One of the most important steps when learning how to identifying trees and plants is understanding phyllotaxy, or the arrangement of the leaves around the stem. The purpose of these arrangements are often to optimize exposure to the sun. Most plants fall into one of three main leaf arrangements which are easy to distinguish from each other.
In an alternate or spiral leaf arrangement, only a single leaf is attached to a particle node on a stem. These types of plants have leaves spread up the stem in a spiral pattern, forming an invisible helix. A plant with an alternate arrangement consistently has a symmetrical pattern with the exact same number of leaves for each turn around the plant's limb. Ivy is one example of a species with an alternative leaf arrangement.
A plant with pairs of leaves attached at each node opposite to each other has an opposite leaf arrangement. This leaf arrangement has two types, decussate and distichous. If the pairs of leaves are rotated 90 degrees as they advance up the stem, the opposite leaf arrangement is considered descussate. Alternately, if the pairs are not rotated, they are called distichous. Plant species with opposite leaf arrangement include maples, periwinkle, most viburnums and chestnut.
Whorled leaf arrangements are identified by three or more leaves attached at a single node. Like opposite leaf arrangements, whorled plants can be decussate if there is a rotation in the leaves as they progress along the twig. Examples of this arrangement can be seen in the catalpa and sweet woodruff.
Other Leaf Arrangements
Although less common than the alternate, opposite and whorled leaf arrangements, there are several other leaf arrangements that happen in nature. Basal is one, characterized by a circular distribution of leaves around the base of the plant, as seen in dandelions. In another, equitant, leaves overlap. This is standard in some iris species.