Flowering plants first appeared around 135 million years ago and have since diversified greatly. They have flowers that attract pollinators like bees, helping the plants reproduce. There are over 500 ways to classify flowers based on their many variations from crossed genes between species. Angiosperms can be divided into two main classes - monocots and dicots - based on their seeds and seedling structure. Monocots have one seed and dicots have two. Angiosperms can also be classified as woody, herbaceous, annual, perennial, or biennial based on characteristics like their stem structure and lifespan.