Structure and function in angiosperm pollen wall evolution

Abstract

Observations and measurements from angiosperm species in more than 150 genera from 15 monocotyledonous and 69 dicotyledonous families show that harmomegathy, changes in size, shape, and volume of pollen grains as a result of water loss or intake are characteristic for flowering plants. Size changes as measured from the exterior wall surfaces average 46%. No particular pollen types or kinds or arrangements of apertures seem to be superior for providing mechanisms to accomodate these volumetric changes and the associated stress. As water is lost from a grain, contractions and invaginations of thinner areas of the wall establish geometric forms related to the positions and numbers of germinal apertures, and to internal and sculptural elements of the exine. As a result of these invaginations and accompanying flexion and arcuation (arching) of interapertural wall sections, configurations are created that permit the grain to develop and withstand enormous stress without breaking. The stress is a measure of negative wall pressure that helps establish a water deficit within the grain that brings the living substance of the protoplast into equilibrium with the desiccating forces of the atmosphere. Stress is accommodated through the inherent strength and resiliency of sporopollenin, enhanced by cellularization (baculation, lamination, plication, and alveolation), and by strutting and reinforcement systems involving massive deposits of sporopollenin at special bearing and stress points, such as costal and oral margins, or the poles. Architectural features that distribute and bear the stress include mechanisms for tension, arcuation, and suspension that carry stress to bearing points. The effect of these accommodations is to permit the pollen grain to arrive at the stigma alive and in condition to respond and grow quickly. It is hypothesized that evolution of structural features that meet harmomegathal stress requirements provides the principal explanation for pollen wall form, composition, organization, and architecture.