Conceptual Overview

Both outer and inner structure of plant organs, to a greater or lesser extent, are influenced by environmental (or ecological) factors. Of these, problems relating to the deficit and excess of water and/or light intensity rank as the most significant. And accordingly, ecological groups such as xerophytes, hydrophytes, sun and shade plants can be distinguished. Leaves are typically the most plastic of organs in their ability to change their structure as an adaptation to a particular ecological factor. The specific structural features of xeromorphic leaves of many xerophytes are thought to be associated with the restriction of water loss during transpiration. Such features are thickened outer cell walls of the epidermis, very thick and waxy cuticle, dense cover of reflecting trichomes, sunken stomata (often on one leaf surface only), compact mesophyll and well-developed sclerenchyma to provide mechanical support and to prevent wilting.

In some dicot xerophytes with hypostomatous leaves, stomata are located in crypts (grooves or depressions) and surrounded by hairs, with the effect of creating a pocket of water vapor, and thus reducing water loss by transpiration, especially in the wind. The epidermis may be more than one layer thick (i.e. a multiple epidermis). Many grasses are capable of rolling or folding their leaves during drought due to the presence of specialized motor (or bulliform) cells in the adaxial epidermis, and/or hinge cells in the mesophyll. If the water supply is sufficient, these cells are turgid and keep the leaf open. In the case of water deficit, the cells lose turgor and shrink. The leaf, like a spring, rolls into a tube with the adaxial surface and stomata oriented to the inside of the tube. Rolling of leaves allows them to reduce transpiration. Some xerophytes have fleshy succulent leaves and tolerate drought by accumulating water in large thin-walled water-storage parenchyma cells that contain only a few small chloroplasts.

The needle-like or scale-like leaves of evergreen conifers are also xeromorphic, even though plants may be confined to mesic habitats. Their leaves have adapted to tolerate water stress in the winter (when the soil is frozen) by reducing transpiration. The surface-to-volume ratio is low. The epidermis consists of pavement cells with thick lignified secondary walls and a heavy cuticle. Stomata are deeply sunken with overarching subsidiary cells. The epistomatal cavity (depression over guard cells of stomata) is usually filled with dense deposits of wax particles which greatly reduce water loss. A sclerenchymatous hypodermis layer underlies the epidermis. It is interrupted only by substomatal cavities. Mesophyll may be homogeneous and compact as in pines or differentiated into palisade and spongy parenchyma as in larch and cycads. Two or more resin ducts are embedded in mesophyll. The center of the needle contains occasional vascular bundles embedded in a unique vascular tissue, termed the transfusion tissue, that is surrounded by a single layer of endodermis.

In submerged hydrophytes, the leaves usually lack cuticle, stomata and intercellular spaces. Supportive tissues and xylem are often reduced in amount or almost absent. In some hydrophytes there is no mesophyll, as in Elodea, or it is thin, one-layered as in submerged leaves of Potamogeton. The Elodea leaves consist mostly of only two epidermal layers which also function as photosynthetic tissue. In other submerged water plants the leaves are finely divided, cylindrical and centric in anatomy as in Myriophyllum. Such structure improves diffusion of carbon dioxide from water into the leaf for photosynthesis in the absence of gas spaces. However, in Vallisneria leaves the mesophyll is represented by aerenchyma (containing large regularly distributed intercellular spaces). Its ribbon-like leaves grow from under the soil surface where gases are available. In Potamogeton, besides submerged leaves, there are also floating leaves. The latter contain abundant intercellular spaces for carbon dioxide diffusion and stomata only on their upper side (epistomatic leaves), through which carbon dioxide diffuses from the air.

Besides availability of water, light intensity is an especially important ecological factor affecting leaf anatomy. There is a difference in the structure of leaves in plants growing under different light intensities. When light intensity is sharply reduced, shade leaves are formed. They are usually thinner, the cuticle and sclerenchyma are not as pronounced, and the palisade tissue is not as developed as in sun leaves. The chloroplasts of shade leaves possess larger grana (i.e. more thylakoids) and contain more chlorophyll. Shade and sun leaves may often be found in different parts of the crown of the same tree.