![]() These signals can be transported to the meristem via the vascular system, which in seed plants is functionally interrelated to the organs, and its spatial structure corresponds directly to phyllotactic patterns ( Figure 1). The second group of theories assumes that the site of the new primordium initiation is determined by internal signals that originate from already differentiated regions. Although interest in phyllotaxis dates to ancient times and has flourished since the 19th century, the mechanisms underlying phyllotactic pattern formation remain incompletely elucidated. There are different types of phyllotactic patterns, such as distichous, whorled, and helical (spiral). Phyllotaxis is a wider term that is also used to describe the arrangement of leaf-like flower elements that are initiated due to the activity of the flower meristem or distribution of flowers within inflorescences (e.g., in capitula). Consequently, the resultant organ arrangement upon initiation creates regular symmetric spatial patterns called phyllotaxis. The subsequent divisions of neighboring cells result in the bulging of a new organ primordium in an established position in reference to existing (older) organs at the apex. However, organogenesis is induced locally when the cells in the sub-superficial layers (L2 and 元 in Arabidopsis) begin to divide periclinally (parallel to the surface) instead of anticlinally (perpendicularly to the surface). This region contains a few layers of cells, and the entire circumference is equally competent in forming new organs. Lateral organs are initiated in the peripheral (organogenic) zone of the SAM, which encircles the central part of the apex, including the initial zone with stem cells and the organizing center ( Figure 1). ![]() SAM is a group of dividing cells, which contain stem (initial) cells and whose perpetual proliferation is the source of cells for all shoot tissues and organs. Subsequent lateral organs, such as leaves, are initiated by the precisely regulated activity of shoot apical meristems (SAM) located at the tip of each stem. The indeterminate growth and development of land plants are related to continuous postembryonic organogenesis.
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