The leaves of extant terrestrial plants show highly diverse and elaborate patterns of leaf venation. One fundamental feature of many leaf venation patterns, especially in the case of angiosperm leaves, is the presence of anastomoses. Anastomosing veins distinguish a network topologically from a simple dendritic (tree-like) pattern which represents the primitive venation architecture. The high degree of interspeci®c variation of entire venation patterns as well as phenotypic plasticity of some venation properties, such as venation density, indicate the high selective pressure acting on this branching system. Few investigations deal with functional properties of the leaf venation system. The interrelationships between topological or geometric properties of the various leaf venation patterns and functional aspects are far from being well understood. In this review we summarize current knowledge of interrelationships between the form and function of leaf venation and the evolution of leaf venation patterns. Since the functional aspects of architectural features of dierent leaf venation patterns are considered, the review also refers to the topic of individual and intraspeci®c variation. One basic function of leaf venation is represented by its contribution to the mechanical behaviour of a leaf. Venation geometry and density in¯uences mechanical stability and may aect, for example, susceptibility to herbivory. Transport of water and carbohydrates is the other basic function of this system and the transport properties are also in¯uenced by the venation architecture. These various functional aspects can be interpreted in an ecophysiological context.

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The Namib grass Stipagrostis sabulicola relies, to a large degree, upon fog for its water supply and is able to guide collected water towards the plant base. This directed irrigation of the plant base allows an efficient and rapid uptake of the fog water by the shallow roots. In this contribution, the mechanisms for this directed water flow are analysed. Stipagrostis sabulicola has a highly irregular surface. Advancing contact angle is 988 + 58 and the receding angle is 568 + 98, with a mean of both values of approximately 778. The surface is thus not hydrophobic, shows a substantial contact angle hysteresis and therefore, allows the development of pinned drops of a substantial size. The key factor for the water conduction is the presence of grooves within the leaf surface that run parallel to the long axis of the plant. These grooves provide a guided downslide of drops that have exceeded the maximum size for attachment. It also leads to a minimum of inefficient drop scattering around the plant. The combination of these surface traits together with the tall and upright stature of S. sabulicola contributes to a highly efficient natural fog-collecting system that enables this species to thrive in a hyperarid environment.

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Variation of stomatal structure may, with unchanged pore size and depth, profoundly affect gas exchange and the pathways of liquid water inside the leaf. Equations for calculation of stomatal conductance which are solely based on stomatal density and pore depth and size can significantly overestimate stomatal conductance.

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There is various evidence that the presence of condensed water, e.g. dew, improves plant water relations. Because particularly plants in drier environments are expected to benefit from atmospheric water, it may be assumed that special adaptations promoting exploitation of dew water evolved in these habitats. Surface characteristics affect dewfall and retention of dew water. Leaf surface characteristics such as hairs (trichomes), which are often present in plants of arid environments, may therefore play a substantial role in dew harvesting. In this study, it is analysed to what extent (1) the presence of hygrophilic aerosoles promotes dewfall by reducing vapour saturation pressure ('early condensation'), (2) dew water can be stored within a dense trichome layer, (3) the presence of dew water increases humidity directly above the leaf surface, thereby reducing water loss from the leaf interior by decreasing the water potential gradient between leaf interior and exterior, and (4) dew evaporation reduces transpiration further by evaporative cooling. A physical framework is provided in this contribution that shows that these processes are in fact possible under natural conditions.