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**Hint:**The histogen is an area or a clearly demarcated zone of primary tissue. It includes the Dermatogen, Periblem and Plerome. These are three distinct meristematic layers or the zones of Histogen.

**Complete Answer:**

- Johannes Ludwig Emil Robert von Hanstein or Johannes von Hanstein was a Botanist who lived in a German city named Potsdam.

- He studied at Gärtnerlehranstalt(Institute of Horticulture) in Potsdam. Later he went to Berlin in order to continue his doctoral studies in Botany.

- Hanstein is best known for his studies in plant anatomy and morphology.

- During 1868, he introduced his well-known Histogen Theory. He tried to explain the versatile and complex behaviour of shoot apex in plants. He even conducted research on the process of fertilization in various ferns so that his findings would be the supplementary help in order to establish his theory.

- This Histogen Theory proposes that the three principal tissues of the root namely vascular cylinder, cortex, and epidermis derive from three groups of embryonic cells, or Histogens, in the apical meristem—Plerome, periblem and dermatogen respectively.

**Note:**In this context, we should mention that the fourth Histogen, the calyptrogen, produces the root cap. The Histogens have been thought to lie in linear order in the apex, with the initial cells of the vascular system toward the older part of the root and those of the cap towards the tip.

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