<P> Arabidopsis thaliana has a gene called AGAMOUS that plays an important role in defining how many petals and sepals and other organs are generated . Mutations in this gene give rise to the floral meristem obtaining an indeterminate fate, and many floral organs keep on getting produced . Roses, carnations and morning glory, for example, that have very dense floral organs . These flowers have been selected by horticulturists for increased number of petals . Researchers have found that the morphology of these flowers is because of strong mutations in the AGAMOUS homolog in these plants, which leads to them making a large number of petals and sepals . Several studies on diverse plants like petunia, tomato, Impatiens, maize etc. have suggested that the enormous diversity of flowers is a result of small changes in genes controlling their development . </P> <P> Some of these changes also cause changes in expression patterns of the developmental genes, resulting in different phenotypes . The Floral Genome Project looked at the EST data from various tissues of many flowering plants . The researchers confirmed that the ABC Model of flower development is not conserved across all angiosperms . Sometimes expression domains change, as in the case of many monocots, and also in some basal angiosperms like Amborella . Different models of flower development like the Fading boundaries model, or the Overlapping - boundaries model which propose non-rigid domains of expression, may explain these architectures . There is a possibility that from the basal to the modern angiosperms, the domains of floral architecture have gotten more and more fixed through evolution . </P> <P> Another floral feature that has been a subject of natural selection is flowering time . Some plants flower early in their life cycle, others require a period of vernalization before flowering . This outcome is based on factors like temperature, light intensity, presence of pollinators and other environmental signals: genes like CONSTANS (CO), Flowering Locus C (FLC) and FRIGIDA regulate integration of environmental signals into the pathway for flower development . Variations in these loci have been associated with flowering time variations between plants . For example, Arabidopsis thaliana ecotypes that grow in the cold, temperate regions require prolonged vernalization before they flower, while the tropical varieties, and the most common lab strains, don't . This variation is due to mutations in the FLC and FRIGIDA genes, rendering them non-functional . </P> <P> Quite a few players in this process are conserved across all the plants studied . Sometimes though, despite genetic conservation, the mechanism of action turns out to be different . For example, rice is a short - day plant, while Arabidopsis thaliana is a long - day plant . Now, in both plants, the proteins CO and FLOWERING LOCUS T (FT) are present . But, in Arabidopsis thaliana, CO enhances FT production, while in rice, the CO homolog represses FT production, resulting in completely opposite downstream effects . </P>

When did angiosperms become the most abundant type of plant on the planet