Notes on the vascular bundle of monocot stem

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Each bundle is oval in shape, conjoint, collateral endarch and closed (cambium is absent hence no secondary growth). Surrounding the bundle there is a sclernchymatous bundle sheath. The phloem is above the xylem. It consists of sieve tubes and companion cells.

There is no phloem parenchyma. The outer cells of phloem (protophloem) are crushed while the inner cells (metaphloem) are healthy. Xylem is reduced; it consists of four (sometimes three) vessels (two meta and two proto) arranged in the form of ‘Y’ Metaxylem vessels are pitted, while protoxylem vessels are annular and spiĀ­ral Sometimes in between the metaxylem vessels, small, pitted tracheas are present. Below the protoxylem a lysigneous cavity is present. It consists water and is known as the water containing cavity. Surrounding this are present cells of wood parenchyma.

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