In land plants (Embryophyta), comprising bryophytes, pteridophytes, gymnosperms and angiosperms, DNA Cvalue data has increased significantly in recent years. Estimates are available for over 3,800 species, ranging over 2000- fold (1C = 0.06 - 127.4 pg). Evaluating the phylogenetic component of this variation is essential to understand its evolutionary significance. Consequently, C-value data were superimposed onto well-supported phylogenies. This showed: (1) Most angiosperms (52%) had small C-values (defined as 1C £ 3.5 pg) within five times the mode (0.6 pg). Very large C-values (1C ³ 35.0 pg) occurred in only two distantly related groups (monocots and Santalales) suggesting that ancestral angiosperms had small genomes, and that very large genomes represented a derived condition that arose independently at least twice. (2) In contrast, extant gymnosperms (sister to angiosperms) typically had larger genomes whose modal Cvalue (15.8 pg) was over 20 times greater than angiosperms. Large C-values (³ 14.0 pg) were even typical in extant cycads (mean 1C = 14.7 pg), regarded as the basal group of gymnosperms. Thus within extant seed plants (gymnosperms and angiosperms), possession of a small genome seems unique to angiosperms. (3) C-value data in pteridophytes and bryophytes are still too sparse to permit analysis of evolutionary trends. However, low C-values (1C = 0.06 - 2.1 pg) in lycophytes, a pteridophyte group sister to all other extant vascular plants, suggest that first vascular plants had small genomes. Further, the narrow range and small size of C-values in bryophytes and Selaginella and Lycopodium (0.06 - 2.1 pg) seem likely to be under tight nucleotypic control. Nucleotypic correlations between cell size and nuclear DNA Cvalues in fossil plants may reveal major trends of plant genome size evolution over geological time. Knowledge of genome size, its origin and consequences will further unify our understanding of the evolutionary processes, structural components and DNA sequences responsible for p lant genome diversity.