LEAF DIMORPHISM IN THE AQUATIC ANGIOSPERM CALLITRICHE HETEROPHYLLA

Abstract

Depending on the position of the shoot tip relative to the water surface, the aquatic angiosperm Callitriche heterophylla produces either ovate land‐form or linear water‐form leaves. This paper is concerned with the developmental basis for the leaf dimorphism of this species. Little significant difference is observed between the apical meristems of submerged vs. emergent shoots; moreover, land‐form and water‐form primordia undergo similar, if not identical, patterns of initial development until they attain a length of 350 to 400 μm. These findings are interpreted to mean that the divergent leaf forms result from the marked sensitivity of the primordia to their respective environments rather than from the mode of their inception. Subsequent growth of the young water‐form leaf emphasizes longitudinal extension, while the immature land‐form leaf continues balanced expansion in both longitudinal and lateral directions. The lateral growth of the land‐form primordium is accomplished in part by a more persistent marginal meristem, but the morphological difference between the two leaf forms is mostly attributable to the difference in the predominant direction of intercalary expansion. In addition, certain anatomical features, such as vasculature, stomates, and cuticle, are much more prominent in mature land‐form leaves than in water‐form leaves. These anatomical differences seem to represent structural adaptations of each leaf form to the specific physiological requirements of its environment.