Phellogen arises from
A. An outer layer of the cortex
B. Epidermis
C. Phloem
D. All of the above

Answer
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Hint: Higher plants undergo two types of growth during their lifespan namely- primary growth and secondary growth. The growth that occurs in earlier life stages of a plant is referred to as primary growth. Increase in the length of the plant body occurs during primary growth. The growth that takes place in later life stages of a plant is referred to as secondary growth. Increase in girth or circumference of the plant body occurs during secondary growth.

Complete answer:
-The internal structure of a typical dicot stem shows certain characteristic features. Parts of a dicot stem are epidermis, hypodermis, general cortex, endodermis, pericycle, vascular bundles, pith and medullary rays.
-The outermost protective layer of stem is called the epidermis. Inside the epidermis, there are present very compactly arranged cells. The layer below the epidermis is called the hypodermis. There are many cells in hypodermis so it is thick and multilayered.
General cortex stores food in its parenchymatous cells. During the secondary growth, a cambium and cork cambium is formed. It is a meristematic tissue that divides laterally and secondary growth takes place.
-All the tissues which are present inside the endodermis constitute the stele. Cork cambium is responsible for the secondary growth in the extra stelar region. It is also termed as phellogen. Cork cambium is responsible for the secondary growth in the extra stelar region. Cork cambium is also termed as phellogen.
-The phellogen or the cork cambium develops in the hypodermal region or the region just outside the general cortex. There are present narrow, thin-walled and nearly rectangular cells in the cork cambium. The secondary tissues in the extra stelar region are formed by the cork cambium.

Hence, the correct answer is option (A).

Additional information:

Note: Cork cambium divides periclinal to form some cells toward the outside (towards epidermis) and some cells towards inside (towards general cortex). The outer layer of cells becomes suberized, thus the cells become dead. These dead cells are known as cork or phellem.
Those cells which are formed towards the inside are differentiated into the parenchyma and may contain chloroplasts. These are called secondary cortex or phelloderm.