2019-03-01 Biology Times

Biology Times (^19)
March
Normal secondary growth
Secondary development may be seen in both roots and stems. In either case secondary development is
observed both in cortex as well as in the vasculature. Secondary growth in the cortex forms the periderm
tissue while secondary growth in the vasculature produces secondary xylem and secondary phloem.
Secondary growth in a dicotyledonous stem
In a dicotyledonous stem, secondary growth is seen in the vascular cylinder as well as in the cortex.
An intrafascicular cambium produces the secondary vascular tissues while a cork cambium produces the
periderm tissue.
Secondary growth in the vasculature
In most of the dicotyledonous stems the primary
vascular bundles are collateral, open and endarch,
arranged in the form of a ring surrounding central
pith. Between xylem and phloem in each vascular
bundle are present strips of cambia. These
constitute the intrafascicular cambia. The presence
of primary medullary rays between the vascular
bundles is responsible for the discontinuity of the
cambial ring. At the initiation of the secondary

growth, the parenchymatous cells of the
medullary rays become meristematic. Thus new
strips of meristems are formed in between the
vascular bundles adjacent to the intrafascicular
cambia. These newly formed cambial strips
constitute the interfascicular cambia, Both intra
and interfascicular cambia join together to form a
completely closed ring of cambium.
Formation of cambium ring:
In dicot stem, vascular bundle is conjoint,
collateral and open type. In this type, cambium
is present between the xylem and phloem.
This cambium is called vascular cambium or
intrafascicular cambium. During secondary
growth certain parenchyma cells of medullary
rays adjacent to intrafascicular cambium undergo
dedifferentiation and become meristematic.
This is called interfascicular cambium. The
intrafascicular and interfascicular cambium join
end to end to form cambium ring.
Formation of secondary xylem and secondary
phloem:
The cells of cambium ring are meristematic, they
divide and produce new cells both towards inside
and outside the cambium ring. The cells formed
towards inside differentiate into secondary xylem
and cells formed towards outside differentiate into
secondary phloem. The cambium ring produces
more secondary xylem than secondary phloem.
The cells of cambium ring are differentiated into
fusiform initials and ray initials. The secondary
xylem and phloem are developed by the fusiform
initials. The secondary medullary rays (xylem
and phloem rays) developed by the ray initials
(the secondary xylem and phloem arise from the
fusiform initials).