Question

Histogen theory states that epidermis is derived from the
A. Periblem
B. Cambium
C. Cortex
D. Dermatogen

Answer 1

Histogen theory was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers.

Complete answer:
Histogen theory proposed three principal tissues of the root—vascular cylinder, cortex, and epidermis. Let us analyze each option to find out the correct option. Let us analyze each option to find out the correct option.

Option A: Periblem is a primary meristem. It is located between the plerome and the dermatogen and it forms the cortex. Periblem gives rise to the cortex. According to histogen theory, the shoot apical meristem consists of three distinct meristematic zones or layers: dermatogen, periblem, and plerome. Periblem is the middle layer which gives rise to the cortex and endodermis. Therefore, this is an incorrect option.

Option B: Cambium is a layer of delicate meristematic tissue. It is present between the inner bark or phloem and the wood or xylem. It produces new phloem on the outside and new xylem on the inside in stems, roots, etc. Therefore, this is an incorrect option.

Option C: Cortex refers to the outermost layer of a structure. It is a tissue of unspecialized cells lying between the epidermis and the vascular, or conducting, tissues of stems and roots. Histogen theory proposed three principal tissues of the root- vascular cylinder, cortex, and epidermis. These three originated from three groups of initial cells, or histogens, in the apical meristem—plerome, periblem, and dermatogen respectively. Therefore, this is an incorrect option.

Option D: According to histogen theory by Hanstein, the three principal tissues of the root, vascular cylinder, cortex, and epidermis originate from three groups of initial cells or histogens, in the apical meristem plerome, periblem, and dermatogen respectively. Therefore, this is the correct option.

Hence the correct answer is option ‘D’.

Note: It is to be noted that the fourth histogen, the calyptrogen produces the root cap. It is a specialized internal meristematic cell derived from the root apical meristem.