Very thin outer layer containing wax amd pectic material
- A peculiar group of carbohydrates of very complex composition.They are usually present as C and Mg salts.In immature fibres there is relatively high amount pectins (6%).
- In mature fibres it is relatively high amount pectins (0.9-1%).
- Decrease in pectin content with parallel increase in cellulose content proves that pectin is the parent substance from which cellulosic is formed.
- Function is to protect the fibre from atmospheric oxidation.
C. Primary wall:
- Composed of cellulose, Pectic and fatty matter.
- Formed in the first phase of growth.
- Cellulose fibrils are disposed transversely or circularly to produce high peripheral strength and also makes it weaker in length wise direction of the fibre and account for the low strength of immature fibre.
- In all native-cellulose fibres, the molecules are highly oriented parallel to one another, but they spiral round the fibre, thus reducing the degree of orientation parallel to the fibre axis.
- In flax, ramie, hemp, and other bast fibres, the spiral angle is small – less than 6° – so that these fibres are highly oriented and give high strength and low extensibility.
- In cotton, however, the spiral angle lies between 20° and 30°, and the fibres can extend more easily by stretching the spiral.
D. Secondary wall:
- Formed during second phase of growth and makes up about 90% of the total weight.
- This wall is composed of successive layers of cellulose deposited on the inner side of the primary wall without increase in diameter.
- Strength of the fibre is determined by secondary wall.
- Central hollow canal whose dimensions varies over a wide range.
- Contains protein, mineral salts and pigments.