vapor
intercalary meristem - meristematic tissue located at nodes and the bases of leaf blades; found only in monocots
internode - region between nodes on the stem
lamina - leaf blade
lateral meristem - also called secondary meristem, meristematic tissue that enables a plant to increase in thickness or girth caused by the vascular cambium and cork cambium
lenticel - opening on the surface of mature woody stems that facilitates gas exchange
meristem - plant region of continuous growth
meristematic tissue - tissue containing cells that constantly divide; contributes to plant growth
node - point along the stem at which leaves, flowers, or aerial roots originate
palmately compound leaf - leaf type with leaflets that emerge from a point, resembling the palm of a hand
parenchyma cell - most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of photosynthesis and starch storage
pericycle - outer boundary of the stele from which lateral roots can arise
periderm - outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm
permanent tissue - plant tissue composed of cells that are no longer actively dividing
petiole - stalk of the leaf
phyllotaxy - arrangement of leaves on a stem
pinnately compound leaf - leaf type with a divided leaf blade consisting of leaflets arranged on both sides of the midrib
pith - ground tissue found towards the interior of the vascular tissue in a stem or root
primary growth - growth resulting in an increase in length of the stem and the root; caused by cell division in the shoot or root apical meristem
rhizome - modified underground stem that grows horizontally to the soil surface and has nodes and internodes
root cap - protective cells covering the tip of the growing root
root hair - hair-like structure that is an extension of epidermal cells; increases the root surface area and aids in absorption of water and minerals
root system - belowground portion of the plant that supports the plant and absorbs water and minerals
runner - stolon that runs above the ground and produces new clone plants at nodes
sclerenchyma cell - plant cell that has thick secondary walls and provides structural support, usually dead at maturity
sessile - leaf without a petiole that is attached directly to the plant stem
shoot system - aboveground portion of the plant; consists of nonreproductive plant parts, such as leaves and stems, and reproductive parts, such as flowers and fruits
sieve-tube cell - (sieve-tube members in angiosperms) phloem cell arranged end to end to form a sieve tube that transports organic substances, such as sugars and amino acids
simple leaf - leaf type in which the lamina is completely undivided or merely lobed
sink - growing parts of a plant, such as roots and young leaves, which require photosynthate
source - organ that produces photosynthate for a plant
stele - inner portion of the root containing the vascular tissue; surrounded by the endodermis
stipule - small green structure found on either side of the leaf stalk or petiole
stolon - modified stem that runs parallel to the ground and can give rise to new plants at the nodes
tap root system - type of root system with a main root that grows vertically with few lateral roots; found in dicots
tendril - modified stem consisting of slender, twining strands used for support or climbing
thorn - modified stem branch appearing as a sharp outgrowth that protects the plant
tracheid - xylem cell with thick secondary walls that helps transport water
translocation - mass transport of photosynthates from source to sink in vascular plants
transpiration - loss of water vapor to the atmosphere through stomata
trichome - hair-like structure on the epidermal surface
tuber - modified underground stem adapted for starch storage; has many adventitious buds
vascular bundle - strands of plant tissue made up of xylem and phloem
vascular stele - strands of root tissue made up of xylem and phloem
vascular tissue - tissue made up of xylem and phloem that transports food and water throughout the plant
venation - pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as in ginkgo biloba)
vessel element - xylem cell that is shorter than a tracheid and has thinner walls
whorled - pattern of leaf arrangement in which three or more leaves are connected at a node
Introduction
Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue or permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.
There are two types of meristematic tissues, based on their location in the plant. Apical meristem or primary meristem contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristem or secondary meristem facilitate growth in thickness or girth in a maturing woody plant. Intercalary meristem is found in some monocots such as grasses. Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue.
Permanent tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Dermal tissue covers and protects the plant, while vascular tissue transports water, minerals, and sugars to different parts of the plant. Vascular tissue is an example of a complex tissue and is made of two specialized conducting tissues: xylem and phloem.
Xylem tissue transports water and nutrients from the roots to different parts of the plant and includes three different cell types: vessel elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve elements (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Gymnosperms lack sieve elements and companion cells. Cells carrying out similar function in gymnosperms are called sieve cells. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other (Figure 1.3.1). In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.
Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.
Any part of a plant has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.
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Dermal Tissue
Dermal Tissue
The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (Figure 1.3.2). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
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Vascular Tissue
Vascular Tissue
The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue (Figure 1.3.3).
Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.
Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-elements (also called sieve-tube members) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.
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Ground Tissue & Cell Types
Ground Tissue
Plant tissues that are not dermal or vascular are considered ground tissue. Cell of ground tisses perform many differnent types of functions, such as photosynthesis, storage, based on their location. In a stem ground tissue mostly contains parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.
Let us look at three types of plant cells, parenchyma, collenchyma, and sclerenchyma cells.
Parenchyma cells are the most common plant cells (Figure 1.3.4). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. These cells are somewhat spherical and have thin primary wall. This help in exchange of raw material and waste products between outside and the inside of the cell. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch. Parenchyma cells rarely show formation of secondary wall.
Collenchyma cells are elongated cells with unevenly thickened walls (Figure 1.3.5). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.
Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin—an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (Figure 1.3.6).
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Dig Deeper
Watch Botany Without Borders, a video produced by the Botanical Society of America about the importance of plants.
Attributions
Title: Browallia americana L.: entire flowering plant with separate parts of fruit and seeds. Coloured etching by M. Bouchard, 1774.
Work Type: Scientific illustrations
Date: 1774
Description: Browallia demissa pedunculis unifloris. H.Cliff.318.t.17. - Hort.Ups.179. - Linn.Sp.Plant.773
Repository: Wellcome Collection
Collection: Open Artstor: Wellcome Collection
ID Number: V0042766ER
Source: Image and original data from Wellcome Collection
License: Creative Commons: Attribution
Use of this image is in accordance with the applicable Terms & Conditions
Biology 2e by Clark Mary Ann, Douglas Matthew, Choi Jung. OpenStax is licensed under Creative Commons Attribution License V 4.0