3.1.3: Plant Tissues

Learning Objectives

• Describe the difference between meristematic and non-meristematic tissues.
• Compare and contrast dermal, ground, and vascular tissue.

Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Plant tissues are composed of cells that are similar and perform a specific function. Together, tissue types combine to form organs. Each organ itself is also specific for a particular function.

Plant tissue systems fall into one of two general types: meristematic tissue, and 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.

Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.

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. Dermal tissue covers and protects the plant. The ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars. The vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue is a simple tissue, meaning that each ground tissue consists of only one cell type. Dermal and vascular tissues are complex tissues because they consist of multiple cell types.

Dermal Tissue

Dermal tissue covers the plant and can be found on the outer layer of roots, stems and leaves. Its main functions are transpiration, gas exchange and defense. The epidermis is an example of dermal tissue (Figure \(\PageIndex{1}\)). It is composed of a single layer of epidermis cells. It may contains stomata and guard cells that allow gas exchange. It may contain root hairs that increase surface area or or trichomes used in transpiration or defense. It may contain a waxy cuticle if found on the upper surface of leaves, to aid with lowering transpiration.

Figure \(\PageIndex{1}\): 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 eudicot. Each stoma is flanked by two guard cells that regulate its (b) opening and closing. The guard cells are more curved when the stoma is open compared to when it is closed. 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)

In woody plants, the epidermis breaks apart into a thick periderm as secondary growth allows the plant to grow in girth. The cork cambium, which makes cork cells, the cork cells (which are dead at maturity), and the phelloderm (parenchyma cells on the inside of the cork cambium) together make up the periderm (Figure \(\PageIndex{2}\)). The periderm functions as the first line of defense for the plant, protecting it from fire or heat injury, dehydration, freezing conditions, and/or disease.

Figure \(\PageIndex{2}\): Cross section of a woody stem. The periderm is composed of the cork cambium, cork cells, and phelloderm. Credit: Kammy Algiers (CC-BY).

Ground Tissue

Often times, tissues that are not considered dermal or vascular tissue are noted as ground tissue. These cells store molecules (such as starch), photosynthesize (such as mesophyll cells), or support the plant. There are three types of ground tissue: collenchyma, sclerenchyma, and parenchyma.

Collenchyma (Figures \(\PageIndex{3-4}\)) is living supportive tissue that has elongated cells and an unevenly thickened primary cell wall. Its main function is the mechanical support of young stems and leaves via turgor.

Figure \(\PageIndex{3}\): 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)

Sclerenchyma is a dead supportive tissue that consists of long sclerenchyma fibers (Figure \(\PageIndex{4}\)) or short, crystal-like cells (sclereids; Figure \(\PageIndex{5}\)). Sclerenchyma fibers occur in groups (bundles). Sclereids may be branched or not and occur individually or in small clusters. Each cell has a uniformly thick secondary wall that is rich in lignin. Its main function is a support of older plant organs, and also hardening different parts of plants (for example, make fruit inedible before ripeness so no one will take the fruit before seeds are ready to be distributed). Without sclerenchyma, if a plant isn’t watered, the leaves will droop because the vacuoles will decrease in size which lowers the turgor. Fibers inside phloem (see below) are sometimes regarded as a separate sclerenchyma.

Figure \(\PageIndex{4}\): Left to right, top to bottom: parenchyma, sclerenchyma (cross- and longitudinal sections) and collenchyma. First three photos from the stem of Helianthus, fourth from Medicago stem. Magnification ×400.

 

Figure \(\PageIndex{5}\): The grainy texture of pears (Pyrus) is due to clusters of stone cells (sclereids), the thick-walled cells that stained pink (left, magnification = 400X). Water lily (Nymphea) leaves contains single, branched sclereids (right, magnification = 400X). Left and right image by Berkshire Community College Bioscience Image Library (public domain).

Parenchyma (Figure \(\PageIndex{4}\)) are spherical, elongated cells with a thin primary cell wall. It is a main component of young plant organs. The basic functions of parenchyma are photosynthesis and storage. They are also important in regeneration because they are totipotent (capable of differentiating into any cell type). Parenchyma cells are widespread in plant body. They fill the leaf, frequent in stem cortex and pith and is a component of complex vascular tissues (see below).

Vascular Tissue

Vascular tissue is the plumbing system of the plant. It allows water, minerals, and dissolved sugars from photosynthesis to pass through roots, stems, leaves, and other parts of the plant. It is primary composed of two types of conducting tissue: xylem and phloem. The veins on leaves are an example of vascular tissue, moving material through the plant in the same manner that our blood vessels carry nutrients through our body. The xylem and phloem always lie adjacent to each other (Figure \(\PageIndex{6}\)). 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.

Figure \(\PageIndex{6}\): This light micrograph shows a cross section of a squash (Curcurbita maxima) stem. 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)

Xylem tissue transports water and minerals from the roots to different parts of the plant. The conducting cells of the xylem are called tracheary elements. Parenchyma cells are also found in the xylem, and sclerenchyma fibers and sclereids are sometimes present.

There are two type of tracheary elements: vessel elements and tracheids (Figure \(\PageIndex{7}\)). Both cell types that are dead at maturity and have thickened secondary cell walls. These cells connect to one another and allow water to be transported through them. Structurally, the vessel elements are wider than tracheids and contain perforation plates between adjacent vessel elements (Figure \(\PageIndex{7-8}\)). Wide openings (slits or pores) in perforation plates allow water to flow vertically between vessel elements, forming a continuous tube. Both types of tracheary elements contain pits, gaps in their secondary cell walls. Adjacent cells have pits in the same locations, forming pit pairs, which allow water and minerals to flow between adjacent cells through the pit membrane (the remaining, thin primary cell walls in these regions; Figure \(\PageIndex{9-10}\)). Therefore, water flows through both perforation plates and pit pairs in vessel elements but only through pit pairs in tracheids. While water can move more quickly through vessel elements, they are more susceptible to air bubbles. An air bubble disrupts cohesion in the column of water moving up the tube of vessel elements preventing use of that particular pathway. In tracheids, an air bubble would only decommission a single tracheid rather than an entire column of vessel elements. Vessel elements are found only in angiosperms, but tracheids are found in both angiosperms and gymnosperms.

Figure \(\PageIndex{7}\): Xylem transports water and minerals through vessel elements and tracheids, which are dead at maturity and have a thin primary and thick secondary cell wall internal to the primary cell wall. In pits, the secondary wall is thin or missing, allowing water to flow laterally. The xylem of angiosperms contains both types of tracheary elements: vessel elements, and tracheids. Vessel elements are stacked up top of each other and contain perforation plates in between cells. Tracheids are thinner and lack performation plates. Pits are thinned regions in the cell wall that allow movement of water between adjacent tracheary elements. Image modified from Kelvinsong (CC-BY-SA).

Figure \(\PageIndex{8}\): Longitudinal section of vessel elements in a Cucurbita (squash) stem (magnification = 400X). Horizontal purple lines represent perforation plates between cells in a column. The rings around the cells are annular cell wall thickenings. Image by Berkshire Community College Bioscience Image Library (public domain).

Figure \(\PageIndex{9}\): Pits are thinned regions of the cell wall (left). Pits of adjacent cells together form pit pairs separated by a pit membrane. On either side of the pit membrane is a pit chamber. The pit aperature is the opening to the pit chamber. The pit membranes of gymnosperms have a thickened central region called the torus (right). 1: The margo is the part of the membrane surrounding the torus. 2: The torus can block the pit opening (aperture) as needed to prevent air bubbles from spreading throughout the xylem. Left and right image by Pagliaccious (CC-BY-SA).

Figure \(\PageIndex{10}\): Bordered pits in tracheids of pine (Pinus) wood appear as bulls eyes. Pit pairs of some species have thickened outer regions (borders). Inside of this is a thinned membrane (margo) and a thickened central portion (torus). Image by Berkshire Community College Bioscience Image Library (public domain).

Phloem tissue transports organic compounds such as sugars from the site of photosynthesis to rest of the plant (Figure \(\PageIndex{11-12}\)). The conducting cells of the phloem are called sieve elements. In comparison to tracheary elements, sieve elements have only primary cell walls (and thus thinner cell walls overall) and are alive at maturity; however, they lack certain organelles, including a nucleus. Sieve-tube elements are the sieve elements found only in angiosperms while sieve cells are found only in gymnosperms while. Both types of sieve elements have pores in their cell walls (sieve areas) that allow transfer of materials between adjacent cells, but these are concentrated at sieve plates in sieve-tube elements and evenly distributed in sieve cells. Because they lack essential organelles, sieve elements rely on specialized parenchyma cells to support them. Companion cells support sieve-tube elements in angiosperms, and albuminous cells support sieve cells in gymnosperms. Additionally parenchyma cells and sclerenchyma cells (phloem fibers) are also found in the phloem.

Figure \(\PageIndex{11}\): Phloem transports sugars and other items. In angiosperms, sieve-tube elements contain the sugar solution. Sieve-tube elements are the conducting cells of the phloem in angiosperms. Sieve plates allow sieve-tube elements stacked on top of each other to connect. Sieve-tube cells are surrounded by various support cells. Companion cells are narrower than sieve-tube elements and each contain a nucleus. They are connected to sieve-tube elements via plasmodesmata and provide them with the molecules they need to function (energy molecules, proteins, etc.) Some companion cells are specialized as intermediary cells, which are between the bundle sheath (see below) and sieve-tube element. Transfer cells are parenchyma cells with cell wall ingrowths, which increase surface area for transport. The bundle sheath cells form the bundle sheath, which surrounds vascular bundles (where the xylem and phloem are located). Within the bundle sheath cell are oval chloroplasts, a nucleus (not labeled), and the central vacuole, which fills most of the cell. Image by Kelvinsong (CC-BY-SA).

Figure \(\PageIndex{12}\): Phloem in a cross section of a Cucurbita (squash) stem, magnified at 400X. Each wide sieve-tube cell has a small, dark companion cell associated with it. (The companion cells are dark because each contains a nucleus.) The cross section cut exactly in between two sieve tube elements in some cases, revealing the sieve plate. Image by Melissa Ha (CC-BY).

The table below summarizes differences between xylem and phloem:

	Xylem	Phloem
Contains mostly	Dead cells	Living cells
Transports	Water & Minerals	Sugar
Direction	Up	Up and Down
Biomass	Big	Small

Meristematic Tissue

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. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.

Attributions

Curated and authored by Kammy Algiers and Melissa Ha using the following sources:

• 30.1 The Plant Body and 30.2 Stems from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 5.1 Tissues from Introduction to Botany by Alexey Shipunov (public domain)