Tissue System in Plants (With Diagram)

Some of the most important types of tissue system are as follows:

All the different type of tissues in a plant that perform similar basic function, irrespective of their location is known as Tissue system.

Sachs (1875) recognized three types of tissue system in plants:

1. Epidermal or Dermal Tissue System

2. Cortical or Fundamental or Ground Tissue System

3. Vascular Tissue System

1. Epidermal or Dermal Tissue System:

It forms the outer protective covering of the plant body. During primary growth the covering of plant body is known as epidermis (in root it is known as epiblema) but in secondary growth the epidermis may be replaced by periderm.

Epidermis:

i. It is the outermost, usually one cell thick continuous layer without intercellular spaces, but in leaf it is interrupted by tiny pores, known as stomata.

ii. The walls of epidemis are unevenly thick and inner radial walls are thick. The additional thickening of epidermal cell is due to deposition of cutin and suberin.

iii. Epidermal cells contain leucoplast, chromoplast and anthocyanin.

iv. Outermostlayerofrootisknownasepiblemaorpiliferouslayerorrhizodermis. It is devoid of lenticel and stomata. Unicellular root hair develops from epiblema.

v. Leaf is protected by upper and lower epidermis.

vi. It is heavily circularized (Cuticle is thick in xerophytes. It is formed by waxy deposition, secreted by epidermal cells) with frequent interruption of pores known as stomata. It helps in gas exchange as well as transpiration.

Function of Epidermal Tissue System:

i. It is mainly an outer protective covering of underlying soft tissue.

ii. It prevents excessive evaporation of water from internal tissue, due to presence of cuticle, wax, or trichomes (stem hairs).

iii. Upper epidermis of monocot leaf carries large, thin walled, vaculated, living motor or bulliform cell. It helps in rolling of leaf to reduce the rate of transpiration.

2. Cortical or Fundamental or Ground Tissue System:

Ground tissue system consists all the tissues which are present inside the epidermis except vascular or complex tissue. It forms the main bulk of the plant body and it extends from below the epidermis to the central core of a plant. It includes hypodermis, cortex, endodermis, pericycle and pith.

1. In dicot stem, cortex is differentiated into three parts i.e., hypodermis (Collenchymatous), middle cortex (thin walled parenchymatous) and endodermis. In young dicot stem endodermis is known as starch sheath because it contains mucilage, tannin and high amount of starch.

2. In monocot stem, cortex is differentiated into two parts: Hypodermis (Sclerenchymatous) and inner cortex. Endodermis is absent.

3. Ground tissue of leaf is known as mesophyll tissue. It is enclosed by upper and lower epidermis. It is undifferentiated in monocot leaf but differentiated into pallisade parenchyma and spongy parenchyma in dicot leaf.

4. Endodermis of root present opposite to phloem tissue becomes thick due to more deposition of suberin and less lignin known as Casparian Strip or Bands. It was first observed by Caspary (1865). This band checks the flow of water towards phloem. The thin walled cells in endodermis of root present opposite to protoxylem are known as passage cells or transfusion eel’s. They help in translocation of water from cortex to xylem. Endodermis of young stem lacks casparian strips and passage cells. But bears starch grains. So often called as starch sheath.

5. Pericycle is composed of thin walled, parenchymatous or sometimes thick-walled sclerenchymatous cells (e.g., Cucurbita); ranging in width from single layer of cells to a few layers. In some stems, e.g., Sunflower, the pericycle is composed of alternating bands of thin- walled and thick-walled cells (heterogenous pericycle). Parenchymatous pericycle stores food but mechanical support to the plant is given by thickwalled pericycle. In angiosperms lateral root originates from pericycle.

6. Pith or Medulla forms the central core of the stem and the root. It is made up of large, thin- walled, parenchyma with inter cellular spaces. It is well developed in dicot stem and monocot root. It is absent in monocot stem and feebly developed in dicot root. It helps in storage of food.

3. Vascular Tissue System:

Vascular tissue system is associated with conduction of water, minerals and food materials. This system includes the vascular bundles (group of xylem and phloem). These are located inside the stele in all vascular bundles. (Fig.9.12).

Radial Vascular Bundles:

Xylem and phloem occur in separate patches on alternate radii. Radial vascular bundles are the characteristic of all types of root. 11 dicot root xylem patch is 2 – 6 (Diarch to hexarch). In monocot root xylem patch is 7 or more (polyarch).

Conjoint Vascular Bundles:

Xylem and phloem are associated with each other and together form a bundle. They are present on the same radius. These are usually found in stem.

It is of two types:

(a) Collateral:

In this type of vascular bundle, xylem is located towards the inner side and phloem towards the outer periphery of xylem. In dicot stem cambium is present between xylem and phloem; such vascular bundles are called open. In monocotstems, the cambium is absent; such vascular bundles are called closed.

(b) Bicollateral:

In these vascular bundles, there are two patches of phloem, one on each side of xylem. Here, there are two strips of cambium one on each side of xylem. Thus the arrangement is outer phloem →outer cambium → xylem → inner cambium and inner phleom, e.g., cucurbitaceae, some members of Solanaceae and Convolvulaceae etc.

Concentric Vascular Bundles:

In this vascular bundle either xylem surrounds the phloem or phloem surrounds the xylem. Concentric vascular bundles are of two types:

(a) Amphicribral (hadrocentric):

The xylem is in the centre surrounded on all sides by phloem e.g., Ferns, aquatic angiosperms and the staminal bundles of many dicots (e.g., Prunus).

(b) Amphivasal (leptocentric):

The xylem completely surrounds the phloem e.g., Dracaena, Yucca.