Do All Eucalyptus Have Opposite Leaves?

Leaf Arrangement

• Botanists categorize leaf arrangement based on how many leaves grow from a single growth node along the plant's stem. If each node has two leaves, they typically occur across from each other. Such plants are said to have opposite leaves. Alternate and spiral leaf arrangements pertain to plants with only one leaf per node, depending on the amount of offset between leaf positions. If each node has more than two leaves, they typically grow around the stem in a circular pattern, and the plant is said to have whorled leaves.

Eucalyptus Heterophylly

• Eucalyptus plants have different types of leaves at different times in their growth cycle, a phenomenon known as heterophylly. For example, the ornamental eucalyptus foliage sometimes seen in florists' arrangements comes from juvenile trees or branches growing from eucalyptus stumps. Eucalyptus specialists have identified four different types of leaves, ranging from seedling to adult types, but leaves of a given type grade into those of adjacent stages.

Juvenile Leaves

• Juvenile eucalypts always have opposite leaves. Furthermore, these opposite leaves are typically decussate: This refers to an opposite leaf arrangement in which each pair of leaves is offset from the previous one by 90 degrees. For example, one pair of leaves projects to the left and right of the stem, but the next pair projects to the front and back, and so on. A few species have opposite leaves with a spiral pattern, where the orientation of leaf pairs slowly circles around the stem. Some eucalypts, such as Eucalyptus macrocarpa, retain juvenile foliage throughout their life cycle.

Non-juvenile Leaves

• As a eucalypt matures, its leaves become less rounded, more regular in outline, and slightly curved, similar to a scimitar. In addition, the leaves take on an apparently alternate arrangement. Works intended for field identification or home gardeners often describe these adult leaves simply as alternate. In a true alternate arrangement, however, each node holds only one leaf. In mature eucalypts, each node gives off two leaves, which originally grow opposite each other. They take on a seemingly alternate appearance due to asymmetric growth of the node's axis, which lies between the bases of the two leaves.