Primordium

A primordium (; plural: primordia; synonym: anlage) in embryology, is 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-be organ and the initial foundation from which an organ is able to grow. In flowering plants, a floral primordium gives rise to a flower.
Although it is a frequently used term in plant biology, the word is used in describing the biology of all multicellular organisms (for example: a tooth primordium in animals, a leaf primordium in plants or a sporophore primordium in fungi.)
Primordium development in plants.
Plants produce both leaf and flower primordia cells at the shoot apical meristem (SAM). Primordium development in plants is critical to the proper positioning and development of plant organs and cells. The process of primordium development is intricately regulated by a set of genes that affect the positioning, growth and differentiation of the primordium. Genes including STM (shoot meristemless) 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 primordium being initiated at the placenta, where the auxin concentration is highest. There is still much to understand about the genes involved in primordium development.
Leaf primordia are groups 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 flowers 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.