Agriculture Reference
In-Depth Information
Chapter 2
Leaves: Evolution, Ontogeny, and Death
Bud burst of Alnus hirsuta
The evolutionary origin of leaves traces back to the gradual modification of branching
systems in the earliest land plants. The vascular plants belonging to the phylum
Rhyniophyta that first colonized land more than 400 million years ago had only
simple dichotomously branching axes without organs we would recognize as either
leaves or roots (Sussex and Kerk 2001). The early evolution of the land plants
involved a combination of progressive changes in branching architecture (overtop-
ping) and the associated flattening (plantation, fusion) of some branch elements to
form laminar photosynthetic organs that we recognize as leaves (Sussex and Kerk
2001; Boyce and Knoll 2002; Donoghue 2005). Over the course of the Paleozoic,
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