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Plant stems, leaves and even seeds have fibrous tissues. There are three major plant fibers made up of tissues having thickened walls and complex permanent dead tissue. They are:

1 (i) Mature xylem tissues composed of tracheids, tracheae, fibers (Figure 2.1), which translocates water and mineral in plants.Walls of tracheids, tracheae and plant fibers of all types are composed of cellulose and lignin. Cellulose is a linear polysaccharide polymer composed of many β-glucose monosaccharide units, i.e., in cellulose the acetal linkage is mainly beta. Alpha glucose monomer unit is also found in the cellulose; Lignin is formed by the removal of hydroxyl groups from sugars, creating phenolic compounds and short-chain alcohol ligands. Lignin polymers are heavily crosslinked. The basic monomer of lignin is 4-alkylcatechol.Figure 2.1 Schematic diagram of Tracheid, Tracheae and Xylem fiber.

2 (ii) Sclerenchyma are long narrow tapering tissues with thick side walls. It has great tensile strength and yet are elastic and gives mechanical support to plants. There are two types of sclerenchyma fibers and scelereids (Figure 2.2).

3 (iii) Phloem Fiber or Bast Fiber: Tissues are composed of long cells with lignified cell walls. Flax and hemp are phloem fibers (Figure 2.3).

A look at the SEM images of plant fibers [20], sclerenchyma and xylem tissues (tracheids, tracheae) and CNF fabricated from them gives an existing correlation between the two (Figure 2.4).

Figure 2.2 Schematic diagram of Sclerenchymatous fiber (left) in L.S. view and right in T.S. view.

Figure 2.3 Schematic diagram of Phloem fiber.

Figure 2.4 (a) Maize fiber and (b) maize fiber after pyrolysis as CNF.