• During the formation of vascular tissue, the innermost cells
of the procambial strands of the stem form the protoxylem.
Cells at the outer side of the procambial strands form the
protophloem. Metaxylem and metaphloem tissues are
formed subsequently.
• Cells of the procambial strand may also differentiate into
fibres, a form of sclerenchyma.
• The formation and enlargement of more robust metaxylem
and metaphloem tends to crush the first-formed vascular
tissue, particularly the protophloem.
primary growth in roots
• The apical meristem of the root is a mass of irregularly
arranged cells, which divide and cut off new cells in all
directions. The cells which cut off ahead of this meristem
form the root cap. In monocots and many other plants,
calyptrogen forms root cap.
• Dermatogen, periblem and plerome form the root tissues in
a fashion similar to stem development.
• The bulk of the cells cut off by the root apical meristem give
rise to the central procambial strand, and to the tissues
of the cortex. Protoxylem and protophloem develop from
the procambial strand cells followed by metaxylem and
metaphloem.
• The central vascular tissue is referred to as the stele. The stele
is surrounded by a single layer of cells, the endodermis, and
immediately within this layer is a layer of cells, the pericycle.• After primary growth is completed and the mature primary
tissues of stem and root are formed, some procambial
strand cells remain in the vascular bundles of the stem
and in the stele of the root. These meristematic cells lie
between the metaxylem and metaphloem, and are known
as cambium. Cambium cells are capable of further growth,
leading to secondary growth of roots and stems.Development of cambium in dicot plants• The developmental phase in plant life when rapid primary
growth occurs is called juvenile phase. This phase begins
with young seedling and continues until the plant begins
reproductive development. In some plants, juvenile phase
extends for a long period, sometimes indefinite, e.g., English ivy.
Juvenile phase is followed by maturity. At maturity plant shows
reduced vegetative growth and active reproductive growth.All the cells of root apical meristems do not divide at the same rate. Determinations of relative rates of DNA synthesis
show that primary roots of Zea, Vicia and Allium have quiescent centres to the meristems, in which the cells divide rarely
or never in the course of normal root growth (Clowes, 1956a, b). Such a quiescent centre includes the cells at the apices of
the histogens of both stele and cortex. Its presence can be deduced from the anatomy of the apex in Zea (Clowes, 1954),
but not in the other species which lack discrete histogens. It is known that by cutting away part of the meristem the cells of
the quiescent centre can be stimulated to divide (Clowes, 1953, 1954) so it is probably their position within the apex which
causes them to be quiescent in normal growth. It is also known that at some stage in embryos and primordia of lateral roots
the cells are all meristematic. The quiescent centre must therefore develop from meristematic cells.- Name the tissue that gives rise to root cap in monocots. What kind of tissue is it?
- What is the significance of quiescent centre to the roots of a plant?
secondaRy gRowth
• In dicots of angiosperms and gymnosperms, the primary
body forms the fundamental structure and increase in girth
and diameter takes place by the formation of secondary
tissue. This formation of secondary tissue is called secondary
growth.
• Secondary growth is the growth in thickness due to the
formation of secondary tissues by lateral meristems. With
the exception of some annuals, most of the dicots and
gymnosperms show secondary growth in their roots.
• It takes place by the production of two types of secondary
tissues: secondary vascular tissues and periderm. These
tissues are formed by meristems, vascular cambium and cork
cambium respectively.
• Secondary growth results in the formation of a large amount
of secondary xylem called wood and an external layer known
as bark.
secondary growth in dicot roots
• Secondary growth in dicotyledonous roots occurs by
the initiation and activity of two secondary meristems–