LEAF DEVELOPMENT AND CROWN GEOMETRY OF 2 IRIARTEOID PALMS

We examined changes in pinnately compound leaf morphology and crown ge ometry that occur during height growth of the iriarteoid palms Socrate a exorrhiza and Iriartea deltoidea in tropical wet forest of Costa Ric a. Although light availability increased with height, the number of le aves per plant was relatively constant. Total leaf area, however, was much larger in taller individuals. Increases in linear dimensions of l eaves (length and width) was responsible for less than half of this gr eater surface area. More important was the transition from a basically dorsiventral display of leaflets in small individuals to a more radia l display in taller plants. Production of leaflets in more than one pl ane resulted in leaves whose surface area was more than twice the hori zontally projected area and whose lateral light interception was great ly enhanced. S. exorrhiza, a faster-growing and more light-demanding s pecies, undergoes this transformation in leaf morphology at heights be tween 3 and 6 m; whereas in the slower-growing and more shade-tolerant I, deltoidea this occurs at heights between 10 and 20 m. Pinnate leav es, with dense radial packing of leaflets along the rachis, are functi onally comparable to branches of dicotyledonous trees and may have bee n important for the evolution of arborescence in palms.