Plant Tissues

What is plant tissue?

Tissues in general 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.

Types of plant tissue

There are typically two types of plant tissue groups to start with: 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.

Fig. 1: Leaf cross-section showing the three different permanent tissue types: epidermal, ground, and vascular. Via flickr.com, edited.

• 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 in plants includes all of the 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.

Fig. 3: An example of ground tissue in plants is the palisade mesophyll inside the leaf. It is a type of parenchymal tissue that photosynthesizes.

The three types of cells associated with the ground tissue, which we describe in the next section, are the collenchyma, the parenchyma, and the sclerenchyma.

Examples of plant tissues

There are many examples that we can delve into showcasing tissue in plants, here we focus on examples of ground tissue in plants: collenchyma, parenchyma and sclerenchyma.

Fig. 4: 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.

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.

Parenchyma

The parenchyma is the plant tissue 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 (Fig. 3). The parenchyma cells can also act as storage sites, especially in the cortex and pith of certain roots.

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).

Vascular tissue in plants

The vascular tissue in plants consists of the xylem and the phloem, which transport water and food/nutrients respectively.

Fig. 5: The vascular tissue form bundles or sheaths that run through the whole plant. The diagram shows vascular bundles inside stems.

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 flowering plants (angiosperms).

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 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 (fig. 5), 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.
• 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).

Plant tissue diagram

A plant tissue diagram illustrating the various permanent tissue types found in vascular plants. Once the plant differentiates, three main tissue types present themselves: dermal, ground and vascular. Each organ of a plant - roots, stems or leaves - contains these tissues for specific reasons as laid out in the sections above.

Fig. 6: Diagram of a leaf section showing the different types of tissues.