The plant genome is organized into chromosomes that provide the structure for the genetic linkage groups and
allow faithful replication, transcription and transmission of the hereditary information. Genome sizes in plants
are remarkably diverse, with a 2350-fold range from 63 to 149 000 Mb, divided into n = 2 to n = approximately
600 chromosomes. Despite this huge range, structural features of chromosomes like centromeres, telomeres
and chromatin packaging are well-conserved. The smallest genomes consist of mostly coding and regulatory
DNA sequences present in low copy, along with highly repeated rDNA (rRNA genes and intergenic spacers),
centromeric and telomeric repetitive DNA and some transposable elements. The larger genomes have similar
numbers of genes, with abundant tandemly repeated sequence motifs, and transposable elements alone
represent more than half the DNA present. Chromosomes evolve by fission, fusion, duplication and insertion
events, allowing evolution of chromosome size and chromosome number. A combination of sequence
analysis, genetic mapping and molecular cytogenetic methods with comparative analysis, all only becoming
widely available in the 21st century, is elucidating the exact nature of the chromosome evolution events at all
timescales, from the base of the plant kingdom, to intraspecific or hybridization events associated with recent
plant breeding. As well as being of fundamental interest, understanding and exploiting evolutionary
mechanisms in plant genomes is likely to be a key to crop development for food production.