THE STRUCTURE AND DEVELOPMENT OF THE COTTON FIBRE
Authors: FLINT E. A. 
Published: January 2008

. The details of the structure of the cotton fibre are not only applicable to the study of cell walls in general but also important in the interpretation of those properties which have technological significance.

The cotton fibre, a tubular outgrowth of an epidermal cell of the cotton seed, has a four‐layered cell wall: the cuticle, the primary wall and the secondary wall, the first layer of which has recently been distinguished from subsequent layers as ‘the winding’. A fifth layer, bordering the lumen, may possibly be differentiated.

The cuticle is usually a characteristic of cells which are exposed to air during some stage of their life cycle; in the cotton fibre it is a mixture of fats, waxes and resins which are released at the surface of the cell during maturation. The cuticle is less than 0–25 μ thick, and although tightly moulded to the primary wall, remains unbroken except over the grossest fibre faults. The long axis of the wax molecules in the cuticle is transverse to the long axis of the cotton fibre.

The primary wall is the only layer, apart from the thin cuticle, enclosing the protoplast during the first 15–18 days of fibre growth; it is composed of pectin and cellulose, the deposition of the latter beginning at a very early stage, possibly on the first day of growth. In its structure, the cellulose of the primary wall is likened to an open mesh‐work of fine thread‐like strands in which left‐ and right‐hand spirals (set at an angle of 65–70o to the fibre axis) are associated with another group of strands (set at right angles to the fibre axis). It is believed that living protoplasm is also present in the primary wall.

The secondary wall consists of concentric layers of cellulose, each layer (lamella or growth ring) being composed of parallel fibrils which follow a spiral course on the inside of the primary wall. The spiral, which contains many reversals, makes an angle of 20–45o with the axis of the fibre. It is doubtful if the pattern of spirals and reversals in the secondary wall is influenced by that of the primary wall.

The first layer of the secondary wall–the winding–is visible in depectinized fibres which have been swollen in cuprammonium hydroxide, as steep spiral strands which make an angle of 20–30o with the long axis of the fibre. Between thirty and fifty reversals occur in the direction of the spiral. The fibrils of subsequent layers of the secondary wall are finer than those of the winding and their pattern of spirals and reversals do not necessarily follow that laid down by the winding.

Between twenty and fifty growth rings, each between 0–4 and 0 12μ wide, are visible when transverse sections of cotton fibres are treated with a swelling agent. There is disagreement as to the cause of growth rings; they may be due to the deposition of a more porous zone at night and a more compact zone during the day. If plants are grown in continuous light, no growth rings are seen in the swollen fibres. In fibres not treated with a swelling agent, however, only ten to twelve growth rings, each about i‐oμ wide, were found, while five to seven growth rings were seen in fibres taken from plants which were grown in continuous light.

Irreversible twisting occurs when the cell wall of a fibre dries. Changes in the direction of the convolutions are determined by the reversal points in the spiral fibrils of the secondary wall.

Differentiation of lint and fuzz is believed to be a continuous process, beginning just before the flower opens, and if fertilization occurs, continuing up to the 28th day after flowering. The fibres at the base of the seed often develop first, and are longer than those which arise at the tip of the seed.

The elongation of the fibre occurs during the 25–30 days after flowering. Towards the end of this period there is a deposition of the secondary wall which may continue up to the 78th day after flowering. The growth in length and the deposition of the secondary wall vary according to the variety of the cotton plant and the environmental conditions.

Although the theory of the deposition of cellulose by ellipsoidal particles is not entirely disproved, it is much criticized and not generally accepted.