Anatomy of Flowering Plants Test - 7

Intrafascicular Cambium is primary meristem.It develops from the procambium of the stem apex.It is located inside the open vascular bundles, between phloem and xylem patches.

The tissues involved in secondary growth are the two lateral meristems namely, vascular cambium and cork cambium.

Wood is a secondary xylem. It is secondary growth by cambium and constitutes the bulk of the plant body in dicot stems and dicot roots.

During secondary growth, cells of meduallary rays, in a line with intrafasicular cambium, become meristematic and form interfascicular cambium.

Therefore, the intrafascicular and interfascicular cambia form a ring which separates the primary xylem and primary phloem, and is known as cambium ring.

In old trees, the greater part of secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem. These substances make it hard, durable and resistant to the attacks of microorganisms and insects. This region comprises dead elements with highly lignified walls and is called heart wood.

As the stem continues to increase in girth due to the activity of vascular cambium, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cell layers. Hence, another meristematic tissue called cork cambium or phellogen develops in the cortex region.

The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem.

The outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm.

Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.These occur in most woody trees.

Scleroids belongs to sclerenchyma complex permanent tissue presentin hard parts of plants. They are generally hard and founds in rough fruits and stem.