Diversity of mechanical architectures in climbing plants: An evolutionary perspective

Mechanical architectures of a wide range of climbing plants are reviewed from a wide phylogenetic range and evolutionary contingencies. They include an herbaceous lycopod (Lycopodiaceae), a woody tropical liana (Apocynaceae), temperate climbers, herbs and shrubs (Ranunculacae), and two representative climbing palms (Arecoideae, Calamoideae). Trends in mechanical properties during development are reviewed and interpreted via changes in anatomical development of the stem and type of connection to host supports. The results indicate that there are some biomechanical features common to diverse climbing plants including (1) phases of relatively rigid stem growth where the climbing stem has to span between supports and (2) a mechanism to achieve greater compliancy towards the base or at points where the slender climbing stem is at risk from excessive mechanical stress. Evolutionary contingencies such as basal plesiomorphic constraint, complexification, simplification and developmental loss can drastically influence ways in which different plants have evolved different biomechanical climbing architectures. Two key developmental features controlling the biomechanics of the climbing stem are (1) the presence/absence of secondary growth and (2) the number, complexity and coordination of development of primary or secondary tissues with varying mechanical properties. Recent research has suggested that evolution of specialized climbing architectures can canalize subsequent evolution of alternative growth forms. The results suggest that the origin and type of climbing architecture can be heavily influenced by ancestral growth forms and architectures. Despite the extremely complex patterns of plant growth form evolution involving "escapes" to and from more specialized or simpler bauplans, selective pressure towards non-self-supporting growth forms is a remarkably persistent and iterative feature of growth form evolution in land plants.