2. ANOMALOUS SECONDARY THICKENING IN
STORAGE ROOTS - BEETROOT
• In many dicots, the root functions as a
storage region.
• The secondary growth in these storage
roots is usually anomalous type.
• Example:
• Secondary thickening in Beta vulgaris
(Beetroot - Chenopodiaceae)
• Secondary thickening in Ipomoea batatas
(Sweet potato - Convolvulaceae)
3. ANOMALOUS SECONDARY THICKENING IN
BEETROOT ( BETA VULGARIS )
Primary structure :
• The primary structure is a typical cotyledons type root with radial - diarch stele. Due to
the storage nature of the roots, the cell number is increased in all the parts.
• In beetroots , the secondary growth takes place by
( 1 ) The abnormal activity of the cambium and
( 2 ) The abnormal position of the cambium
4. Secondary Structure :
• The secondary growth is initiated by the formation of cambial ring.
• The cambial ring is formed from the cells of pericycle outside the proto - xylem and
parenchyma between the xylem and phloem.
• The primary cambial ring behave abnormally, it produces:
• Inner side : Secondary xylem and parenchyma in alternate groups.
• Outer side : Secondary phloem opposite to the secondary xylem and parenchyma
opposite to inner parenchyma.
• This result in the formation of a ring of conjoint, collateral vascular bundles embedded
in parenchymatous cells.
5. • The parenchyma cells
function as the storage
region.
• The cambium after
sometime ceases its
activity.
• Then, a second ring of
cambium (accessory
cambium) arises just
outside the first (from the
phloem parenchyma cells
or outer ring of
pericycle).
• The second ring of
cambium also behaves
abnormally as the first
ring.
6. • Produce a ring of conjoint , collateral vascular bundles embedded in
parenchymatous cells .
• These vascular bundles are separated by wider parenchymatous gaps which
function as the storage region .
• The first accessory ring of cambium also ceases its function and additional
cambial rings are successively formed .
• Towards the later stages , the developing cambial rings enclose a few layers
of pericycle cells with are meristematic in nature.
• These meristematic cells divide rapidly to form multilayered parenchymatous
cells.
• These parenchymatous cells function as the storage region.
7. • They get filled with the food materials.
• With the passage of time, more and more proliferative parenchymatous cells are formed.
• Due to the formation of proliferative parenchyma , the vascular bundles get separated
further apart.
• Subsequent cambial ring formation results in the production of vascular bundles and
proliferative parenchymatous cells.
• All these proliferative parenchyma cells get filled with reserve food materials .
• In a cross - section through the storage portion of the root, the parenchymatous ring
appears as dark red (due to the presence of anthocyanin pigments) whereas, the vascular
bundle ring appear as light in colour .
• In the mature root , bark formation also occurs.
