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Flowers are modified branches. Like vegetative branches they are composed of leaves attached to a stem (Receptacle). In most cases there is little internodal elongation between successive flower parts and they are tightly inserted one above the other. Flower parts may have a spiral arrangement. However, they usually occur in whorls. A Whorl occurs at a Node when three or more appendages are attached at the same level.
 Flowering Apex of Ranunculus: The Sepal and Petal Primordia have begun to develop. The Stamen Primordia are just visible. Carpel Primordia will eventually develop at the summit of the apex. |
The initial stages of development at the shoot apical meristem are similar for leaves and floral organs. Subsequent development is obviously different. This is especially true for Carpels because they form a hollow structure with enclosed ovules.
Vegetative shoot apical meristems are said to be "Indeterminate" which means that they have "unlimited" growth potential. The sequence of events which lead to leaf production involve alternating patterns of LOCALIZED Cell Divisions which produce leaf primordia, followed by a restoration of the SAM and another highly localized leaf initiation event. It is important to remember that cell divisions do NOT occur throughout the entire vegetative SAM and that the SAM is relatively small.
When a vegetative SAM becomes a Flowering Apical Meristem (FAM) the first event is often a marked elongation of the stem (axis). This is especially evident in plants like Banana, Agave and many Composites which have a rosette of leaves.
 Vegetative Shoot Tip of Banana: Note the flat nature of the Shoot Apex and the absence of Internodal Elongation. SAM = Shoot Apical Meristem; LP = Leaf Primordium.
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| Banana Flowering Shoot Tip at various Magnifications: Note the Bracts which subtend each Flower Bud. FAM = Flowering Apical Meristem. |
The next step may be a widening and flattening of the FAM. A thick surface layer of highly meristematic cells develop and produces floral structures in an Acropetal succession.
(Acropetal
= from the Base towards the Tip;
[i.e. Sepals -> Petals -> Stamens -> Carpels]).
