Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes

Leaf-stem size relationships over ontogeny were studied here in three diffe rent lineages of hollow-stemmed myrmecophytes in order to understand how a new stem function affects morphology. In each of six taxa, the primary cross-sectional area of a terminal intern ode and the area of the leaf borne by it were measured on plants representi ng all stages of ontogeny. Cross-sectional areas of both the cavity and the ring of wood were determined. The leaf-stem relationship over ontogeny was allometric, in contrast to the isometry previously found in solid-stemmed relatives. Stem cross-sectional area was initially larger relative to leaf area than for solid-stemmed spe cies, increasing less than proportionally with increasing leaf size. Because mechanical stability requires a minimum ratio of t (thickness of th e solid ring) to R (external radius of the cylinder), cross-sectional area of the ring of wood must vary with that of the cavity; both contributed to leaf-stem allometry. Relative to leaves, both are initially large and incre ase more slowly over ontogeny, suggesting that domatia are particularly cos tly for plants early in development.