Anomalous Secondary Thickening in Bougainvillea Stem

Bougainvillea (belongs to Nyctaginaceae family) is a thorny ornamental bushy plant extensively grown as ornamental hedges. The inflorescence of Bougainvillea consists of large colourful petal-like bracts which surround three simple flowers. A large number of colour variants for the bracts are available. Anatomically Bogainvillea is peculiar that its secondary thickening pattern is Anomalous type. In this post, we discuss the Anomalous Secondary Growth in Bougainvillea Stem with diagram.

Download the PPT: Bougainvillea Stem

Secondary thickening in Bougainvillea occurs by the Formation of Successive Rings of Accessory Cambia. This result in the formation of rings of vascular bundles embedded in the sclerenchymatous conjunctive tissue. Here the cambial ring is originated form the extra-stelar region. This extra-stelar cambium cut secondary xylem and thick walled conjunctive tissue internally and secondary phloem and thick-walled conjunctive tissue externally. Thus, in the mature stem, the vascular bundles appear to be embedded in a mass of thick-walled conjunctive tissue.

Source: Flickr_Dennis Jarvis (https://www.flickr.com/photos/22490717@N02/34849123086)

Let’s see the detailed anatomy of Bougainvillea Primary and Secondary growth

Primary Structure

$. Epidermis is single-layered and parenchymatous with thick external cuticle.

$. Hypodermis is few-layered and composed of collenchymatous cells.

$. The cortex is large and parenchymatous.

$. Endodermis is present but not distinct.

$. Pericycle is heterogeneous type, composed of parenchymatous cells alternating with sclerenchyma patches.

$. Vascular bundles are irregularly distributed in the ground tissue; these bundles are the leaf traces.

$. Primary vascular bundles very rarely undergo secondary thickening (increase the size)

Anomalous Aecondary Growth in Bougainvillea Stem

$. The formation of the first cambial ring is extra-stelar in origin.

$. Cambium arises from the pericycle.

$. The first cambial ring is short-lived.

$. Successive cambial rings formation also occurs.

$. Cambial ring cut:

#. Internally: Secondary xylem alternating with secondary parenchyma.

#. Externally: Secondary phloem above the secondary xylem and parenchyma above secondary parenchyma.

$. The cells of the secondary parenchyma are fusiform (spindle-shaped) type, radially elongated and develop lignified secondary wall.

$. This tissue is represented as the conjunctive tissue.

$. Successive rings of cambia are formed.

$. Successive cambial rings behave exactly the same manner as described above.

$. Thus, successive concentric rings of vascular bundles are formed embedded in the thick-walled conjunctive tissue.

$. This layering pattern will be prominent due to the prominent vessels in the xylem and the cursed older phloem.

$. In most of the cases, the new cambium does not arise as a complete ring around the periphery of the vascular cylinder, but it forms strips which join with the older cambia.

$. In some species of Bougainvillea, the conjunctive tissue composed entirely of sclerenchyma and it cannot be distinguished from the xylem of the embedded vascular bundles.

$. Hence, the secondary phloem appears in the form of islands (isolated groups) surrounded by the conjunctive tissue.

$. It should NOT be mistaken as included phloem.

Reference:

Prakash, E. John Jothi., 2000, Textbook of Plant Anatomy. Emkay Publishers, India

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