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Plant Stem Structure

Industrial hemp fibre is obtained from the stem, which has two main parts: surrounding fibres i.e. bast fibres and the middle woody part called a shive. 

The outside of the stem is covered with the cuticle and the epidermis, which is covered with many little hairs. Inside of it there are the collenchyma tissue; the main tissue with circles of fibre bundles; the cambium; the woody part with vessels of different sizes and the pith. The cells of the pith disintegrate gradually by the flowering stage and a cavity forms in the centre of the stem (Reimets, E., 1976).

Bast-fibre bundles consist of thick-walled intertwined elementary fibres that are connected with each other through pectin 'tiles'. Such pectin tiles also connect the fibre bundles with the surrounding main tissue. The cross-section of the elementary fibres is a polyhedron with walls structured in layers. The length of an elementary fibre is usually 30 to 50 (10 to100) mm and the diameter is 18 to 25 µm (Reimets, E. 1976). It consists of cellulose, hemicellulose, lignin etc.


Cross-section of a hemp stem

The fibre bundles at the top of the stem are situated more densely and the fibre content is larger than at the bottom. The outer bundle circle contains the most fibre. Moving inwards, the fibres become more woody, less elastic and shorter and the fibre bundles are smaller. 

Hemp shives are made up of tracheids, parenchyma cells and wood fibres, which transport water and nutrients and ensure that the stem remains upright and rigid.
 
Data on the lignin and cellulose content of bast fibres and shives are by and large the same in different sources but specific figures depend on the growth stage and growth conditions. The spectroscopic analysis of fibre shows that as the plant ages, the lignin content increases and pectin content decreases (Bullard, M. 2000) – the stems of the plant start to lignify. Because of this, it is more difficult to separate bast fibre from shives in later growth stages and the risk of harming the fibres may increase.
The following table shows the average content of the most important components of bast fibre and shive in the stem of adult plants.
 Fibre length (mm)Cell width (µm)Ash content (%)Alpha-cellulose (%)Hemicellulose (%)Lignin (%)
Bast fibre 5 to 55
Average: 30
18 to 25
Average: 22
5 62 to 67 8 to 15 4
Shive 0.5 25 N/A 35 to 38 18 to 31 18 to 20

So far, bast fibres have been economically more important and widely used in different fields (textile industry, composite materials etc), where fibre has more added value than the shive.
 
The merit in using bast fibre in different ways depends largely on its properties and quality after decortication i.e. the mechanical separation of fibre from the lignified parts i.e. shives. The main quality indicators evaluated in case of fibre are thinness (measured as linear density by Tex – mass in grams per 1000 meters), E modulus i.e. Young modulus i.e. elastic modulus (N/mm²), breaking strength i.e. tensile strength (cN – max tensile elongation at break) and toughness (tensile strength and linear density ratio N/tex).
 
 Separability and the resulting fibre properties (elasticity, tensile strength) are better in the earlier growth stage of the plant and deteriorate as the plant ages and the lignin content increases. Therefore, an optimal balance between the fibre quantity and quality ought to be found. Keller (2000) points out that fibre is easier to separe from unsoaked stems and the quality of the fibre is better if the yield is harvested before the seeds mature. Seed maturation is also considered the critical point when the stem starts to lignify and the quality of the fibre decreases (fibre becomes weaker). This harvesting time necessary for obtaining the best quality fibre is also referred to as the 'technical maturity' of hemp (Van der Werf, 1991, cit. Barron et al. 2003).

Mechanical properties of fibre separated from preprocessed hemp stems
 Thinness (Tex)Tensile strength (N)Toughness (N/Tex)
Unretted hemp 14.9 46.9 33.8
Vähe Slightly retted hemp 8.1 64.3 33.9
Retted hemp 7.4 72.6 43.2