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A primordium (//; plural: primordia) 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 is the simplest set of cells capable of triggering growth of the would-to-be organ and the initial foundation from which an organ is able to grow.
Although it is frequently used term in plant biology, the terminology is used in biology of all multicellular organisms (for example: primordium of tooth in animals, primordium of leaf in plants or primordium of sporophore in mushroom.)
Plants produce both leaf and flower primordia cells at the shoot apical meristem (SAM). Primordia developments in plants is critical to the proper positioning and development of plant organs. The process of primordia development is intricately regulated by a set of genes that affect the positioning, growth and differentiation of the primordium. Genes like the STM (shoot meristem less) and CUC (cup-shaped cotyledon) are involved in defining the borders of the newly formed primordium.
The plant hormone auxin has also been implicated in this process, with the new primordia being initiated at the placenta where the auxin concentration is the highest. There is still much to understand about the genes involved in primordia development.
Leaf primordia are a group of cells that will form into new leaves. These new leaves form near the top of the shoot and resemble knobby outgrowths or inverted cones. Flower primordia are the little buds we see at the end of stems, from which a flower will develop. Flower primordia start off as a crease or indentation and later form into a bulge. This bulging is caused by slower and less anisotropic, or directionally dependent, growth. (See anisotropy)