Certain megaspores from the genus Selaginella (Lycophyta) and similar
fossil genera of Cretaceous and Tertiary age are known to demonstrate
a remarkable, iridescent appearance. Recent work on the spore wall ult
rastructure shows that this iridescence is produced by a complex, orde
red, particulate organization which is analogous to that of iridescent
virus aggregates, precious opal and other synthetic materials. Our re
sults suggest that this effect is produced by the accumulation of the
spore wall material in the form of a monodisperse colloidal crystal. C
olloidal organizations can also account for adjacent non-iridescent wa
ll layers, the transition zones between these and the colloidal crysta
l and for spore walls with no hint of regular organization. The brief
time in which the distinctive ordered exine ultrastructure develops an
d its occurrence around non-viable spore protoplasts add weight to the
hypothesis that the exine is largely self-assembling. This new model
of spore wall formation may well have wider implications in terms of c
urrent concepts of exine organization and development.