Abnormal Secondary GrowthPREPARED BY:AZKA JAVED

Secondary Growth

An increase in plant girth due to vascular cambium or cork cambium is called secondary growth.

Secondary growth & lateral meristems

Anomalous

Whereas the development, arrangement, activity of the vascular cambium in most woody dicotyledonous and Gymnospermous plants tends to be very similar, there are some alternatives which produce new secondary tissues that do not follow a normal pattern. As a result, the secondary plant structures that are formed are termed  anomalus. Most anomalous growth is associated with the formation of multiple cambia.`

Abnormal Secondary growth

Abnormal secondary growth does not follow the pattern of a single vascular cambium a thick hair clit producing xylem to the inside and phloem to the outside. Some dicots have anomalous secondary growth, e.g. in Bougainvillea a series of cambia arise outside the oldest phloem.

http://en.wikipedia.org/wiki/File:Monocot_Stem.jpg

Abnormal growth in monocot

Most monocots either have no secondary growth or else anomalous secondary growth of some type;

Monocotyledons  

Dracaena (the Dragon's blood tree) is the only monocot which has been shown to also have secondary growth in roots.

Anomalous secondary growth in dicot system

Secondary growth in dicot systems characterized by the following;

By the formation of new cambium strips in the interfascicular region by the initiation of meristematic activity in the parenchymatous cells of primary medullar rays adjacent to fascicular cambium.

These interfascicular cambium strips join with the fascicular cambium to from a complete ring of vascular cambium.

Position of vascular cambium ring always lies between the primary xylem and phloem.

Cells of this cambium cut secondary xylem towards inner side and secondary phloem towards outer side.

Normally single vascular cambium ring remains active and no extra cambium rings are formed except a ring of cork cambium which develops extra stellar.

Anomalous Secondary Growth: type of secondary growth which deviates from the normal type is regards as abnormal or anomalous secondary growth.

In Dicot stem anomalous secondary growth occurs due to following reasons –

Normal position of cambium with abnormal activity.

Abnormal position of cambium with normal activity.

Abnormal position of cambium with abnormal activity

Abnormal secondary growth in dicot stem

Cross section of Boerhaavia stem, showing  successive rings of xylem and phloem

Abnormal secondary growth in different plants

Beta vulgaris  root & stem.

Beta TS root

Abnormal secondary groth in plants

Bougainvillea is a member of the Nyctaginaceae and is an example of a dicotyledonous stem which displaysanomalous secondary growth.  In this TS, near the centre of the stem, you will see some primary vascular bundles embedded in lignified pith parenchyma. Move the slide towards the outer regions, and you will notice that there has been fairly extensive production of secondary vascular tissue. Look for the vascular cambium. Secondary phloem and secondary xylem lie on either side of it. The secondary xylem is composed of tracheids, fibers and narrow-diameter vessels. Interspersed with the secondary xylem you will be able to see small pockets of phloem and what look like large- diameter of metaxylum.

Daucus (carrot) root;

Carrot root

Campsis TS stem

Bougainvillea

     6. Serjania stem                                                    

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