The following points highlight the two types of secondary tissues in dicot root of Plants. The types are: 1. Secondary Vascular Tissues 2. Secondary Ground Tissues or Periderm.
Type # 1. Secondary Vascular Tissues:
Stage I:
Conjunctive parenchyma cells lying on the inner edges of the primary phloem bundles become meristematic. They give rise to small quantity of secondary xylem on the inner side and secondary phloem on the outer side. In the process, these cambial strips and primary phloem bundles are pushed slightly to the outside.
Stage II:
Conjunctive parenchyma cells on the lateral sides of the phloem bundles as well as pericycle cells lying outside the protoxylem ends become brick-shaped and meristematic. This gives rise to a wavy band of a vascular cambium (Fig. 6.24 C).
The vascular cambium of the root is a complete secondary meristem. It continues to form secondary xylem on the inner side and secondary phloem on the outer side. Secondary phloem consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Secondary xylem is similarly made of vessels, xylem parenchyma and xylem fibres.
Stage III:
Initially vascular cambium derived from the pericycle gives rise to only ray cells. Hence wide multi-seriate primary vascular rays (also called medullary rays) are formed opposite the protoxylem points. However, the formation of ray cells is slower than the formation of secondary vascular tissues. As a result the depressed parts of vascular cambium move outwardly and ultimately the cambium becomes circular (Fig. 6.24 D).
Stage IV:
The ring of vascular cambium produces secondary xylem on the inner side and secondary phloem to the outside. Both of them are in the form of rings (Fig. 6.24 D-E) (c.f, primary vascular bundles). The primary phloem gets crushed by the growth of secondary vascular tissues.
The older secondary phloem is also partially destroyed as the new phloem becomes functional. The primary and secondary xylems persist. Primary xylem is distinguishable by its exarch nature and central position.
As compared to the primary xylem, the vessels of the secondary xylem are broader and thinner. Annual rings are not very sharp because unlike aerial climate, the climate of the soil does not vary much during different seasons.
The secondary phloem is made up of sieve tubes, companion cells and phloem parenchyma. Sclerenchyma fibres are rare. The secondary xylem is formed of vessels, tracheids and xylem parenchyma.
At places the vascular cambium possesses ray initials. They produce vascular rays. The rays are made up of two parts, xylem or wood ray (present in secondary xylem) and phloem ray (present in secondary phloem). They help in radial conduction of substances.
Type # 2. Secondary Ground Tissues or Periderm:
The pericycle layer, either directly or after a few divisions becomes converted into a secondary meristem called cork cambium or phellogen (Fig. 6.24 D-E). Rarely phellogen appears in the cortex.
The cells of phellogen divide both towards the outside as well as inside. The tissue formed towards the inner side is parenchymatous and known as secondary cortex or phelloderm. It is only a few layers in thickness.
The cells formed on the outside by the phellogen are rectangular and compactly arranged. They soon become dead. Their cavities get filled up with tannin and their walls become suberized. They are described as cork cells. The tissue of cork cells is spoken as cork or phellem (Fig. 6.24 F). The cork is impervious to water.
It protects the interior from mechanical injury and entry of bacteria. Primary tissues present outside the cork undergo starvation and get shriveled. Under the impact of secondary growth in the interior, the outer layers of the cork are also peeled off occasionally.
The phellem, the phellogen and the phelloderm are collectively called secondary ground tissue or periderm. At places the phellem or cork bears lenticels for exchange of gases (Figs. 6.24 E, 6.25).
