91. Summary
Miocene angiosperm leaf tissues of Betula, Hydrangea,
Platanus, and Quercus, and of Castanea, Persea, and gymnospermous tissues from
offshore and onshore lacustrine sediments, respectively, reveal various states
of ultrastructural detail in preservation which correlate with their
paleobiochemical profiles. Excellent cytologic preservation of membrane-bound
organelles in offshore tissue samples is correlated with relatively mild
chlorophyll and fatty acid degradation (chlorins, unsaturated hydrocarbons),
while progressive deterioration of ultrastructure seen in onshore specimens is
concomitant with extensive chlorophyll degradation and saturation of
hydrocarbons. Comparative studies between fossils found in pyroclastic (Succor
Creek Flora, Miocene) and lacustrine (Clarkia Flora) environments indicate that
the degree of pre- and postdepositional hydration of tissues is a less
significant factor in determining preservation than redox potential and
secondary metabolites (tannic acids, chlorophyll derivatives). A sequence of
organelle degradation in leaf tissues is given, and it is suggested that
chloroplasts and cell walls are the most stable cellular constituents, while the
endoplasmic reticulum, nuclei, and mitochondria are the most labile.