Question

Explain the process of secondary growth in stems of wood Angiosperms with the help of schematic diagrams. What is its significance?

Answer 1

Hint: The growth of roots and stems in length with the help of apical meristems is called the primary growth whereas the growth of roots and stems in girth with the help of lateral apical meristems is called the secondary growth.

Complete Answer:
> Angiosperms include Dicotyledons and monocotyledons. Primary growth is seen by all plants whereas secondary growth is seen only in dicotyledonous plants. Monocots do not show secondary growth due to absence of vascular cambium in them.
> Intrafascicular cambium, interfascicular cambium and cork cambium are the lateral meristems that result in secondary growth of dicots.

In Dicot stems, the secondary growth takes place as follows :
Formation of cambial ring :
- In dicot stems, vascular bundles are arranged in a ‘ring’ like manner around the central pith. The vascular bundles are conjoint and open.
- The bundles are open as they possess the cambium, also called as fascicular cambium or intrafascicular cambium.
- In young stems, it is present in the patches. It is between xylem and phloem but later these different patches join and form continuous rings.
- Medullary rays lie in between the vascular bundles.
- When their cells become dedifferentiated, they give rise to the new cambium which is called an interfascicular cambium as this lies between intrafascicular cambium.
- Interfascicular and intrafascicular together form the vascular cambial ring.

Activity of cambial ring :
- The vascular cambial ring becomes active and begins to divide to form new cells both towards inner and outer sides.
- As xylem is towards the pith and phloem is towards the periphery, the formation of cambium breaks the xylem and phloem.
- Thus the cells towards the pith form secondary xylem and that towards periphery forms secondary phloem.
- Secondary xylem forms outer to primary xylem bur secondary phloem forms inner to primary phloem.
- The cambium is generally more active on the inner side than on the outer and as a result, the amount of secondary xylem produced is more than secondary phloem.
- Thus the cambial ring expands towards the periphery.
- The primary and secondary phloem gets crushed and primary xylem remains intact, thus secondary xylem keeps on increasing.
- At some places, the cambium forms a narrow band of parenchyma which passes through the secondary xylem and the secondary phloem in the radial directions. These are known as medullary rays. These conduct food, water and minerals from centre to the periphery of the organ.

Note: The activity of cambium rings is not uniform throughout the entire year but is mainly affected by climate, temperature etc. In the spring season, the cambium is very active and produces spring wood or early wood. In winter, cambium is less active and forms autumn wood or late wood.