Chemical barriers at these surfaces include specialized soluble substances that possess antimicrobial activity as well as acid pH.

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Anomalous secondary growth

Several dicotyledons show secondary growth that is different from normal  secondary growth. The deviating methods of secondary thickening are called  abnormal or anomalous secondary growth. 

e.g., Bougainvillea (Dicot)

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Dicot stem anomalous secondary growth occurs due to following reasons:

Anomalous position of cambium 

e.g.,Serjania sp. and Bauhinia sp.

Abnormal behavior of normal cambium

e.g., Bignonia, Aristolochia, Tinospora, Clematis etc.

Extrastelar cambium formation

e.g., Amaranthus sp. 

Interxylary phloem or included phloem

e.g., Entada sp (box bean), Combretum sp (Combretum), Strychnos sp.

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Dicot stem anomalous secondary growth occurs due to following reasons:

Intraxylary phloem

Absence of vessels in the xylem

Scattered vascular bundles in dicot stem

Presence of medullary bundles

Presence of cortical bundles

Secondary growth in monocot

e.g., Dracaena 

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Secondary growth in monocotyledons

Due to lack of cambium in monocotyledons, secondary growth is absent. But Very few plants in monocots shows anomalous secondary growth such as Dracaena, Yucca, Aloe, Sansevieria and Agave