Leaf Morphology: Arrangement

Since my guidebook was intended for nature-lovers, not necessarily plant specialists, I took care to define and explain the technical terms used in the descriptions of the plants in my book, choosing simpler English synonyms when possible. But it’s not always possible. So I thought it might be helpful to dedicate a few blog posts to delving into some of these technical terms a bit more, deepening our understanding and looking at some specific examples from our desert plants. And I thought I’d start with the terms used in leaf morphology.

In botany, morphology is the study of the size, shape, and structure of plants. Plant biologists use these characteristics for the descriptions, classification, and identification of plants. Having some understanding of these different characteristics will help you to recognize and identify the plants you see while wandering through wadis.

In leaf morphology, one of the key characters studied is leaf arrangement, the number and placement of leaves along the stems. This arrangement of leaves is called phyllotaxy and we’ll talk today about four general categories – alternate, opposite, whorled, and rosette – although there are various levels and ways of categorizing these patterns.

Leaf morphology (Debivort) CC BY-SA 3.0

A node is the point where the leaf emerges from a stem or twig, and arrangement is always regular.

In the alternate pattern, sometimes called spiral, each leaf or leaflet grows from a different node.

In the opposite arrangement, two leaves or leaflets grow per node, on opposite sides of the stem.

If, in this opposite pattern, the successive leaf pairs grow at right angles, it is called decussate. These perpendicular pairs of leaves are typical of plants in the mint (Lamiaceae) family, like the ones pictured below.

In the whorled arrangement, three or more leaves or leaflets are connected at one node. Blepharis attentuata, pictured below, grows whorls of four leaves.

When the leaves of the plant emerge from the base in a whorled arrangement, spreading out in a circle, it is called a rosette.

Understanding the various patterns of leaf arrangement will help you to understand plant descriptions that you read in my book and other sources. And if you come across a plant you do not know, take note of its leaf arrangement (Photos are a great way to document this.) because it could be an important characteristic to consider when identifying the plant. In the next few posts, we’ll learn about other key characters in leaf morphology that will help us identify the plants.

One of my favorite desert plants is the caper bush; its leaves grow in an alternate pattern. I also love germander (Teucrium sp), both for its delicious fragrance and its neat geometrical leaf pattern. Do you recognize these leaf patterns in any of your favorite desert plants?

Desert Plant Adaptations (IV) ~ Leaf Adaptations

Getting back to my series on desert plant adaptations, let’s discuss some of the ways that leaves have adapted to help plants survive the hot and dry conditions of the desert wadis.

Besides dropping all of their leaves like drought-deciduous plants, some plants simply have smaller or fewer leaves. Smaller leaves means there is less surface area open to the wind and sun, and so less water is lost through transpiration. Semi-shrubs and shrubs may have large leaves in the winter that are shed during the dry season and are replaced with smaller leaves in the summer. Artemisia herba-alba and Phlomis aurea both have smaller summer leaves.

Another strategy that helps conserve water is folding or rolling up the leaves so that the stomata are facing inwards, reducing the surface area subject to transpiration. This adaptation is employed by Helianthemum species, Fumana thymifolia, and many perennial grasses.

But it’s not just transpiration that leads to water loss. Leaves also lose water through their cell walls. To combat this, the leaves of some desert plants have a waxy coating or a thick cuticle, the film covering the surface, which helps seal in and protect moisture already in the leaves. Most xerophytes have a thick cuticle.

Hair on the leaves and stems also helps reduce water loss by providing shade and trapping water vapor near the plant’s surface. Artemisia herba-alba, Phlomis spp., Majorana syriaca, and other plants utilize this adaptation.

Plants needs energy from the sun to photosynthesize. What energy they don’t use to make food is used to heat the leaf up. That is very useful for plants growing in cold climates whose leaves need to be a bit warmer to photosynthesize but not so necessary for desert plants, which must reflect some of this solar radiation or risk over-heating. How do they do this? Some plants, like Capparis sinaica, have a waxy coating on their summer leaves which gives them a lighter – and more reflective – color than their winter leaves.

Leaves of Atriplex species also have a different color depending on the season, but they don’t use a waxy coat. Instead, the leaves are covered in vesicular hairs that contain a salty solution. In the winter, the hairs are full and transparent, allowing more absorption of the sun’s energy. In the summer, the water evaporates and the hairs dry out causing the leaves to be a lighter color that reflects the sun’s rays.

You can read more about each of the species photographed above in my book. In the next post in this series, we’ll learn about stem adaptations. Stay tuned!