Leaves and Environmental Adaptations

The most pervasive challenge to the survival of leaves comes from the fact that there is always a tremendous moisture gradient between leaves and the atmosphere. The internal atmosphere of a leaf is saturated with water. Even at high relative humidity there is a steep H2O concentration gradient between the internal and external atmospheres. This might not be a problem except that mesophyll cells need carbon dioxide for photosynthesis and CO2 occurs at extremely low concentrations in most environments. Leaves need to let CO2 in while preventing water loss.

The Cuticle prevents excessive evaporation but it also blocks the entry of CO2. When it comes to the cuticle, thicker is better for water retention.

The Stoma is one of the most important adaptations for terrestrial plants. The opening and closing of Guard Cells allows the leaf to regulate gas exchange physiologically. However, there are other adaptations which are beneficial for the function of leaves in dry (Xeric) environments. It is important to realize that almost all environments appear xeric to the leaf.

Some plants grow in wet (Hydric) habitats. This presents another type of problem. Most gases are less soluble in H2O than in air. This is especially true for Oxygen. Submerged organs have adaptations which are directly related to the low oxygen concentrations they encounter. The presence of Aerenchyma is common in floating or submerged leaves. This provides internal reservoirs for Oxygen and CO2.

Some leaves face the worst of both worlds. Waterlily leaves are scorched by the sun on one side and swamped by water on the other side!

The term Xeromorphic has been applied to traits that appear to protect leaves from excess evaporation, heat and/or light. This terminology grew out of early work on desert plants which grow in xeric conditions. However, Stressomorphic might be a better term because many of these adaptations are seen in plants that are exposed to excessive wind or cold, or to mineral deficiency or even to waterlogged soils. This is important because many tropical leaves can have Xeromorphic traits even though they live in apparently Mesic (middle of the road) or moist environments.

Mesomorphic is the term used to describe leaves that you might expect to find in a mild and moist environment. Sun and Shade leaves from the same plant are used for comparison.

Sun Leaves would have more Xeromorphic traits while Shade Leaves would be more Mesomorphic. The theoretical Mesomorphic leaf is somewhere between these two. It is much easier to define Xeromorphic and Hydromorphic traits than it is to describe Mesomorphic traits.

Another consideration which applies to islands, like Hawaii, is Genetics. Island flora may be genetically poor because only a few genotypes may have founded its populations. Consequently, the genes present in any species have a dominant effect on leaf morphology regardless of the habitat in which individual plants grow. Phenotypic Plasticity is a term which is used to describe the range of phenotypes which can be produced by a species or population. Plants with little phenotypic plasticity will look the same regardless of their immediate environment. Conversely, species with great phenotypic plasticity can display a wide range of phenotypes over a range of environments. Metrosideros polymorpha (Ohia' lehua) displays a range of leaf morphologies. It has been studied locally to see how much leaf anatomy is due to phenotypic plasticity or genetic constraint.