| Leaf epidermal anatomy of 46 species of basal angiosperms, representing Amborellaceae, Nymphaeales, Austrobaileyales, and Chloranthaceae, was investigated. Leaf clearings, cuticles, and unmacerated samples were examined with light and scanning electron microscopy. Characters pertaining to specialized structures such as stomata, ethereal oil cells, etc., were constructed, coded, and their evolution was traced. The results of this and comparison to early angiosperm fossils and other living and fossil seed plants were used infer ancestral character states, character polarity and evolution, and to answer questions about homology, development, and related issues.; An investigation of stomatal architecture---the number, form, and position of specialized epidermal (subsidiary) cells associated with stomatal guard cells---indicates that the ancestral state of angiosperms is anomo-stephanocytic, a system in which stomatal complexes lacking subsidiary cells (anomocytic) intergrade with those having weakly or strongly defined rosettes of cells (e.g., stephanocytic). From this system, tangential divisions arose in cells of stephanocytic or anomocytic precursors in early Cretaceous fossil taxa, and in extant Amborellaceae, Austrobaileyales, and Chloranthaceae, to form more derived stomatal types with strongly specialized subsidiaries such as laterocytic and paracytic. Stomatal architecture in Nymphaeales became little modified. Other lineages of living and fossil seed plants also display evidence of modification through tangential divisions.; Examination of other specialized leaf epidermal structures provides strong evidence that ethereal oil cells of Austrobaileyales, hydropotes of Nymphaeales, and uniseriate nonglandular trichomes in various genera, and their associated rosettes of epidermal cells are homologous with, and evolutionarily derived from, stomatal complexes. Oil cells, and basal portions of hydropotes and trichomes are hypothesized to be homologous with the stoma or guard mother cell.; Levels of variability in stomatal architecture in these and more derived angiosperms were quantified with three different metrics, including a new one devised here. These were used to infer that the ancestral angiosperm condition was characterized presence of relatively few stomatal types, but a great variability in stomatal architecture, and an inferred lack of developmental canalization. The results cast doubt on the idea that levels of variability in early angiosperm fossils and extant basal taxa are highly atypical of angiosperms as a whole. |
