Phyllotaxy (aka Phyllotaxis)

Below are shown examples of the most common types of phyllotaxy seen in nature.

In a decussate (opposite) pattern, leaves are arranged along the stem in opposite pairs, with each successive pair oriented at 90 degrees.  Example, cyprus.

In a distichous (alternate) pattern, single leaves alternate on either side of the stem.  Example, maize, what we call corn. Note that in both the decussate and alternate, the successive leaves or leaf pairs maximize the distance between themselves.

Spiral phyllotaxy, single leaves are offset by an angle of about 137.5 degrees.  Examples: Arabidopsis, Hosta, many other plants.

In a whorled pattern, three or more leaves, or organs in the case of flowers, are affixed to a single node with maximal separation between them.  This is the normal phyllotaxy of the floral organs in most flowers.

In plants phyllotaxy can change during the course of development.  In maize, leaves on the main stem of the plant are arranged in alternate phyllotaxy, whereas, the husks on the ear are arranged in a spiral phyllotaxy.   In Arabidopsis, the SAM produces leaves in a decussate (first two leaves) then spiral phyllotax (seedling), the inflorescence produces flowers still in the spiral phyllotaxy, while the floral meristem produces organs in a whorled phyllotaxy.  This is essential from an evolutionary view, because the various phyllotaxies of leaves permit maximal light harvesting, ie because the leaves are not so shaded by those above, whereas the whorled petals provide enhanced visual impact for pollinators, and the whorling of stamens and carpels is essential for the mechanisms of pollination and fertilization.

 Phyllotaxy isn't evident at the embryonic stage but is quickly established during postembryonic growth; this is seen in Arabidopsis where the two first primordia are formed opposite one another during embryogenesis and successive leaves are offset by about 137.5 degrees.