The mode of arrangement of leaves on the stem is known as phyllotaxy (Gk. Phyllon = leaf ; taxis = arrangement). Phyllotaxy is to avoid over crowding of leaves and expose the leaves maximum to the sunlight for photosynthesis. The four main types of phyllotaxy are
In this type there is only one leaf per node and the leaves on the successive nodes are arranged alternate to each other. Spiral arrangement of leaves show vertical rows are called orthostichies. They are two types.
i. Alternate spiral: In which the leaves are arranged alternatively in a spiral manner. Example: Hibiscus, Ficus.
ii. Alternate distichous or Bifarious: In which the leaves are organized alternatively in two rows on either side of the stem. Example: Monoon longifolium (Polyalthia longifolia).
In this type each node possess two leaves opposite to each other. They are organized in two different types.
i. Opposite superposed: The pair of leaves arranged in succession are in the same direction, that is two opposite leaves at a node lie exactly above those at the lower node. Example: Psidium (Guava), Eugenia jambolana (Jamun), Quisqualis (Rangoon creeper).
ii. Opposite decussate: In this type of phyllotaxy one pair of leaves is placed at right angles to the next upper or lower pair of leaves. Example: Calotropis, Zinnia, Ocimum
In this type there are three leaves attached at each node. Example: Nerium
In this type more than three leaves are present in a whorl at each node forming a circle or whorl. Example: Allamanda, Alstonia scholaris.