Anatomy of Flowering Plants

Secondary Growth

Do you know how the age of a tree is calculated? Well, we simply count the annual rings that form in its trunk due to the wood growth. This lateral expansion or growth of a tree is called Secondary Growth. Lets us learn about it here.

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Secondary Growth

Growth in plants occurs in two ways: primary and secondary.

  • Primary growth causes the plant to grow in length, both below and above the ground, due to the apical meristems that are actively dividing into these regions.
  • Secondary growth causes the plant to grow in width due to the presence of lateral meristems or cambium layer which actively divides to bring about this kind of growth.

We need to understand that secondary growth occurs in both stems as well as roots.

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Secondary growth in stems

Secondary-growth-in-stems

   (Source: BiologyDisscussion)

As mentioned earlier, secondary growth occurs due to the lateral meristems that divide similar to the apical meristems.The cells of the lateral meristems divide rapidly and grow outwards laterally rather than apically as in case of primary growth.  The lateral meristems that cause secondary growth are known as cambium. This layer of cambium is present in dicots but absent in monocots.

Two layers of Cambium

Cork Cambium: The cork cambium makes a tough, insulating layer of cells called as cork.  These cells have wax in them, which helps them protect the stem from water loss. Cork is also a part of the bark.

Vascular Cambium:  The vascular cambium produces vascular tissue (xylem and phloem), which provide additional support for the shoot system in along with transporting water and nutrients. The xylem and phloem that arise from the vascular cambium replace the original (primary) xylem and phloem, and so are called as secondary xylem and secondary phloem. They add to the width of the plant. The vascular cambium is only single layer in thickness and adds xylem on the inside and phloem on the outside of it. In trees, the secondary xylem forms the wood and the secondary phloem forms the bark.

In cases of monocots, who lack cambium, secondary growth is not seen. The stems do not get as wide as in the dicots.

Secondary growth in roots

Secondary-growth-in-roots

(Source: UCD)

Secondary growth in roots leads to increase in the thickness of the root. This happens by the addition of vascular tissue.

Initiation of secondary growth takes place in the zone of maturation soon after the cells stop elongating there. The vascular cambium differentiates between the primary xylem and phloem in this zone. The pericycle is a cylinder of parenchyma or sclerenchyma or cells that lies just inside the endodermis and is the outer most part of the stele of plants.The pericycle cells divide simultaneously with the procambium. As a result, a cylinder of cambium is formed that encircles the primary xylem. Similar to the stem, the xylem is formed on the inside and the phloem towards the outside of the cambium.

Some roots form an outer protective layer called the periderm which originates from the pericycle and replaces the epidermis. The pericycle resumes its meristematic character and begins to divide periclinally again. At this point, it is called the phellogen or the cork cambium. This cork cambium forms cork cells towards the outside of the plant. They are suberized which makes the cells impermeable to water.

Solved Example for You

Q: Which part of the vascular cambium forms the bark in trees?

(a)    Endodermis        (b) Secondary phloem

(c)     Pericycle         (d) Secondary xylem

Sol. (b) Secondary phloem

It is the secondary phloem of the vascular cambium of dicot stems that forms the bark of trees. The secondary phloem forms the wood.

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