The histogens are classified on the basis of
a)	Cells they contain	b)	Cells they give rise to future tissue
c)	Meristematic activity	d)	Cell division

(b)

The histogens are the group of cells, which give rise to future tissues. Major histogens of stem are as follows

1. Dermatogen forms external layers like epidermis.

2. Plerome forms central core of tissues like pith and vascular bundles.

3. Periblem forms the region between central core and epidermis such as endodermis, cortex, etc.