Sclerenchyma Tissues

The lignified tisries .vhich lack protoplast at maturity are called sclerenchyma. Hey have thick secondary wall. These are main strengthening tissues of plant. Sclerenehyma cells are found in all parts of plant. They: are present in both ground tissues and vascular tissues. Sclerenchyma has two types: Fibers and sclereids

(a) Fibers

Very long and narrow sclerenchymatous cells with tapering ends are fibers. The length of the fibers varies greatly. Fibers are found in all parts of the plants. They are present in stern, roots and leaves. Fibers are also present in xylem and phloem. They may form component of vascular tissues. Or they are present^– outside the vascular tissues.

Types of fibers

There are three types of fibers: Xy Ian. bast and septate fibers.

I.  Xylary fibres or wood fibers: They are present in xylem. They form major pall of the xylem. Xvlary fibers have three main types:

• Libiform fibers: These fibers have very thick Y all and simple pits. They are longer than the tracheids of that plant.

• Fihro-tracheids: They have intermediate thickness between tracheids and libilorm fibers. They have bordered pits.

• (e) Mucilaginous fibers: The innermost laver of these fibers is rich in cellulose. Ii has greater Nater holding capacity. Therefore, these fibers absorb %%ate’: and swell. These fibers are common in elastic Yood.

2.  Bast fibers: These litsers are present in phloem and cortex tissues. In cortex, they form uninterrupted hollov. cylinders in !tic ground tissues. In phloem. the) form fiber sheath.

3   Septate fibers: These arc found in both xy lem and phloem. Ii hers have interim I septa. These are found in septate yood ti hers.

Development of fibers

Fibers develop from different meristems like procambium, cambium and ground meristem. Fibers also develop from parenchyma cells. The initials of the primary fiber appear earlier. They grow in length and form fibers. The secondary •fibers develop in the fully grown tissues.

Commercial fibers

Phloem or bast fibers have great economic importance. These fibers give commercial fiber. These Fibers are used in rope and cloth making. Some important plants which give commercial fibers are: Hemp. Jute, Kenaf, Flax etc. Sonic commercial fibers are obtained from the leaf of monocot plants. These plants are: Manila hemp, Bowstring hemp, New Zealand hemp and pineapple fibers.

(b) Sclereids

The variable shaped sclerenchymatous cells with strongly lignified wall having simple pits are called sclereids. Any non — fibrous sclerenchymatous cell is sclercid. Sclereids are found in different parts of plants. ^–1 hey are present in epidermis, ground tissues and vascular tissues. The are present in the form of hard mass of cells. The seed coats of many seeds are entirely composed of sclereids. Sonic non-functional parenchyma cells are present in kascular tissues. The walls of these parenchyma cells become thick and they become sclereids.

Types of Sclereids

There are four types of sclereids:

1. I. Brachysclereids or stone cells: They have isodiametric shape. They are found in phloem. cortex, bark of stem and fruits.
2. Macrosclereids: They are rod shaped. They are found in testa of seeds.
3. Astrosclereids: They are star shaped. They are mainly found in leaves.
4. Osteosclereids: They are hone or spoon shaped. They are present in seed coat.

Development of Sclereids

The development of sclereid is coordinate and intrusive.

I. Coordinated: In this ease, the cell wall continues to grow uniformly. This uniform growth on all sides without separation from neighbouring cells is called coordinated growth.

2. Intrusive: Later the growth becomes localized at certain points. It produces several processes. These processes grow outward and form branches. These branches penetrate into the middle lamella of the neighbouring cells. This type of growth is called intrusive growth.