In this article we will discuss about the stages of secondary growth in dicotyledonous root with the help of suitable diagrams.
The roots of some herbaceous dicotyledons and of all gymnosperms and woody dicotyledons show secondary growth in thickness. The tissues of secondary origin in the dicotyledonous roots are basically similar to those of the stem, but the way of formation is different.
In dicot roots there are limited number of radially arranged vascular bundles with exarch xylem. Pith is very little or absent. A group of parenchyma cells beneath each phloem patch become meristematic and thus form strips of cambium which divide and produce secondary xylem inside and secondary phloem outside.
The cells of the uniseriate pericycle external to each protoxylem group start dividing to form a few layers. The first-formed cambium internal to each phloem patch now extends both ways and joins with the innermost derivatives of the pericycle and finally a continuous wavy ring of cambium is formed. The cambial cells produce much more xylem at a higher rate than phloem and thus the wavy cambium cylinder ultimately becomes circular.
Due to continued secondary growth the primary xylem is pushed towards the centre and the primary phloem is pushed towards the periphery. Finally, the centrally located exarch primary xylems are fused to give a star-like appearance in transverse section. The strands of secondary vascular tissues remain collaterally arranged like those of the stem.
The sieve elements of the primary phloem often get crushed. The cambial cells originating from the pericycle against protoxylem groups function as ray initials and produce broad bands of vascular rays which are also called medullary rays. These rays — running between xylem and phloem through the cambium are characteristic of the roots (Fig. 5.144).
To cope up with the pressure generated by the secondary growth, the periderm is formed in the outer region. Phellogen arises outside the pericycle. It produces cork cells on the outer side and some phelloderm inside. The pressure due to secondary growth ruptures the endodermis and cortex, which are ultimately sloughed-off. Lenticels may occur as transversely elongated rough areas.
The arrangements of tissues exhibited by the majority of the plants are considered as normal, variations from which found in minority are considered as anomalous. The anomalous structures found in the primary body are defined as primary anomalous structures, which occur in both monocotyledons and dicotyledonous stem, and may persist throughout the life of the plant.
Following are the common anomalous structures found in stems:
Occurrence of atactostele in dicotyledonous stem:
Normal arrangement of the vascular bundles in the dicotyledonous stem is more or less in a ring. In Thalictrum (Ranunculaceae), Podophyllum peltatum (Podophyllaceae), Papaver orientate and P. somniferum (Papaveraceae), Bougainvillea (Nyctaginaceae), Piper betle and Peperomia langsdorfii (Piperaceae), Nymphaea (Nymphaeaceae) etc. the vascular bundles remain irregularly scattered throughout the ground tissue.
Occurrence of arranged vascular bundles in monocotyledonous stem:
In monocot stems the vascular bundles are scattered in the ground tissue. But in Coix, Triticum, Oryza (Poaceae), Tamus communis (Dioscoriaceae) etc. the vascular bundles are more or less arranged in one or two rings.
Occurrence of cortical bundles:
In dicotyledonous stem the vascular bundles remain encircled by the endodermis to form the stele. But in some dicotyledonous stems accessory vascular bundles occur in the cortex in addition to stellar bundles. They remain scattered in the cortex and are referred to as cortical bundles. In most of the species the cortical bundles are the leaf trace bundles.
In Bombax malabaricum (Bombacaceae), Calycanthus occidentalis, Chimonanthus (Calycanthaceae) four pericyclic bundles (leaf traces) occur with internal phloem and external xylem.
In Campanula pyramidalis (Campanulaceae), Crepis, Senecio, Vernonia (Compositae), Cotyledon (Crassulaceae), Momordica (Cucurbitaceae), Armeria, Plumbago europaea (Plumbaginaceae), Peltiphyllum (Saxifragaceae), petiole of Eryngium campestre, Siler trilobum (Umbelliferae), Nyctanthes arbortristis (Oleaceae) four inversely oriented cortical vascular bundles occur with xylem outside and phloem inside.
Occurrence of medullary bundle:
The extra vascular bundles may be found in the pith of some dicotyledonous stem in addition to the normal ring of bundles, which are referred to as medullary bundles. In Acanthus (Acanthaceae) collateral and inversely oriented medullary bundles occur. In Achyranthes aspara (Amaranthaceae) two medullary bundles at internode and in Cyathula prostata four medullary bundles are observed.
Occurrence of internal bundles:
This type of bundles is found below the normal ring of bundles but above the medullary bundles. These bundles are referred to as internal bundles. In Rumex crispus, R. orientalis etc. this type of bundles are observed.
Occurrence of internal phloem or intraxylary (perimedullary, medullary) phloem:
The phloem that occurs at the periphery of the pith in the form of a continuous cylinder or strands is referred to as intraxylary phloem or internal phloem or medullary phloem. In Wrightia, Vinca, Thevetia, Landolphia (Apocynaceae); Calotropis (Asclepiadaceae); Convolvulus, Ipomaea, Evolvulus (Convolvulaceae); Acanthus, Barleria (Acanthaceae); Solanum, Nicotiana (Solanaceae) etc. such type of bundles are found.
Absence of vessels in angiosperm:
Normally the xylem of angiosperms contains vessels but in certain members of the families Tetracentraceae (Tetracentron), Trochodendraceae (Trochodendron) and Winteraceae (Drymis, Pseudowintera, Bubbia, Zygogynum) vessels are absent.
Occurrence of polystele in dicotyledons:
Usually the vascular bundles of dicotyledonous stem remain encircled by a common endodermis to form monostele, but variations occur when each vascular bundle becomes encircled by an endodermis and thus forms a polystelic structure. The nodes of Nymphaea (Nymphaeaceae) and Parnassia palustris (Parnassiaceae) form polystele.
Dianthera americana forms seven steles and each stele is represented by a vascular bundle completely enclosed by an endodermis. One stele is located at the centre and the rest towards the periphery.
Occurrence of separate xylem and phloem bundles:
Normally, xylem and phloem together form the conjoint bundles. In some abnormal cases, vascular bundles may consist of either only xylem or phloem in addition to the normal bundles.
In Cuscuta fully developed phloem strands occurs interspersed with other normal collateral bundles. Incomplete phloem bundles are found in Rumex crispus, Ricinus communis, Xanthium strumarium, Antigonon leptopus, Achyranthes aspera, Mirabilis jalapa, Boerhaavia diffusa etc.