The flowering plants (angiosperms) are the most diverse and most recently evolved plant group with more than 250 000 species compared to second most diverse group, the ferns (pteridophytes) with ~10 000 species. The angiosperm radiation, diversification and rise to dominance occurred over a very short period of geological time. During this radiation the angiosperms to moved away from there ancestral aquatic/ moist habitat to more arid environments. Explaining the underpinng mechanisms behind this event remains one of the key unanswered questions in the palaeobotanical research. Advances in our understanding of the genome of living plants shows that many angiosperms may have undergone several episodes of genome duplication (polyploidy) during their history. Duplication of the genome leads to an increase in plant vigour and fitness, when compared to their ancestors. A well known example of this phenomenon is the hybrid vigour of modern crop plants when compared to their wild relatives. Dating of genome duplication events in the model plant Arabidopsis thaliana shows one of these events occurred during the initial angiosperms radiation. This suggests that changes in genome size may have played an important role in driving this diversification. However, to date this hypothesis has only been tested using the genome of one extant plant, furthermore no direct measure of the size of the genome of fossil plants exists. Consequently, although fascinating, this idea remains speculative. Recent work has shown the length of guard cells (specialised cell on the leaf surface that control gas exchange) are a strong positive predictor of genome size of living plants. Leaves have an excellent fossil record and obtaining the length of guard cells from fossil specimens is straight forward. This suggests that guard cell length could be used as a proxy measure of genome size on fossil plants. I plan to use this relationship to test the hypothesis that increase in genome size accompanied the diversification of the early angiosperms. A suite of experiments are designed to run along side this fossil plant investigation to assess how polyploidy impacts on plant health when grown in drought conditions at elevated atmospheric CO2 concentrations relevant to when the angiosperms originated. This work is relevance and importance to the wider scientific community given that a large proportion of crop plant are polyploids.