Anomalous Secondary Growth

In most of the angiosperms normal secondary growth takes place. But many dicotyledons show deviation from the normal type of secondary growth. The growth which is in contrast to the normal secondary growth is known as the abnormal or anomalous secondary growth.
Anomalous secondary growth is commonly seen in tropical plants than in temperate plants. Haberlandt recongnized two types of anomalous secondary growths – adaptive and non-adaptive.
The anomalous secondary growth that takes place due to its functional requirements is called adaptive secondary growth. It takes place to fulfil the mechanical requirements of the plants as per their morphological requirements as per their morphological structure e.g., Aristolochia, Bignonia, Draceana.
The anomalous secondary growth that takes place due to environmental requirements and has no functional significance to the plants is called as non-adaptive secondary growth. Eg., Amaranthus, Achyranthes, Boerhaavia.
Usually, secondary growth does not take place in monocotyledonous plants. However, certain monocots are arborescent plants. To suit to their woody habit and to produce a strong stem, anomalous secondary growth occurs in these plants; e.g., Yucca, Agave, Dracaena.
In storage roots to produce the required tissue to store the food, adaptive type of anomalous secondary growth takes place; e.g., Beta vulgaris, Daucus carota etc.

Anomalous secondary Growth in Achyranthes Stem
The young stem has a wavy outline with alternate ridges and furrows.
Epidermis: Made up of single row of tubular cells. The cells are closely arranged with thick outer walls coated with lignin, followed by cutin. Several multicellular hairs are present over the cells of the epidermis.
Cortex: The peripheral hypodermal region is made up of collenchymas below the ridges and chlorenchyma below the furrows.
Endoermis: consists of single row of tangentially elongated parenchyma cells.
Pericycle: Made up of sclerenchyma, parenchymatous cells are also present.
Vascular Bundles: arranged in form of a ring.  They are conjoint, collateral, endarch and open. Medullary rays are present between the vascular bundles.
Apart from the primary vascular bundles, two medullary vascular bundles are present in the pith region. The medullary vascular bundles are conjoint, collateral, endarch and closed. These two bundles lie and grow opposite to each other.
Secondary Growth and Medullary Bundles:
In the pericycle region, extrastelar cambium strips develop which produce secondary vascular bundles. Cambium also produces the conjunctive tissue  between the vascular bundles. Secondary vascular bundles and conjunctive tissues are present without any sharp limits. So phloem of the secondary vascular bundles appears in the form of patches. This phloem is the included phloem.

Anomalous secondary Growth in Boerhaavia
In the primary structure, the stem can be distinguished into epidermis, cortex and stele
Epidermis:
 Single-layered epidermis consists of small, radially elongated cells. Multicellular epidermal hair arise from some cells. A thick cuticle is present on the epidermis. Some stomata are also present.
Cortex:
 It  well-differentiated and consists of few- layered collenchymatous hypodermis followed by chlorenchyma. Collenchyma is 3 to 4 cells deep, but generally it is only one-layered near stomata.
Chlorenchymatous cells are thin-walled, oval, full of chloroplasts and enclose many intercellular spaces.
Endodermis is clearly developed and made up  many, tubular, thick-walled cells.
Pericycle:
 Inner to the endodermis is present parenchymatous pericycle but at some places it is represented by isolated patches of sclerenchyma.
Vascular Bundles:
Vascular bundles are present in three rings. In the innermost ring are present two large bundles; in the middle ring the number ranges from 6 to 14 while the outermost ring consists of 15 to 20 vascular bundles.
 Vascular bundles of innermost and middle rings are medullary bundles. All the Vascular bundles are conjoint, collateral, endarch and open.
Two vascular bundles of the innermost ring are large, oval and lie opposite to each other with their xylem facing towards centre and phloem outwards.
Vascular bundles of inner and middle rings may show a little secondary growth. The cambium produces only a little amount of secondary xylem to the inner side and secondary phloem to the outer side. As a result the vascular bundles of these rings become slightly enlarged. Interfascicular cambium never develops between these bundles.
Anomalous Secondary Growth:
In the vascular bundles of outer ring, strips of cambia are formed between the vascular bundles, which fuse with the strips of the fascicular cambium and thus form a continuous ring of cambium. The fascicular cambium produces secondary xylem to the inner side and secondary phloem to the outer side. The inter fascicular cambium give rise to the conjuctive parenchyma to the inner side and secondary parenchyma to the outer side.
Very soon the activity of this cambial ring stops.
Later a cambial ring develops secondarily from the pericycle and becomes active. It cuts secondary phloem towards outer side and secondary xylem towards inner side. In between them conjuctive tissue is produced. As a result, a ring of collateral bundles separated by conjuctive tissue is formed. After a period of activity, this cambium also stops to function. Another ring of cambium arises outside, which behaves in the same pattern.
Thus, in this stem several rings of cambia arise successively in a centrifugal manner. The abnormal cambial ring produces xylem and conjuctive tissue on the inner side and phloem and parenchyma on the outer side. The resulting tissue gives the appearance of concentric rings of vascular bundles embedded in the conjuctive tissue.

Anomalous secondary Growth in Bignonia Stem
Bignonia is a woody climber or lianas. The mechanical requirements of woody lianas are different. The stems of lianes require flexibility to twin around the support. The secondary xylem formed through normal secondary growth is organized in the form of a cylindrical pole. This type of secondary growth is not useful to the lianeous plants. Hence, by the anomalous activity of the cambium, furrows are produced in the secondary xylem of these plants.
Primary Structure:
In T.S, the young stem exhibits the ridges and furrows in outline.
Epidermis:
 Single-layered epidermis consists of rectangular cells. A thick cuticle is present.  A few multicellular hair are also arising from some cells.
Cortex:
It is well-differentiated into collenchyma and parenchyma. Collenchyma is present below the epidermis in the ridges in young stem but at maturity there develops sclerenchyma.
 Parenchyma is present below the sclerenchyma or collenchyma in the ridges and directly below the epidermis in the grooves. Endodermis is undistinguishable from cortical cells. The cells lack casparian strips.
Pericycle:
 The pericycle shows alternate bands of sclerenchyma and parenchyma.
Conjoint, collateral, endarch and open vascular bundles are arranged around the pith in form of a ring.
Anomalous Secondary Growth
The interfascicular and intrafascicular cambium form the cambial ring. The activity of cambium is normal and it produces more secondary xylem to the inner side and little secondary phloem to the outside. But soon, at four places, the cambium produces more amount of secondary phloem on the outside and relatively small amount of secondary xylem on the inner side. As a result four deep furrows of phloem projecting into the secondary xylem are formed.
These furrows become deeper as the secondary growth advances. Each phloem wedge is provided with bars of sclerenchyma, which give them mechanical support and secondary phloem is not crushed. Although only four furrows are formed initially, their number increases gradually as the stem matures.
Though these stems appear to be strong, these are adaptive to bend and twine around due to the presence of furrows. A solid cylinder of xylem is likely to break under pressure of bending and twining. The furrows of the phloem present in the xylem act as shock absorbers and allow the stem to bend.
Hence, the anomalous secondary growth in the stem of Begnonia is a adaptive type.

Anomalous Secondary Growth in Dracaena
Monocotyledons normally do not show secondary growth due to absence of vascular cambium, but few plants like Dracaena, Yucca, Aloe etc show anomalous secondary growth.
Primary Structure:
Dracaena is an arborescent plant, belonging to the family Liliaceae. The young stem shows typical monocotyledonous structure.
Epidermis: Outer most layer made of single layer of cells with thin cuticle on the outside.
Cortex: Several rows of parechymatous cortex is present.
Stele: Several vascular bundles are scattered irregulary in the ground tissue. The vascular bundles are collateral, and closed.
Anomalous Secondary growth:
During the initiation of secondary growth, the parenchymatous cells of the cortex, external to the primary vascular bundles, become meristematic and forms the cambium in a ring. The cells of cambium divide and produce more cells towards the inner side and few cells towards outside. The cells produced on the inner side develop into vascular bundles and conjuctive tissue.
Each vascular bundle develops from a single cambial initial. The initial divides first by anticlinally to form a row of two or three cells. These cells undergo periclinal division, but the division soon become irregular. This results in the formation of a group of cells.
The peripheral cells develop into xylem elements, whereas the central cells differentiate into phloem elements. This leads to formation of amphivasal vascular bundles.
Formation of cork:
After certain period of secondary growth, the parenchymatous cells below the epidermis give rise to the meristematic initials. The initials divide several times periclinally. The cells thus formed become suberised and form the cork or phellem. This cork, with suberin coated cells and radial or storied arrangement is called as storied cork.

Anomalous Secondary Growth in Beta Vulgaris Root:
In the root of Beta vulagaris adaptive type of anomalous secondary growth is found. In this root additional tissue are required to store food and these are produced through secondary growth.
In Beta root the primary vascular system is diarch and exarch. As in the normal roots the xylem and phloem are arranged radially and alternately.
Anomalous Secondary Growth
A normal cambial ring is formed from the pericycle cells external to the protoxylem and the cells of the conjuctive tissue below the phloem.
This primary cambial ring exhibits anomalous activity, since it produces secondary xylem and parenchyma alternately on the inner side, instead of producing only xylem on the inner side and phloem on the outer side.
It forms secondary phloem opposite to the secondary xylem and parenchyma opposite to the existing parenchyma on its outer side.
Thus the vascular bundles formed are collateral in nature and arranged in a ring. These bundles are separated by narrow strips of parenchyma. This cambium becomes inactive after some time.
Then later, the cells of the pericycle undergo periclinal divisions and one of the inner layers behaves as cambium. This can be recongnised as the first additional cambium. Due to the activity of the additional cambium a ring of secondary vascular bundle is developed. These bundles are separated by wide radial channels of storage tissue.
Very soon the second additional cambial or the third cambial ring is formed from the phloem parenchyma or pericycle and organizes the third vascular bundle ring.
In this way, several rings of additional cambia are formed successively and produce the vascular bundle rings. In all these, parenchyma is present between the vascular bundles. Several parenchymatous layers are formed. These cells store food material. Storage sugars and anthocyanin pigment are also present in these cells.
In this way the thickness in the Beet root is increased by the activity of the concentric rings of cambium.
In a well developed Beet root, a phellogen is differentiated in the peripheral region, which produces the suberin coated, dead cell of the cork to the outside and secondary parenchyma to the inner side. The latter is known as the secondary cortex.
These secondary cortex and cork with cork cambium constitute the protective cover known as periderm.