Page 304 - Handbook of Properties of Textile and Technical Fibres

278 Handbook of Properties of Textile and Technical Fibres

section into an oval shape. Leblicq et al. (2015) show that, through bending tests, the
presence of coreerind increases the effective bending resistance of the stem. More-
over, the existence of a low specific gravity core drastically reduces the possibility
of brazier-type buckling (Schulgasser andand Witztum, 1992). For a flax stem, during
bending the shive limits the risk of ovalization and local buckling of the bundles. The
3
wooden part of flax has a low volume weight (about 110 kg/m ). It is used for the pro-
duction of particleboard, insulating bricks for building construction (most often mixed
with lime), or as agricultural mulch.
The varietal selection of flax (L. usitatissimum L.) has always focused on specific
criteria fulfilling requirements of farmers and textile workers. Stem transverse
sections from four varieties selected from the 1940s to 2011 have been compared
by Goudenhooft et al. (2017). Anatomical changes over time are highlighted. The
general organization of the stems has not changed. The most important point
involves the size of the gap between fiber bundles (Fig. 8.2) and the amount of fibers,
which can be improved by selection (the gap size decreasing toward zero and the
fiber area increasing from 7.8% to 13.4% of the tissue area per section). This
trend coincides with the increase in biomass production over time expected from
the selection work.

8.2.2 Plant development
Plant development happens in five stages: germination, stem growth, flowering,
formation, and maturation of seeds after which plant senescence and cell death occur.
In Western Europe, flax plants generally develop within 100 days, which means

variations of meteorological conditions in terms of temperature (a minimum of 5 C
being needed for flax growth), rain, sun, and light/night period can have significant
effects on fiber quality. Such an environmental impact can be observed along the
stem length when looking for example at the distance between two leaves (nodal
length) or at the diameter and length of fibers (Lefeuvre et al., 2015) (see Section
8.4.1). Flax bast fibers are primary phloem fibers and originate from procambium,
close to the apical meristem. Bast fiber initiation is coordinated with other tissue
formation, including xylem components and leaf primordia (Evert, 2006).
Throughout the growth of the plant, fibers develop around the outside and in three
distinct steps:

1. Fiber elongation occurs in the top 3e5 cm of the stem. Elongation of the individual fiber
cell is very fast and takes place within 2e3days at a rate of 1e2 cm per day. It is mostly
an intrusive elongation (Snegireva et al., 2010; Ageeva et al., 2005). At thesametime,
the cell diameter grows at most two- to threefold, while the length increases 3000e4000
times.
2. Fiber expansion and thickening of the walls occur below the snap point, a zone defined in
Gorshkova et al. (1997), located 6e8 cm from the top of the plant, where the stem changes
its mechanical properties (Gorshkova et al., 2003). Above this point the stem is soft, whereas
below it, the stem is stiff. The snap point is well pronounced during the fast growth stage
(approximately 3 weeks in the middle of flax plant development). Below the snap point,
cell wall thickening takes place in the fibers, with no further elongation.