Biology Today — December 2017

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.