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A Handbook of Biology

Function: It gives mechanical support to stem.

Heartwood does not conduct water.

Sapwood: It is the peripheral region of secondary xylem. It is living and lighter

in colour. It is involved in the conduction of water and minerals from root to

leaf.

ÇØRK ÇÅMBÏÜM

As the stem continues to increase in girth due to the activity of vascular

cambium, the outer cortical & epidermis layers get broken. It is to be replaced

to provide new protective cell layers. Hence, another meristematic tissue called

cork cambium (phellogen) develops, usually in the cortex.

Phellogen is a couple of layers thick. It is made of narrow, thin-walled and nearly

rectangular cells.

Phellogen cuts off cells on both sides. The outer cells differentiate into cork

(phellem) while the inner cells differentiate into secondary cortex (phelloderm).

Cells of secondary cortex are parenchymatous.

The cork is impervious to water due to suberin deposition in the cell wall.

Phellogen, phellem, and phelloderm are collectively known as periderm. Due to

activity of cork cambium, pressure builds up on the remaining layers peripheral

to phellogen and ultimately these layers die and slough off.

Bark is a non-technical term that refers to all tissues (such as periderm &

secondary phloem) exterior to the vascular cambium. It is of 2 types:

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Early (soft) bark: It is formed early in the season.

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Late (hard) bark: It is formed towards end of season.

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Lenticels: At certain regions, phellogen cuts off closely arranged

parenchymatous cells on outer side instead of cork cells. These cells

rupture epidermis, forming a lens shaped openings called lenticels. They

occur in most woody trees.

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Function: Lenticels permit gas exchange of between the outer

atmosphere and the internal tissue of the stem.

(NEET 2013)

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Lenticles are filled up with loosely arranged cells called complementary

cells.