Secondary growth in stems leads to increased stem width. This exerts pressure on outer cell layers, eventually rupturing the epidermis. Cork cambium then develops in the exposed cortex and produces cork tissue. Cork is impermeable and protects the living inner tissues from pathogens and environmental conditions. It is composed of dead cork cells and living secondary cortex cells. Lenticels also form and allow for gas exchange, compensating for the lack of stomata beneath the impermeable cork layer.

1. Secondary growth in extra stellar region due
to activity of Cork cambium

2. Formation of cork
• The addition of secondary xylem and secondary phloem in the stem
results an increase in the width of the stem.
• Secondary xylem which is produced on the inner side of the cambial
rings exerts pressure on the overlying cambial ring.
• The cambial ring exerts pressure on the secondary phloem which
exerts pressure on the primary phloem and so on.
• Ultimately the pressure is exerted on upper most layer of the cells, the
epidermis.
• As a result of this pressure epidermis stretched and get rupture from here
and there.

3. • This rupturing of epidermis appears as the cracks and
under such conditions,
• The underlying living tissue of cortex is now directly
exposed to the environment.
• The environmental conditions like temperature and pH
can directly effect this exposed living tissue.
• Microorganism/pathogens like bacteria and fungi can
directly infect this living tissue.
• Due to above mentioned facts it becomes necessary that the living
tissue of cortex must be protected.

4. Extra-stelar Secondary Growtb or tbe Dicot Stem
• Under these conditions, the outer most layer ot the exposed cortex changes
into meristematic tissue and is known as cork cambium orphallogen
• The cells of phallogen divide activately and new cells are produced both on
upper side as well as on lower sides.
• The cells which are produced on the lower side remain living and are known
as secondary cortex or phelloderm.
• The cells which are produced on the upper side of phallogen are non
living/dead and are known as cork or phellem

5. Cork possesses the following characters
• It is a nonliving/dead tissue.
• It is a bad conductor of heat.
• Microorganism like bacteria and parasitic fungi cannot attack on the dead tissue.
• It is impermeable to water.
• Due to the formation of cork the inner living tissues of the stem are well
protected.
Formation of periderm
• Cork cambium (phallogen), secondary cortex (phalloderm) and cork (phellem) are
collectively known as periderm
• It develops in the regions of cortex and is a secondary tissue by origin.
• In some plants the periderm persists for a number of years, adding new cork cell each
year.

6. • But in many trees cork cells are broken and new periderm layers are deeper in the
stem.
• Periderm layer generally develop in the region of the cortex but ultimately these also
occupy the secondary phloem region.
• Dead tissues on the surface of the stem known as bark
• In the botanical sense bark is the tissue lying external to the living phellogen.

7. • But in medicinal term bark is the, cork cambium, secondary cortex, primary
cortex, endodermis, pericycle, primary phloem and secondary phloem, primary
xylem and secondary xylem (wood).
• In some trees bark comes out in the form of scale and is known as scaly bark as
in ova.
• In some other trees bark comes out in the form of a ring and is known as ring
bark as in wine.

8. CORK VERS US BARK

9. Formation of lenticels
• In the epidermis of a young dicot stem minute pores are
present here and there which are known as stomata
• These stomata serve for the exchange of gases and release of water in
the form of vapors.
• As a result of secondary growth an increase in the width of stem takes
place and cracks are produced in the uppermost lying layer of epidermis
of a dicot stem.
• In such cases when cork is produced by the activity of cork cambium,
the epidermis and Its stomata dies off because cork is a dead/non
living/lmpermeable tissue.
• Under such circumstances small dots/cells appear on the surface of stem
which are known as lenticels.
lenticels

10. Estra.stelar Seeoodary Growtb or the Dicot Stea•
• Lenticels are generally produced from the phellogen just
beneath the places in the epidermis where stomata were
present.
•Phellogen at these places produce thin walled parenchymatous
cells which are more or less rounded in shape and develop large

11. inter cellular spaces among themselves instead of giving rise to
a cork tissue.

12. Estra.stetar Secoodary Growtb or the Dicot Steuy
• Each lenticels consists of a group of loose and thin walled
cells which are living in nature and are known as
complementary cells
• These complementary cells are separated from each other
by intercellular spaces and through these intercellular spaces

13. exchange of gases and evaporation of water in the form of
vapors can easily takes place, and this compensate the
absence of stomata.