Since the root lies underground, it is not subjected to variations of aerial conditions and therefore unlike stem, annual rings are
rarely formed in roots. Though the secondary wood forms the main bulk, the primary xylem bundles remain intact and can be
easily recognised under a microscope.
Fig.: T.S. of root of Tinospora
(Diagrammatic)
Cork
Primary phloem
Cortex
Primary xylem
Medullary ray
Pith
Secondary phloem
Secondary xylem
Cambium
Fig.: Detailed structure of a part of
T.S. of root of Tinospora
Cork
Cortex
Primary phloem
Cambium
Secondary phloem
Secondary xylem
Primary xylem
Pith
Medullary ray
Bark
All the dead tissues lying outside the active cork cambium constitute the bark of plant. It includes epidermis, lenticels, cork and
sometimes hypodermis or portion of cortex depending upon position of cork cambium. The deeper the origin of cork cambium,
the thicker is the bark. Bark protects against parasitic fungi and insects, prevents water loss by evaporation and guards against
variations of external temperature. It is insect repellent, decay proof, fire proof and is used in obtaining drugs or as spices.
Ring bark
When cork cambium appears in form of complete ring.
E.g., Betula.
Scale bark
When cork cambium appears in strips, it takes the form of scales.
E.g., guava.
It may be of two types
SECONDARY GROWTH IN DICOT STEM
The dicotyledonous stems are characterised by presence of fascicular (or intrafascicular) cambium (primary meristem) derived
from procambium of shoot apical meristem between xylem and phloem. Interfascicular cambium (secondary meristem) arises
from the cells of medullary rays which occur at the level of intrafascicular strips. This joins on to the fascicular cambium on
either side and form a complete ring known as cambium ring or vascular cambium.
The cells of cambium mostly divide by periclinal divisions. Out of the two cells produced from single cell of cambium, one
differentiates into secondary tissue and the other remains cambial cell. In this way the cambial cells, by repeated divisions, add
new cells either towards centre or towards periphery and the cambium still remains single layered.
Fig.:Diagrammatic representation of various stages
ofsecondary growthindicot stem
Primary phloem
Secondary phloem
Cambium
Secondary
xylem
Medullary ray
Primary
xylem
Pith
Fas cularci
cambium
Formation of
interfascicular
cambium
Cortex
Endo ermisd
Pericycle
Primary
phloem
Cambium
Epidermis
Metaxylem
Protoxylem
Primary xylem
Pith
Primary phloem
Secondary phloem
Vascular
cambium
Secondary xylem
Primary xylem
Pith
Fas cularci
cambium
Formation of
interfascicular
cambium
Cortex
Endo ermisd
Pericycle
Primary
phloem
Cambium
Epidermis
Metaxylem
Protoxylem
Primary xylem
Pith
Primary phloem
Secondary phloem
Vascular
cambium
Secondary xylem
Primary xylem
Pith
Fig.: Diagrammatic representation of various stages of secondary growth in dicot stem
A B C
Vascular cambium is composed of two types of initials - elongated spindle shaped fusiform initials and shorter isodiametric
ray initials.