Plant Tissues

What do you have in common with a plant? Well, besides needing sunlight and lots of water, you have organs that are made up of tissues. That’s right, tissues make up you, me, non-human animals, and plants. Tissues help both plants and other organisms carry out specific functions, like transporting water, minerals, and oxygen throughout their bodies!

What is plant tissue?

Tissues are a group of cells that work together to perform a function. So, when tissues work together, organs are formed. Plant tissues make up the organs of plants, including the leaves, the stem, and the roots.

Plant tissues are typically divided into two groups to start: meristematic tissue and permanent tissue. Meristematic tissue is plant growth tissue that contains cells that are undifferentiated meaning they have no specialized shape, function, or size yet. Meristematic tissue cells differentiate to become a certain type of tissue with certain qualities. When it differentiates, it becomes permanent tissue, which makes up the three main classifications of plant tissue we are concerned with.

Types of plant tissue

Plant tissue is divided into three groups: dermal, vascular, and ground. Dermal tissue is the outermost layer or layers of tissue that surrounds the plants, the vascular tissue is the tissue associated with the vascular system, and ground tissue is the tissue that is not epidermal or vascular and makes up a large percentage of the plant tissue.

Dermal tissue in plants

Did you know that "dermis" means skin? The dermal tissue of the plant is pretty cool and has a similar function to your own skin, protecting the plant organs and helping to prevent water loss!

Dermal tissue in plants is made up of epidermal cells that are usually small, round, and may or may not have a vacuole. These epidermal cells make up the outer cell layer(s) of plant organs and their main function is to protect the plant organs from the climate or pathogens. Epidermal cells also secrete a cuticle, which is a waxy layer that provides extra protection against water loss and UV rays from the sun.

Epidermal cells may become specialized in their later development, depending on the organ they protect, and may become trichomes, stomatal guard cells, or root hairs. Trichomes are protective projections on stems and leaves, the stomatal guard cells control gas exchange in leaves, and root hairs help increase root surface area.

Ground tissue in plants

Ground tissue includes all of the plant tissue that is not vascular or dermal. This means a lot of the plant tissue is actually ground tissue. Ground tissue in itself is diverse and has various functions, including:

• Storage

• Photosynthesis

• And structural support.

The three types of cells associated with the ground tissue are the collenchyma, the parenchyma, and the sclerenchyma (Fig. 1).

Examples of ground tissues in plants

Collenchyma

The collenchyma cells are typically long and fibrous and they provide support for the non-woody parts of the plant including the stems of herbaceous plants and petioles (the part that attaches the leaf to the branch). Unlike the other ground tissue cell types, the collenchyma cells do not have a secondary cell wall, making them more flexible.

The parenchyma is mostly involved in photosynthesis. In fact, the mesophyll tissue in leaves is made up of parenchyma cells, where photosynthesis occurs, thus some parenchyma cells contain chloroplasts. The parenchyma cells can also act as storage sites, especially in the cortex and pith of certain roots. For example, the cortex of carrots is parenchyma tissue.

Parenchyma cells help in the wound healing process for plants because unlike other cells after differentiation, parenchyma cells retain the ability to divide.

Sclerenchyma

The sclerenchyma cells are mainly for support. They have a thick secondary cell wall and are fortified with lignin. Lignin is a rigid polymer that helps provide support in woody plants and in some nuts and seeds. Oftentimes, these cells will undergo programmed cell death (apoptosis) and exist merely as structural support. The two types of sclerenchyma cells are fibers (support for woody stems) and sclereids (protection in nuts and seeds).

Figure 1: Ground tissue is found in many different plant parts including the cortex of the roots (carrots), the pits of fruits (plums), and the stems (celery). Source: pixabay.com, edited.

Vascular tissue in plants

The vascular tissues in plants are the xylem and the phloem, which transport water and food/nutrients respectively.

Vascular tissue exists in the roots, stems, and leaves, connecting them to ensure that each plant organ can get the necessary nutrients and water it needs for survival (Fig. 2). Water and minerals only flow through xylem tissue in one direction, and food and nutrients flow in both directions through the phloem.

The xylem tissue of most plants is made up of:

• Tracheids: Tracheids are long cells that are fortified with a strong polymer called lignin and are typically dead at maturity (they undergo apoptosis). Tracheids are thought to be the earliest conductive cell in the vascular system.

• Vessel elements: Vessel elements are shorter and wider than tracheids, and are not fortified with lignin. They are specific to the flowering plants (angiosperms).

The apoptosis that the xylem tissue cells undergo allows the cells to have holes in their cell walls so that water and minerals can pass through easily.

The phloem tissue of most plants is made up of:

• Sieve elements: In flowering plants, the sieve tube elements are long cells connecting at the end that have cytoplasm but few organelles. The ends have plasmodesmata which become bigger to allow for nutrient transport, these ends are known as sieve plates. Unlike in cells of the xylem tissue, these cells do not undergo apoptosis when they mature.

Plant tissue diagram

Figure 2: Leaf cross-section showing the three different permanent tissue types. Source: Fayette A. Reynolds, Bioscience Image Library Berkshire Community College via flickr.com, edited.

Plant vascular tissue: monocots versus dicots

The arrangement of tissues in plant organs may vary depending on the group of plants. The monocots and dicots, the two main groups of flowering plants, have different arrangements of vascular tissue throughout their roots, stems, and leaves.

In the stem, the vascular tissue of monocots is spread out, with vascular bundles (xylem and phloem tissue) being dispersed throughout the stem. In dicots, the vascular bundles are organized in a circular pattern around the stem (Fig. 3).

In the leaves, the vascular issue of dicots is often branched (think of a maple leaf), and in monocots, it is often unbranched and in parallel lines (think of a grass leaf). Figure 3: Vascular bundle arrangement in a monocot (left) and dicot (right). Source: Fayette A. Reynolds, Bioscience Image Library Berkshire Community College via flickr.com, edited.

Primary versus secondary growth

All vascular plants undergo the primary growth of stems and roots. Primary growth is characterized by cell division and cell growth that will increase the length of the plant. Typically, plants that are considered herbaceous will only undergo primary growth.

Woody plants may undergo another round of growth known as secondary growth. Secondary growth is lateral growth that increases the width of a root or stem. This occurs through the production of the new vascular tissue (secondary xylem and phloem) which is produced from the lateral meristem (cork cambium and vascular cambium). The older xylem does not function for transport but continues to support the plant, allowing plants exhibiting secondary growth to grow larger.