The term 'bark' refers to
a. Phellem, phelloderm and vascular cambium
b. Periderm and secondary xylem
c. Cork cambium and cork
d. Phellogen, phellum, phelloderm and secondary phloem

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Hint: Bark is a term used to define all the tissues outside vascular cambium. It’s differentiated into outer bark and inner bark. Outer layer of bark peels off regularly for the formation of new secondary vascular tissues in the interior. Bark is insectifuge, decay proof, fire-proof and acts as a heat screen.

Complete answer:
The outermost covering of stems and roots of woody plants is known as bark. It is also used to refer to all tissues outside the wood. The bark is composed of periderm, cortex, and phloem. Inner soft bark also known as bast is produced by vascular cambium which consists of secondary phloem tissue which carries food from leaves to the rest of the plant. The outer bark, which is usually dead tissue, is the product of the cork cambium (phellogen). Bark is usually thinner than the woody part of the stem or root.

The dead cork cells are lined with suberin, which makes them highly impermeable to gases and water. The gaseous exchange between inner tissues of bark-covered roots, stems and the surroundings takes place through lenticels within the cork. Cork cambium gives rise to new cells which form the inner layer of periderm. Included also within the bark is the phloem, which is the conductive tissue liable for the translocation of food materials. Phloem tissue includes fibres for structural support. These fibres are harvested for commercial uses.

Hence, the correct answer is option (D).

Bark as a habitat: Cracks in bark provide great habitat. The deep fissures and crevices within the bark of an old oak area occupied by several species of insects and spiders. These invertebrates attract birds like treecreepers and crested tits.
Food for wildlife: Bark does an excellent job of protecting the tree. Although there are some very determined creatures that are keen to urge to the nutritious cambium, or the wood beneath it.