Primordium

A primordium, in embryology, is defined as an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells.

A primordium can be considered as the simplest set of initial conditions capable of triggering growth.

Primordium development in plants
Primordial development in plants is a very critical process for proper positioning and development of different plant organs. Different primordial types like the leaf and flower primordia arise from the shoot apical meristem. The process is intricately regulated by a set of genes that affect the positioning, growth and differentiation of the primordium.

The plant hormone Auxin has also been implicated in this process, with the new primordia being initiated at the site where the auxin concentration is the highest. Genes like SHOOTMERISTEMLESS (STM), CUP-SHAPED COTYLEDON (CUC) etc are involved in defining the borders of the newly formed primordium.