What is shoot apical meristem ? Explain the theories of structure organisation of shoot apex

Shoot apical meristem : Shoot apex occurs at the tip of stem and its branches as terminal bud. It is conical or dome-shaped and is always covered by young leaves, arising from its sides, called shoot apical meristem.
Theories of structural organisation : Several theories have been put forward to explain the activity of shoot apical meristem.
(a) Apical cell Theory : (i) According to Nageli, the activity of single apical cell leads to the development of entire plant body. (2) This theory is applicable to algal forms and cryptogams.
(b) Histogen Theory : (i) According to Hanstein, the apical meristem consists of three distinct histological zones called histogens. (ii) The outermost mantle like layer of cells is called dermatogen. (iii) The underlying layers of cells form periblem. (iv) The central zone of cells is called plerome. (v) The dermatogen gives rise to epidermis, periblem to cortex and endodermis, while plerome to vascular cylinder including pith.
© Tunica-Corpus Theory : (i) This theory was proposed by Schmidt (1924). (ii) It recognizes only two zones in the apical meristem-tunica and corpus, (iii) The cells of tunica are small. They are divided in anti-clinal plane only and, therefore, take part in surface growth, (iv) The cells derived from tunica, rise to epidermis of both stem as well as leaves, (v) The cells of corpus are comparatively larger, (vi) They divide in different planes, (vii) They derived from corpus to give rise to procambium and ground meristem. (viii) Procambium gives rise to primary phloem, primary xylem and intrafascicular :cambium, (ix) Ground meristem differentiate into pith in the centre, pericycle, endodermis, cortex and hypodermis respectively towards the outerside.