The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions or other circumstances. However, there is relatively new fossil evidence of flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Angiosperms represent a single origin of related organisms, the phylum Anthophyta, that experienced an exceptional radiation in species. As of 2019, there are approximately 370,000 known extant species. Most of the plants that you see, eat, and otherwise interact with in your daily life are likely to be in this group.
Angiosperms can be distinguished from other plant groups by the production of flowers. These collections of modified leaves allowed angiosperms to attract pollinators and increase the chances of successful fertilization. Over time, angiosperms evolved different flower morphologies, smells, and colors that corresponded to their particular pollinators. These sets of characteristics, called pollination syndromes, allow scientists to predict the pollinators for different plants.
In the xylem, this group of plants evolved large diameter conducting cells for rapid water uptake called vessel elements, though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements with their associated companion cells, increasingly specialized for transportation of photosynthates.