2. INTRODUCTION:
• The roots of gymnosperms and most dicots undergo
secondary growth,Ex.Sunflower. Most of the dicot
roots shows secondary growth in thickness, similar
to that of dicot stems. However the roots of extant
vascular cryptogams and most monocot do not show
any secondary growth; they remain entirely primary
throughout their life.
• The secondary tissue developed in the dicot roots
are fundamentally quite similar to that of dicot
stems, but the process initiates in some different
manner. Certain dicot roots do not show secondary
growth. The secondary vascular tissue originate as a
result of the cambial activity. The phellogen gives
rise to the periderm.
3. • The process of secondary growth in dicot roots is accomplished in 2 steps:
(a)Formation and activity of the vascular cambium
(b)Formation and activity of the cork cambium
A. The formation and activity of vascular cambium
The vascular bundles are arranged in a radial
manner.
The xylem and phloem are distinct from each other
in separate patches.
The xylem is of exarch type.
The vascular cambium forms as a result of
secondary growth hence it is not present right from
the beginning.
Conjuctive tissue becomes meristematic below
phloem bundles and from separate curved strips of
vascular cambium during the secondary growth in a
dicot root.
4. Now the cells of the pericycle lying opposite to protoxylem also become
meristematic to form additional strips of cambium.
In the way a complete ring of vascular cambium is formed due to joining of these
two.
The shape of the ring of vascular cambium is wavy in the beginning, but later on
it becomes circular due to the pressure of the secondary xylem.
The activity of vascular cambium of root is similar to activity of vascular
cambium of stem.
Vascular cambium forms secondary xylem towards the inner side and secondary
phloem towards the outer side.
The portion of vascular cambium which is formed by the pericycle is responsible
for the formation of pith rays. These are made up of parenchyma. These pith rays
are known as primary medullary rays(Multiseriate).
A few medullary of pith rays are also formed from remaining vascular cambium.
These are called secondary medullary rays(Uniseriate).Thus two types of
medullary rays are found in the secondary structure of roots.
5.
6. B. The formation and activity of the Cork cambium
The pericycle cells divide to generate the cork
cambium.
Therefore the cork cambium gives rise to the
periderm, which is responsible for replacing
reptured epidermis and cortial layers.
The cork cambium in root is a lot similar to the
cork cambium in the stem. This cambium forms a
dead cork towards the periphery and the living,
parenchymatous secondary cortex towards the
centre.
Pressure builds upon the peripheral layers of the
phellogen, that is the epidermis and the cortial
layer as a result they die and finally slough off.
