Plants have to
- Synthesise their food through the process of photosynthesis.
- Protect the internal structure from the external environment.
- Transport all the food materials and water throughout the plant body
- Prevent the loss of water under extreme conditions.
- Be flexible to prevent the damage of plant parts.
- Be rigid and strong.
To perform all these functions, plants need several tissues to work together.
In flowering plants, several tissues work together to perform a particular function in the form of a unit. This tissue forms a system called a tissue system.
In \(1875\), Sachs classified three types of tissue systems based on their structure and location.
- Dermal or epidermal tissue system
- Ground or fundamental tissue system
- Vascular or conducting tissue system
The plant tissue systems
1. Dermal or epidermal tissue system:
The epidermal tissue system forms the outermost covering of the whole plant body and made up of,
- Epidermal outgrowths of epidermal appendages
The epidermis is the plant's outermost layer, consisting of a single layer of cells. Epidermal cells are parenchymatous and have a large vacuole. Thus epidermal cells form a continuous layer with no intercellular spaces to protect the inner tissues of the plant. In addition, the outer and sidewalls of epidermis tissue are thicker than the inner wall. It covers the entire plant's surface.
Furthermore, the outer side of the epidermis is frequently covered with a thick, waxy, water-resistant layer of cutin known as the cuticle, which helps regulate water evaporation. This layer, in turn, protects the plant's surface from water loss, mechanical injury, and parasitic fungi invasion. There is no cuticle in the roots.
Stomata are structures that can find in the epidermis of leaves. It is surrounded by two kidney-shaped guarding cells (dumb-bell shaped in grasses), required for gas exchange in the leaf. They also aid in transpiration, a process by which loss of excess water in the form of water vapour through leaves' surface. The outer wall of guard cells is thin (away from the stomatal pore), while the inner walls are thickened (towards the stomatal pore). The guard cell contains chloroplasts and regulates stomatal opening and closing.
Figure left to right: (a) Depicting the process of transpiration in stomata (b) Depicting the stomata of monocot (grass) and dicot (bean) plants
- A few epidermal cells near the guard cells become specialised in shape and size and are called subsidiary cells.
- Stomatal apparatus refers to the stomatal aperture, guard cells, and surrounding subsidiary cells.
- Stomata on a dicot leaf's lower surface are usually more numerous than those on the upper surface. In contrast, on the upper surface of a monocot leaf, they are equal in number.
Stomatal apparatus in grasses
Root hairs and trichomes are the epidermal outgrowths.
Root hairs: The epidermis cells have a lot of hairs. Root hairs are unicellular elongations of epidermal cells that aid in absorbing minerals and water from the soil.
Root hair cell collecting mineral nutrients and water from the soil
Trichomes: The epidermal hairs present on the stem are called trichomes. In most cases, the trichomes in the shoot system are multicellular.
They might be soft or stiff, and they can be branched or unbranched and may be secretory in function. It aids in the prevention of water evaporation due to transpiration.
Ground tissue system:
It is the largest tissue system. It includes all tissues of the plant body such as cortex, endodermis, pericycle, and pith.in early stems and roots, except epidermal and vascular tissues. The mesophyll, which is made up of thin-walled chloroplast-containing cells, is the ground tissue in leaves.
Iris germanica root transverse incision
The above picture is showing the cross-section of the root. Ground tissue parts are labelled and are explained below,
- The cortex and the pith are the two parts of a stem's ground tissue. Parenchyma cells make up the majority of the cortex and pith.
- The cortex is an outer layer of a plant's stem or root, located under the epidermis but outside the vascular bundles.
- The pith is a soft region in the centre of the stem and the roots of some plants, made up of parenchyma cells. The pith's principal role is to transport nutrients throughout the plant and store them within its cells.
- The endodermis is the central, innermost layer of the cortex. It aids in regulating water, ions, and hormones entering and exiting the vascular system.
- Next to the endodermis are a few layers of thick-walled parenchymatous cells referred to as pericycle. The pericycle contributes to the vascular cambium in secondary-growth plants, often diverging into a cork cambium.
Cross section of root