In most dicotyledonous stems and roots, distinct secondary growth is visible, which increases the diameter or thickness of the stems and roots.
Secondary tissues are formed due to the activity of the lateral meristems, i.e. cambium in the stellar region and cork-cambium in the extra-stellar region respectively.
The increase in thickness due to the addition of secondary tissues cut off by the cambium and the cork cambium in the stellar and extra-stellar regions respectively is spoken of a secondary growth.
In general secondary growth is not marked in monocot stem. However the stems of Dracaena. Agave Yucca etc. exhibit secondary growth. Secondary growth in dicot stems
Formation of Cambium ring the strip of cambium present between xylem and phloem of a vascular bundle is known as fascicular or vascular cambium. At first the parenchymatous cells of the modularly .rays in a line with cambium becomes maristematic.
This joins to the fascicular cambium on either side and forms a complete ring. The cambium ring is thus partly primary and partly secondary meristem in origin.
(A) Activity of the cambium and formation of secondary tissues
The cambial ring becomes active and begins to cut off new cells, both to-wards the inner and outer sides. The cells cut off to-wards the outer side get differentiated into the secondarily phloem. The cells to-wards the inner side get differentiated into the secondary xylem.
The secondary phloem consists of sieve-tubes, companion cells, phloem parenchyma and often patches of bast fibers. Many of the textile fibers of commerce such as jute, hemp, flax etc. are the bast fibers of secondary phloem.
The secondary xylem consists of scalar form and pitted vessels, tracheids, wood fibers and wood parenchyma.
The cambium is always more active on the inner side than on the outer. As I result, the xylem increases more rapidly than the phloem and soon forms a hard compact mass.
This forms the main bulk of the plant body. Due to the continued formation of secondary xylem, both the primary and secondary phloem of the earliest years gets gradually crushed.
At places, cambium forms some narrow bands of parenchyma which pass through secondary phloem and the secondary xylem. These are the secondary modularly rays.