This theory was proposed by Hanstein in 1870. According to this theory the shoot apex consists of three distinct meristematic zones or layers. Each layer consisting of a set of initial is called a Histogen. This theory designates the three histogens;
Dermatogen: outermost uniseriate layer and give rise to the epidermis of stems and epiblema of roots.
Periblem: (middle layer) is composed of isodiametric cells. The cells divide actively and give rise to the primary cortex.
Plerome: innermost histogen layer gives rise to the vascular cylinder, i.e., primary xylem and phloem, medullary rays and medulla or pith.
The histogen theory is not accepted because; there is no sharp and clear distinction between dermatogen and Periblem, the tissues destined to be produced by a particular histogen were sometimes formed by another histogen also, i.e., the dermatogen which is supposed to produce epidermis also forms a part of the cortex destined to be formed by Periblem.
Apical cell theory was proposed by Hofmeister in 1857 and supported by Karl Wilhelm Von Nageli in 1878. According to them there is a single pyramidal cell in the shoot apex of most vascular cryptogams or the single apical cell is responsible for the shoot formation and successive growth of plant. The apical cell theory may hold good for higher algal groups, bryophytes and a few pteridophytes. Higher vascular plants do not possess a single apical cell. Hence, this theory is not applicable to gymnosperms and angiosperms.