Phyllotaxis or phyllotaxy refers to the arrangement of leaves along a stem. Phyllotaxis originates at the shoot apex where the directions for growth are genetically programmed in the meristematic tissues. It is yet unknown how the genetic code provides the information for the location of a new leaf.

      The apical meristem proceeds through minimal and maximal phases. The apical dome is smallest (in its minimal phase) just after a new leaf primordium has originated. The dome then expands to its maximal size and on its flanks a new leaf primordium will form as a mound of cells. The dome returns to its smallest size and the cycle is repeated.

Spiral Phyllotaxy

      Spiral phyllotaxy is the classification of leaf arrangement where by one leaf primordium is produced at a time by the apical meristem. The generative spiral governs the position of successive leaf primordia. Besides the generative (genetic) spiral, other spirals called contact parastichies, can be recognized at the shoot apex. The contact parastichis are so-called because they are formed where leaf primordia are in contact (touching) each other. Parastichies of all spirals follow the Fibonacci series of numbering. This series of numbers is created by adding the first number to the second number to create a third, then adding the second number to the third to create a fourth, then adding the third number to the fourth to create a fifth, and so on. The resulting series of numbers would be: 0,1,1,2,3,5,8,13.... Therefore, when the number of counter-clockwise parastichies is five, the clockwise number of parastichies will be eight, and the plant is said to have a 5+8 phyllotactic pattern. A phyllotactic fraction (i. e. 5/13) is defined as the number of turns divided by the number of intervening leaves.

      Divergence angles form between the successive leaves in the genetic spiral. When one leaf is produced at each node the resulting divergent angle equals 137°. Some exceptions include a distichous spiral in which the divergent angle is 180° and the result is an alternate phyllotaxis. Plants exhibiting this type of phyllotaxy will always have two orthostichies. Tristichous phyllotaxis results from leaves emerging at 120° divergent angles. Plants with tristichous phyllotaxis will always have three orthostichies.

      The spiral pattern is a common phenomenon in plant development. Many inflorescences, such as sunflower, pepperomia, and pineapple incorporate identical angles of divergence which produce spirals of flowers along the stem or receptacle.

Rosette


Alternate


Contact Parastichies

      Whorled systems occur when two or more leaves emerge from each node. Opposite phyllotaxy results when there are only two allowable positions for growth on the apical meristem. These positions lie 180° apart and the result is leaves that emerge opposite from one another. These plants will always have two orthostichies.

      In many plants, two leaves will emerge from each node in an opposite fashion, but at the next node these positions rotate 90°, and so on, resulting in a decussate or cruciate pattern of phyllotaxis. These plants will always have 4 orthostichies.

      When three or more leaves emerge from each node, they are termed verticillate. A particular pattern is seen in this classification of phyllotaxis in which bijugate or pairs of leaves emerge 67.5° apart.

      Ontogenetic changes may occur during a plants life cycle that change the phyllotactic pattern. This can happen when a plant reaches a certain age, becomes reproductive, or changes orientation.