Plant Tissues and Cell Types
Overview
Public Domain Mark
Description
Browallia americana = (Browallia demissa)
Publication/Creation
[Romae (Rome)] : [Bouchard et Gravier], [1774]
Physical description
1 print : etching, with watercolour ; platemark 37.2 x 22.9 cm
Contributors
Lettering
Browallia demissa pedunculis unifloris. H.Cliff.318.t.17. - Hort.Ups.179. - Linn.Sp.Plant.773
Reference
Wellcome Library no. 17029i
Type/Technique
Languages
- Latin
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.
Keywords
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
dermal tissue - a protective plant tissue covering the outermost part of the plant; controls the gas exchange
ground tissue - plant tissue involved in photosynthesis; provides support, and stores water and sugars
parenchyma - most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of photosynthesis and starch storage
phloem - tissue responsible to transport sugars, proteins, and other solutes
sclereids - a type of sclerenchyma cell with thick and highly lignified walls
sclerenchyma - plant cell that has thick secondary walls and provides structural support, usually dead at maturity
sieve cells - (sieve-tube members in angiosperms) phloem cells arranged end to end to form a sieve tube that transports organic substances, such as sugars and amino acids
sieve tube member - (sieve cell in gymnosperms) phloem cell arranged end to end to form a sieve tube that transports organic substances, such as sugars and amino acids
tracheids - a type of xylem cells that specialize in water conduction
vascular tissue - tissue made up of xylem and phloem that transports food and water throughout the plant
vessels - xylem cells that are shorter than tracheids and have thinner walls
xylem - tissue responsible for long-distance transport of water and nutrients
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 contains meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristem or secondary meristem facilitates 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. Primary and secondary meristem is discussed in more detail in lesson 3: Stages of Growth.
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 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 functions 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 1.4.1). 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.
Figure 1.4.1. This light micrograph shows a cross-section of a squash (Curcurbita maxima) root. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue. (credit: modification of work by "(biophotos)"/Flickr; scale-bar data from Matt Russell). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Dermal Tissue
The dermal tissue of the stem consists primarily of the 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 1.4.2). 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.
Figure 1.4.2. Openings called stomata (singular: stoma) allow a plant to take up carbon dioxide and release oxygen and water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked by two guard cells that regulate its (b) opening and closing. The (c) guard cells sit within the layer of epidermal cells. (credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth College; credit b: modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
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 1.4.3).
Figure 1.4.3. In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and the phloem is located toward the exterior. Sclerenchyma fibers cap the vascular bundles. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue. Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
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.
Ground Tissue & cell types
Ground Tissue
Plant tissues that are not dermal or vascular are considered ground tissue. Cells of ground tissues perform many different types of functions, such as photosynthesis, and 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 the 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 1.4.5). 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 a thin primary wall. This help in the exchange of raw material and waste products between the 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 the formation of a secondary wall.
Figure 1.4.5. The stem of common St John's Wort (Hypericum perforatum) is shown in cross-section in this light micrograph. The central pith (greenish blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) is composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. (credit: Rolf-Dieter Mueller). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Collenchyma cells are elongated cells with unevenly thickened walls (Figure 1.4.6). 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.
Figure 1.4.6. Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures. (credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
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 1.4.7).
Figure 1.4.7. The central pith and outer cortex of the (a) flax stem is made up of parenchyma cells. Inside the cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. The (c) flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the source of linseed oil. (credit a: modification of work by Emmanuel Boutet based on original work by Ryan R. MacKenzie; credit c: modification of work by Brian Dearth; scale-bar data from Matt Russell). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Attributes
Biology 2e by Clark Mary Ann, Douglas Matthew, Choi Jung. OpenStax is licensed under Creative Commons Attribution License V 4.0