Shoot Apex Theories & Organisation of Shoot Apex
Many theories have been given to explain shoot apex organisation, such as
- Apical theory
- Histogen Theory
- Tunica-Corpus theory
(1) Apical Cell Theory
It was proposed by Hofmeister and Nageli. According to this theory, a single apical cell leads to the development of entire plant body. This theory is applicable to algae, as well as to most of the bryophytes and pteridophytes.
(2) Histogen Theory
It was proposed by Hanstein. According to this theory shoot apex (as well as root apex) consists of following histogens
- Dermatogen. Outermost layer. It forms epidermis (skin) and epidermal tissue system.
- Periblem. It gives rise to the tissues between epidermis and stele, i.e., cortex and endodermis
- Plerome. Innermost layer. The central mass of cells which gives rise to central stele.
3. Tunica-Corpus Theory
It was proposed by Schmidt (1924). It is based on plane of division of cells. According to this theory, shoot apex consists of two distinct layers:
- Tunica. It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus and divide by anticlinal divisions mostly.
- Corpus. It represents central core with larger cells. The cells divide in all planes (anticlinal and periclinal).
Sometimes, tunica is multilayered, then only the outer layer forms epidermis and the remaining layers with corpus form cortex of shoot.