Types of tissue
Summary
Plants, like all living things, are made of cells. Plant cells are distinguished from those of animals by a rigid cell wall, the presence of chloroplasts, and the presence of a single large vacuole. A collection of cells that together carry out a particular function is known as a tissue. The majority of a plant is made up of ground tissue, whether soft (parenchyma cells), semi-rigid (collenchyma) or rigid (sclerenchyma). In addition, plants have vascular tissue, consisting of xylem and phloem, which transports water and dissolved organic compounds throughout the plant; and dermal tissue, which protects the plant from its environment. At key growth points, plants have meristem tissue, which is capable of differentiation into other types of tissue. Knowledge of plant tissues is important to be able to understand key processes within plants, such as transpiration; and to understand plants’ interactions with their environment, such as why plants that are exposed to air movement grow stronger stems, or why particular garden pests attack plants in the way that they do.
So far, we have only been scratching the surface, barely getting dirt beneath our fingernails. It’s time now to dig a bit deeper, in the first of our intermediate articles here at Getting Dirty. If it’s been a while since you last had a science class, this one may be a little heavy going… but getting your head around this will really help in understanding some of the key processes in plants. Transpiration, for example, takes place through the xylem tissues in the plant’s vascular system, and it is the structure and specialisation of these tissues that allows transpiration to take place.
Cells and tissues
All living things are made of “cells”, the basic unit of life (except possibly viruses, which don’t have cells - but there is an active scientific debate as to whether viruses are alive or not). Some organisms are single-celled, such as bacteria and amoebae. Others, such as plants, animals and fungi are multi-cellular.
Different types of organism have different types of cell. The most fundamental difference is between the two main domains of life (domain is at the top level of the Linnaean taxonomy), the “prokaryotes” (bacteria and some other single-celled organisms) and the “eukaryotes” (everything else). Prokaryotic cells have a much simpler structure and are much smaller than eukaryotic cells. Eukaryotic cells are divided into compartments (called “organelles”), separated by membranes. Each organelle is responsible for specific activities within the cell. The most important is the “nucleus”, which coordinates the activities taking place within the whole cell. The nucleus also contains the genetic material (the DNA) that provides the ‘instructions’ for the organism to conduct all of its living processes (including to reproduce itself).
More relevant to Getting Dirty is the difference between plant and animal cells. For the most curious, I’ve included a couple of diagrams from Wikipedia at the end of this post showing the difference in structure between a typical animal cell and plant cell - these are too deep for Getting Dirty (at least right now…).
The key differences between plant and animal cells are threefold:
(a) plant cells have a rigid cell wall surrounding the cell’s outer membrane whereas animal cells do not;
(b) plant cells contain “chloroplasts”, ancient bacteria that have been incorporated into the plant cells but retain their own genetic material (DNA) and in which photosynthesis takes place, whereas animal cells do not (though both plant and animal cells contain “mitochondria”, similarly captured ancient bacteria in which respiration takes place), and;
(c) plant cells have a single large, fluid-filled “vacuole”, whereas animal cells have numerous tiny vacuoles (the vacuoles play a lot of important roles in plants, such as maintaining the internal pressure necessary for a plant cell to remain rigid, and storing many of the chemicals necessary for the plant to function).
Cells within organisms are most often specialised for particular purposes (for example, neurons and red blood cells in humans). All cells start off as “undifferentiated” cells, capable of reproduction by dividing themselves into two new, identical cells (called “stem cells” in animals and “meristem” cells in plants). These cells then “differentiate”, changing their size, shape, internal structure, activity etc to carry out their specific function. Differentiated cells generally lose the ability to reproduce, and when it is retained, they can only produce copies of themselves. Undifferentiated stem/meristem cells are said to be “totipotent”, simply meaning that they can differentiate into specialised cells.
Cells are organised into “tissues”, groups of cells that perform a single function for the organism. These may be “simple”, containing only one type of cell, or “complex”, containing multiple different cell types. In plants, there are four main types of tissue: meristematic tissue, dermal tissue, vascular tissue and ground tissue (everything else).
Ground tissue
The ground tissue of a plant forms the bulk of the plant, and is formed of three types of cell: “parenchyma”, “collenchyma”, and “sclerenchyma”.
Parenchyma
Of all the ground tissues cells, parenchyma are the least specialised. They have thin, flexible cell walls, allowing them to take on different shapes; make up the majority of the soft tissues of a plant; and carry out most of a plant’s essential processes. They typically have air spaces between the cells, the size of which depends on the shape of the parenchyma cells; which in turn depends on their function.
Not all parenchyma cells have chloroplasts; those that do are called “chlorenchyma”, and it is in these cells that photosynthesis takes place. When appropriately stimulated, parenchyma cells are able to revert to being meristematic; allowing them to be used by the plant to repair wounds. Parenchyma cells are also used by plants to store starches, proteins, fats, oils and water (for example in roots and tubers). They may also be able to secrete these substances, for example pine resin or maple syrup. In aquatic plants, parenchyma can become further specialised (then being known as “aerenchyma”) , resulting in very large air gaps between the cells, giving the plant buoyancy.
Collenchyma
Plants need structural support as well as soft tissue, and collenchyma cells provide this within areas of the plant that are still growing. Their cell walls are thickened in different places in irregular patterns, and they take on an elongated shape, allowing them to fit together with no gaps between them. The pattern and extent of thickening is highly influenced by the movement of the plant and the stresses this puts on its tissues; this is why plants that are exposed to air movement and wind become much stronger than those kept in motionless air. The stringiness in celery stalks is caused by collenchyma tissues.
Sclerenchyma
The final type of ground tissue provides structural support in places where the plant has stopped growing. Unlike parenchyma and collenchyma, mature sclerenchyma cells are no longer living. As the cells grow, their cell walls continue to thicken until they make up almost all of the cell volume, at which point the cell dies and becomes rigid. Sclerenchyma can either be long and slender, organised into bundles known as “fibres”, or short and taking on a variety of different shapes, in which case they are known as “sclereids”. Examples of fibres include flax, hemp and jute. Examples of sclereids include the hard shells of nuts and the seed cases of stone fruits. Small clusters of sclereids are also responsible for the tougher tissue in the core of apples, and the gritty texture of some pears.
Vascular tissue
The vascular tissue of a plant forms the system that the plant uses to transport water, minerals and nutrients throughout the plant. It consists of three components: “xylem”, “phloem”, and “vascular cambium”.
Xylem tissue
The xylem of a plant is a complex tissue responsible for the transport of water and minerals from the soil to the leaves of the plant. Xylem cells (or “vessels”) themselves resemble thin tubes, open at both ends, and connected in long columns, all the way from the base of the plant to its leaves. Like sclerenchyma, mature xylem vessels are dead tissue, whose walls provide rigidity to the plant. The xylem vessels also have small holes or “pits” in their sides, allowing water to move sideways between individual cells (and thus enabling water to continue to flow in the event that some of the xylem vessels become blocked or severed).
Xylem tissue consists of the xylem vessels themselves, together with sclerenchyma fibres that give it additional structural support, and parenchyma cells (the only living part of the xylem tissue), which are able to supply the energy needed to transport substances into and out of the xylem tissue. A further group of cells, called “tracheids” sit alongside the xylem vessels. These are closed at the ends, which taper, and like xylem vessels, perforated by pits. Only angiosperms have xylem vessels; gymnosperms and pteridophytes rely on tracheids for water transport.
This tissue should properly be called “primary xylem”; woody plants also produce “secondary xylem” tissue. This is produced laterally as the plant thickens (so called “secondary growth”, with primary growth being the lengthening of the plant) and forms the central supporting structure of the plant. The difference in xylem tissue between gymnosperms and angiosperms is reflected in the strength of this structure: the wood of coniferous trees (gymnosperms) is known as softwood, whereas that of angiosperm trees is known as hardwood.
Phloem tissue
Like xylem tissue, phloem tissue is a complex tissue, in this case responsible for the transport of sugars and other substances (together forming the plant’s “sap”) throughout the plant. Unlike xylem, which can only transport water in one direction, phloem is able to transport its cargo in both directions.
Phloem’s primary function is achieved through highly specialised cells called “sieve elements”, supported by “companion cells”. Like xylem vessels, sieve elements are elongated and joined end to end. Unlike xylem vessels, they retain their end walls, which are perforated by pores, to allow the passage of sap from element to element. Taken together the sieve elements form structures called “sieve tubes”, and the perforated end walls are known as “sieve plates”. Phloem cells have become so specialised that they have lost the ability to manage their own internal processes; each relies on a companion cell, a parenchyma cell that manages its functions. Gymnosperms lack companion cells, but have “albuminous cells”, which carry out a very similar function.
Phloem tissue (again properly “primary phloem tissue”) consists of the sieve tubes (including their companion cells) together with sclerenchyma cells for structural support, and additional parenchyma cells that provide immediate storage for the soluble organic compounds in the sap before they are transferred to other tissues where they can be used.
Secondary phloem is formed laterally during secondary growth (thickening), and becomes the “inner bark” of a woody plant.
Vascular cambium
The vascular cambium of a plant is a thin layer of meristem cells that sits between the xylem tissue and the phloem tissue, with the three together being arranged in a “vascular bundle”. The primary xylem and phloem of a plant are formed from apical meristem tissue as the plant grows (see below); the vascular cambium is where secondary growth occurs, as it differentiates into both secondary xylem and secondary phloem. In a woody plant, the vascular cambium is easily seen in a cross section of a stem; it is the dividing line between the wood and the bark.
Herbaceous perennials also have vascular cambium, but unlike in woody plants where the vascular bundles form a contiguous ring, in herbaceous perennials, the vascular bundles do not lie immediately next to one another. This allows stems to thicken during a particular growing season, but not to develop the woody tissue needed to survive harsh winter weather.
Dermal tissue
The dermal tissue of a plant is the tissue that protects it from the external environment. There are two key components of dermal tissue, only one of which is present in non-woody plant, the “epidermis”. The other is the “periderm”, which is formed of “cork” and the “cork cambium”.
Epidermis
The epidermis of a plant is typically only one cell thick, and forms the outermost layer of all parts of the plant. The epidermis consists of several different types of specialised cells:
Most epidermal cells (“pavement cells”) are flattened, with a wavy shape, and tightly linked to one another, providing a strong barrier against external factors. In the above ground parts of the plant, these cells contain a waxy substance called “cutin”, which forms a “cuticle”, reducing water loss to the atmosphere. In some plants, a further layer of wax may sit atop the cuticle, providing additional protection from sunlight and wind. Epidermal cells typically lack chloroplasts, making them transparent, allowing light to reach the inner tissues of the plant.
The epidermis is covered in pores (or “stomata”), which allow the plant to exchange gases (oxygen and carbon dioxide) with the environment, and to release water vapour into the surrounding air. “Guard cells” are specialised cells that are located around the stomata and can become more or less rigid, opening and closing the stomata, and allowing the plant to regulate gaseous exchange and the loss of water.
The surface of the epidermis also has specialised hair-like cells called “trichomes” that protrude above the waxy cuticle. These provide a way for the plant to secrete substances through the epidermis, such as aromatic oils (which may deter herbivores, for example) or toxins (e.g. the sting of nettles, Urtica dioica). Trichomes are responsible for giving some plants a soft, hairy feel (e.g. Stachys byzantina, commonly known as lamb’s ears), which may limit water loss and protect against strong sun exposure. Prickles on roses are also modified trichomes. Root hairs, however, are not trichomes, but are elongated sections of epidermis.
Periderm
Secondary growth (thickening) in woody plants increases the surface area needed to be covered by the epidermis; however epidermal cells are differentiated and cannot reproduce. Accordingly, woody plants have a layer of meristem cells just below the surface. This “cork cambium” layer produces “cork” cells (also known as “phellem”); the two together are called the periderm, and this replaces the epidermis. Mature cork cells contain a waxy substance called “suberin”, which protects the plant from water loss and attack by pests and pathogens. As the plant ages, the cork cambium continues to produce cork, thickening the periderm, whose outer cells die (along with the cells of the epidermis). These cells together form the bark of the plant.
The bark of a plant is impermeable to gases and water, and so contains “lenticels”, which develop underneath the stomata of the epidermis. These are porous tissues, with large gaps between their cells, allowing gaseous exchange to take place.
Meristematic tissue
The final group of tissues in a plant are the meristematic tissues, which consist of undifferentiated meristem cells, and occur in three locations in the plant:
“Apical meristems” occur at the tips of roots (in the apical bud) and shoots, and differentiate into the tissues needed for growth of the plant. The buds that form at the axils of a plant (axillary buds) contain apical meristem tissue, but their growth is limited (or in some cases prevented) by a chemical produced by the terminal bud on the main stem (a plant hormone called “auxin”).
“Intercalary meristems” only exist in monocot plants, and occur at the base of the internodes. These allow the plant to regrow when tissues above the intercalary meristem are severed, and so, for example, allow grass to recover from being mown, whereas eudicot plants would need to regrow from their roots, and would be substantially weakened.
“Lateral meristems” make up the vascular cambium and cork cambium discussed above.
Why do we need to know this
Well done for getting through this one! You may be wondering why I subjected you to a lengthy article with lots of technical anatomical terms. The answer is that the understanding you now have allows you to work out why plants respond in particular ways to their environment. For example:
Removing the bark in a ring around a tree strips away the primary phloem (just inside the secondary phloem), preventing sap from being transported through the plant. Such “girdling” of a tree results in the death of the plant.
Air movement during the growth of a young plant thickens the cell walls of the collenchyma, resulting in stronger plants.
As we build an understanding of critical processes within plants (including photosynthesis, respiration and transpiration), our knowledge of plant tissues will be called upon again and again.
And finally…
Below you can find the diagrams I promised (from Wikipedia) of a typical animal cell and a typical plant cell. We won’t be going into these structures in any detail, so feel free to dig deeper on your own…
Structure of a typical animal cell
Structure of a typical plant cell
Image credits
The diagrams of animal and plant cells in this post have been released into the public domain by their creator:
Credit: Lady of Hats (Mariana Ruiz). Image source (animal cell). Image source (plant cell).
