Growth in length of the axis due to the activity of apical meristem is called primary growth. The increase in girth or thickness of the axis is called secondary growth. It is formed by the activity of the two lateral meristems, namely Vascular cambium and the Cork cambium.
Vascular Cambium
- Dicot stems usually have open vascular bundles.
- There is a meristematic layer between the xylem and phloem tissues. It is called fascicular cambium or intra–fascicular cambium.
- In young stem, it is seen as small patches between the xylem and phloem tissues. Later, it forms a ring.
Formation of Cambial Ring
- In dicot stems, some of the medullary rays (mostly which are present in line with fascicular cambium) becomes meristematic and forms a strip known as inter-fascicular cambium.
- This joins with the intra–fascicular cambium to form a complete ring known as the cambial ring or the vasccular cambium.
Activity of Cambial ring
- When the cambium becomes active, it forms new cells towards the inner and outer sides.
- The cells that are formed towards the pith become secondary xylem.
- The cells that cut off towards the periphery develop into secondary phloem.
- The cambium is more active on the inner side than the outer side. Hence the amount of secondary xylem formed is more than that of secondary phloem.
- Gradually the secondary xylem and phloem gets crushed due to the continuous formation of new secondary tissues. However, the primary xylem remains intact around the centre.
- At some areas, the cambium forms a narrow band of parenchyma that passes through the secondary tissues in radial directions. These are called secondary medullary rays.
Spring Wood and Autumn Wood
The activity of cambium is periodical. In most cases, it is seasonal and hence, the secondary xylem rings usually form the growth rings.
- The activity of cambium is under the control of various physiological and environmental factors.
- In temperate regions, the climate is not uniform throughout the year.
- In spring season, the cambium is very active and produces a large number of xylary elements.
- The spring is the most favourable season for plant growth. The wood is less dense with numerous large xylem vessels. It is called early wood or spring wood.
- In winter, the cambium is less active.
- The xylem is denser with fewer xylary elements having narrow vessels.
- This wood is called autumn wood or late wood.
Thus, with each season, new layers are formed in the xylem. They appear as rings in cross section and are called growth rings. The spring wood and autumn wood produced in one year together form an annual ring or growth ring. The count these annual rings can determine the age of a tree. This process is called dendrochronology.
Heartwood and Sapwood
- In old trees, the central region of secondary wood is filled with organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils.
- These substances make the central wood brownish, hard, durable and resistant to the attack of micro-organisms and insects.
- The vessels also become plugged with balloon like ingrowths formed from adjoining parenchyma cells through the pits called tyloses.
- This region is called heartwood. It gives mechanical support to the stem.
- The outer region is lighter and is composed of secondary xylem tissues.
- It is the functional part of the secondary wood.
- The conduction of water and minerals from root to leaves occurs through this part.
- This region is called sapwood or laburnum.