secondary growth

1. Secondary growth of dicot stem
Manoj Das

2. Secondary growth in dicot stem
• The secondary growth can be defined as an increase in the
diameter or girth of the axis by the addition of secondary tissue
in extra stelar and stelar regions due to the activities of vascular
cambium and cork cambium. It is the diagnostic character of
gymnosperms and dicots. It is absent in monocots due to the
lack of cambium except Yucca, Dracaena palms etc.
• The secondary growth in dicot stem involves as-
• I. By the activity of vascular cambium
• 1. In dicot stem, the vascular bundles are conjoint, collateral
open type and arranged in a ring. The vascular cambium is
present in the form of a thin strip between the primary xylem
and phloem known as fascicular cambium. In the medullary
rays, it is present as

3. interfascicular cambium between the neighbouring vascular
bundles.
2. The joining of fascicular and interfascicular cambia leads to
the formation of a complete ring called as cambium ring.
3. The fascicular cambium produces secondary phloem to the
outer side and secondary xylem to the inside by mitotic
divisions. The amount of secondary xylem produced is much
more than the secondary phloem because the cambium is
more active on the inner side than the outer side.
The cambium cells divide radially to increase the growth in
circumference and tangentially to form secondar xylem and
secondary phloem. Primary phloem gets pushed outwards
and primary xylem inwards towards the pith.

5. 4. The secondary xylem occupies the major portion of the
stem forming a hard compact mass. The secondary medullary
rays are the living cells that run radially from pith to cortex.
They form a connection to transport sap and food radially
across the stem. These may also used for storage of food
during winter.
Primary xylem get pushed toward the center and become
non functional, the pith gets reduced.
Primary phloem gets crushed and is pushed outwards.
With the increase of secondary xylem, some of the peripheral
tissue also get stretched. It brings about the rupture of

6. endodermis, cortex and epidermis. These are replaced by cork
that develops from the cork cambium.
Some related terms
Dendrochronology- It is the branch of plant anatomy that
determine the age of plants by counting of annual rings.
Annual ring= spring wood + autumn wood
Spring or early wood-It is the wood formed during spring or early
summer. The vessels have large cavities and thin walls to increase
the rate of transport.
Autumn or late wood- It is the wood formed during autumn or
winter. Vessels are fewer in number. They have narrow cavities
and thick walls.

7. Annual ring- It is formed by combination of spring wood and
summer wood of a year.

8. • Porous and non porous wood
• In the gymnosperms, vessels are absent and the wood is
entirely made of tracheids. Such wood is called as non
porous wood.
• In dicots, the wood made up of tracheids and vessels is
called as porous wood.
• Heart wood and sap wood
• In the older trees, two types of wood heart wood and sap
wood are present. Differences between heart wood and sap
wood are as

9. Heart wood or duramen
• It occupies the central region of
wood.
• It is dead part of wood.
• It provides mechanical support
to the plant body.
• It is dark in colour due to the
deposition of tannin, gum, resin.
• It is resistant against termites,
microbe fungi etc.
Sap wood or alburnum
• It lies at the peripheral region of
wood.
• It is living part of the wood
• It helps in conduction of sap and
food materials.
• It is lighter in the color.
• It is suspectable to the termites,
microbes fungi etc.

11. Soft wood and hard wood
• Soft wood- It occurs in the gymnosperms mainly in conifers and
made up of entirely tracheids and non porous.
• Hard wood- It occurs in dicots and made up of both tracheids
and vessels. It is porous wood.

14. II. By the activity of cork cambium
• With the growth of secondary xylem, the tissue on outer side
become compressed and stretch side ways. Ultimately, the
epidermis ruptures. The cells of meristematic tissue i.e cork
cambium or phellogen which lie just below the epidermis produce
cork or phellem to outside and secondary cortex or phelloderm to
the inside. The epidermis is relaced by periderm which is made of
cork cambium or phellogen, cork or phellem and secondar cortex
or phelloderm.

16. Some related terms
• a. Cork- the cork cells are dead, prism like in structure and
highly suberinised. It is brown in colour. Being light,
waterproof and highly compressible, it is used as stopper for
bottles.
• b. Bark – All the tissues outside the vascular cambium form
bark. It includes periderm, phloem and cortex. It may be-
• Ring bark- If the cork cambium forms a complete cylinder of
bark around the stem, the bark is called as ring bark and
occurs in Betula utilis commonly known as bhojpatra.
• Scale bark- If the cork cambium produces small strips of bark
is called as scale bark and occurs in Psidium guajava
Eucalyptus etc.

17. • Lenticels- These are small opening pores in the periderm
that allow gaseous exchange. They appear as scars or raised
portions on the surface of stem. These usually appear below
the stomata.