ON DETERMINATION OF LEAF FORM IN RANUNCULUS AQUATILIS

Ranunculus aquatilis L. is an amphibious plant that shows a spectacular kind of heterophylly; the leaf primordia possess the genetic competence to develop three different kinds of leaf: i, the submerged divided leaf which is made up of long, cylindrical, capillary segments; 2, the terrestrial divided leaf which is made up of short, flattened, capillary segments; and 3, the entire leaf which has a normal expanded blade borne on a petiole. An account is given of the structure and development of these leaf forms. The effects of temperature, photoperiod, light intensity and emergence or submergence in water on the determination of leaf form were studied. Temperature and light intensity unless exceptionally high or low played no role in the determination of these leaf forms. It was found that the submerged divided kind of leaf was developed in terrestrial or submerged conditions under short photoperiods (10 hours or less) but under long photoperiods (14 hours or more) it developed only on submerged stems. The terrestrial divided leaf was formed only under long photoperiods in terrestrial conditions while the entire kind of leaf, which occupies the air‐water interface, was developed only under long photoperiods in submerged conditions. Photoperiod, therefore, determines the epigenetic ability of the stem apex to develop different kinds of leaf but it is the presence or absence of water per se that regulates the phenotypic expression of this epigenetic ability. The ecological and evolutionary significance of this kind of leaf form determination are discussed. Copyright © 1969, Wiley Blackwell. All rights reserved