Angiosperms, or flowering plants, arose about 130 million years ago and gave rise to more than 250,000 extant species that make up nearly 90% of plant life on Earth. As the name suggests, their most distinguishing feature is the flower itself, the color and structure of which exhibit tremendous diversity across species. Despite this variation, flowers have some basic structural similarities, which include stamens (male reproductive organs) and carpels (female reproductive organs). Like gymnosperms, angiosperms are seed plants, but differ in that the unfertilized egg or ovule is protected in layers of plant tissue. Fertilization occurs by male gametes passing into the ovary via a pollen tube. After fertilization, the ovule develops into the seed, while the ovary wall develops into the fruit. Angiosperms have traditionally been divided into two subclasses: monocots (one seed leaf per seed) and dicots (two seed leaves per seed). However, new data from both molecular studies and fossils now suggest that this simple division does not reflect true evolutionary relationships, and therefore needs rethinking.