Agriculture Reference
In-Depth Information
of the stem is grooved and its diameter increases
from the plant's apex to its base. Morphological
characteristics such as height and diameter are,
however, very dependent on the species, envi-
ronment and stage of development of the plant.
Male plants, for example, are between 10 and
15% taller than their female homologues,
whose stems are wider. Fine stem diameters are
promoted by a low sowing density, which also
is propitious for the development of tall plants
(Bocsa and Karus, 1997).
The formation of the stem tissues starts with
the generation (division and differentiation) of
cells derived from the meristem (Esau, 1977;
Robert and Catesson, 2000) (Fig. 3.2). The
meristematic tissues are composed of undiffer-
entiated cells that support the production
of new cells capable of acquiring a range of
specific functions (protection, metabolism, sup-
port, sap transport). Two types of meristem
are recognized. The primary meristem is
located at the apices of the stem and is respon-
sible for the primary growth of the internode
as well as the formation of leaves. This growth
arises through division and growth of the
different parenchymal, conducting and protec-
tive tissues. Primary growth essentially allows
organs to lengthen and results in the gener-
ation of primary tissues. Lateral growth (widen-
ing) of the stem ('secondary growth') comes
from the laterally located secondary meristem-
atic tissue (cambium). The vascular cambium
produces secondary xylem towards the inside
of the stem and secondary phloem towards the
outside of the stem. The cambium arises from
the procambium, which itself is derived from
the primary meristematic tissues that give rise
to the primary xylem and primary phloem.
Cambial activity is thus responsible for the
production of secondary xylem and is a key
component of the stem of the hemp plant.
The xylem which supports the weight of
the plant is responsible for the transport of min-
erals. These functions are carried out by the
fibrous walls and lumen respectively (Esau,
1977). The anatomy of the xylem of the hemp
plant is very similar to that found in other dicoty-
ledenous annual plants such as lucerne, although
the cellular distribution seen in flax shows some
variation (Day
et al
., 2005). The bark is made
up primarily of long fibres (sclerenchyma) asso-
ciated with phloem tissue, parenchyma and sur-
face epidermis. Two types of fibre can be
distinguished, forming two rings around the
central xylem (Esau, 1977) (Fig. 3.2):
1. The primary fibres arise from primary
phloem cells, derived from the primary
meristem.
2. The so-called secondary fibres are gener-
ated from the cambium.
The primary fibres can be distinguished
from the woody (pith) fibre by their length
3.3.3
Anatomy and origins of stem
tissues
The principal products of hemp are derived
from the stem, for it is a source of fibres with
remarkable physico-chemical properties. The
anatomy of the stem tissue will therefore be
presented in detail. The stem is produced as a
result of the sequential addition of internode
sections (the section of stem between the inser-
tion of two successive sets of leaves).
The constituent tissues of the stem - their
nature and origin
The stem of the hemp plant, as with other
annual dicotyledons, demonstrates two tissue
zones: a central woody area (pith) and an exter-
nal epidermis, or bark. Together, these perform
a number of special functions (Bowes, 1998):
•
The epidermis forms a protective barrier
around the stem and modulates exchanges
between the plant and its environment.
The parenchymal tissues responsible for
•
metabolic processes such as photosynthe-
sis are poorly represented in the stem of
mature plants.
The supporting tissues that provide the
•
plant and its organs (collenchyma, scleren-
chyma) with its rigidity and suppleness are
found in the stem's outer layer.
The tissues responsible for the movement
•
of sap are of two types: xylem tissues move
water and soluble mineral nutrients from
the roots through the plant, whereas phloem
tissues transport organic nutrients from
the site of photosynthesis (leaves) to non-
photosynthetic tissues around the plant.
