Since the first endosymbiotic event that enabled a free-living photosynthetic organism to take up residence in a eukaryotic cell, the organisms that we know of as plants have evolved in a dramatic and highly varied way. In particular, plants have evolved from their single-celled ancestors into complex multicellular structures. A key characteristic of these multicellular plants is that they contain cells of different types, which are distinguishable from each other in the functions that they perform within the whole complex organism. With increasing complexity of form through the evolution of Bryophytes, Monilophytes and into the higher plants, Gymnosperms and Angiosperms, large numbers of different types of cells have been developed such that, in the more complex members of the flowering plants, the Angiosperms, there are over 50 different types of cells. As a result of this diversification of cell types in plants, the original endosymbiotic plastid found itself being manipulated by the host cell to take on a variety of different roles in these different types of cells. Plastids evolved from their original photosynthetic function after the original endosymbiosis to take up a key role in the cell as a whole, particularly in relation to biochemical interactions in the cell's metabolism. As a result, in modern-day higher plants, there are a variety of different types of plastids, which fulfil different roles in different types of plant cells. The situation, however, is not clear-cut. There is significant interaction between different plastid types in different types of cells in that the plastids can interconvert between different types according to molecular and environmental signals.