The opening chapter offers a brief introduction to paleontological methods and an overview of the evolutionary timescale from a botanical viewpoint. This is followed by a brief account of the first prokaryotes and the origins of eukaryotes. The Evolution of Plants then proceeds chronologically in five chapters, each covering the fossil evidence from a period, the origin of key groups and their distinctive morphological features and innovations, phylogenetic reconstructions, tectonic and environmental history, and biogeography and the biomes present.
The middle chapter of these begins by surveying the broad environmental changes during the Permian (290-248 million years ago): the formation of Pangaea and warming and aridification with increasing levels of atmospheric carbon dioxide. It covers the evolution of cycads, bennettites, ginkgos, and glossopterids, looking at their distinctive features and the relationships between them. It outlines the biogeographical distribution of vegetation during the middle Permian, when there were seven biomes: tropical summerwet, tropical everwet, subtropical desert, warm temperate, mid-latitude desert, cool temperate, and cold temperate. And it describes the radiation of the conifers and the changed biogeographical distribution during the early Jurassic.
The two chapters preceding that cover the colonization of the land (and the rise of vascular plants) and the first forests (the rise of trees). The two following cover the origins of flowering plants (angiosperms and their coevolution with insects) and the past 65 million years (which saw aridification and the evolution of grasses and C4 and CAM photosynthesis).
Willis and McElwain find no clear evidence in the plant fossil record of mass extinctions to match those among marine and terrestrial fauna; they also consider some of the possible reasons for the persistence of relatively unchanged plants over long periods of time. They look at the contested claims and debates over the recovery of ancient DNA, and touch on the use of other biomolecules and carbon isotope ratios in probing the past. And in a final chapter they fit plant evolution into broader evolutionary theory, considering punctuated equilibrium and possible links to orbital variations, pulsation tectonics, and so forth.
One disappointment is the cursory treatment of chloroplast origins, with only a passing mention of their endosymbiotic origins as cyanobacteria. And there's not much developmental biology: Willis and McElwain ask why lineages such as angiosperms and grasses developed "so late", but they only consider environmental explanations, not the possibility that developmental constraints were significant. These are minor complaints, however.
The Evolution of Plants assumes no knowledge of plant anatomy or taxonomy: occasionally technical terms are used without explanation or an entry in the glossary — "entire-margined", "cotyledon" — but that seems more indicative of sloppy editing. Excellent use is made of maps, anatomical diagrams, timelines and charts, and so forth: these are done simply, and in black and white, but they are clear and effective and engaging. The only "textbook" element is a summary at the end of each chapter.
The result is not just for botany students, but for anyone curious about plants. The Evolution of Plants should be perfect for amateur botanists who enjoy popular books on evolution and evolutionary history.