Physical structure of cotton

Under microscope cotton looks like a bean or lace and it is not circular in cross section. Physical structure of cotton comprises of several cell walls. The structure is described as follows:

Cuticle: The outermost layer of cotton is known as the cuticle. The cuticle is a few molecules thick and covers the primary wall with a waxy film. The waxy film is a mixture of fats, waxes, and resins.

Primary cell wall: The primary wall is the original thin cell wall and has felt-like reticular structure and consists of crossed cellulose crystalline fibrils. Fibrils are bundles of linear polymer which form the small fibrous structure. In primary cell wall fibrils are about 20 mm thick .

Winding or transition layer: This is the first layer after primary cell wall and between the primary and secondary wall. It differs in structure from either the primary wall or the remainder of the secondary wall. Its fibrils are more wider angle about 20-35 degree than secondary wall and in this region the fibrils periodically reverse the direction of the spiral.

Secondary cell wall: The secondary wall has several layers of spiraling fibers, which make up most of the weight of the cotton fiber. Secondary cell wall is situated inside the winding layer, accounts for the bulk of the cellulose content and in this case, the fibrils are in the form of helices with an angle of 20-30 degree.

In very mature fibre the Tertiary layer can be found adjacent to the linning of the lumen and it consist of the mineral salt and the proteins.

Lumen: The hollow central core of cotton fibres is known as the lumen. Initially there is no lumen in cotton fibre. When cotton becomes mature then lumen is noticed but it gradually begins to smaller and contains the dried out protoplasm and nucleus when the cotton boll development is complete.