Reducing size to increase number: a hypothesis for compound leaves

Abstract

Adaptive advantages of the compound leaf form in flowering plants have so far remained elusive. A novel idea—proposed here—is that there are no direct advan- tages; the compound leaf evolved instead as a trade-off of selection in favour of something else: greater leafing intensity. Producing more leaves per unit of supporting shoot or plant body size generates a larger ‘bud bank’: an aggregate of axillary meristems available for optimal deployment in strategies for growth, survival, and reproduction, and also facilitating capacity (through DNA replication errors) for generating novel adaptive mutations that can be transmitted through the germ line. But, higher leafing intensity requires that individual leaf mass be smaller. Transition from a simple leaf (the ancestral form in angiosperms) to a compound leaf, therefore, may represent one mechanism whereby indiv- idual leaf mass was reduced within some angiosperm lineages. Compound leaves, however, are rare in angio- sperms, I suggest, because more parsimonious mechan- isms for mass reduction in a simple leaf are likely to involve straightforward reductions in overall dimensions (i.e. length, width), or increased lobing.