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Around 400 million years ago, in the Silurian Era, the first plants appeared on land. Most similar to what are known today as Bryophytes, they descended from early water dwelling alga. They lacked vascular (circulatory) systems and complex physical characteristics, but their appearance marked a great step in the development of Earth. The world they populated was far different from that which we enjoy today: rocky, exposed and barren, with no sign of the rich and diverse life which was to later make our planet so unique. Only the oceans, from which the first plants came, teemed with organisms. As time passed, the environment caused the first plants to change and modify their structures to be able to meet their life needs. Further impetus for evolution arrived with the first terrestrial animals, as the first links in the great chain of animal-plant codependence were forged. With change came specialization and increasing complexity in structure, and it is based upon these characteristics that plants are classified or "ranked" today in the study of phylogenetics. Classification begins with comparing the primitive features of the earliest terrestrial plants to those of other plants. In this method, we can organize terrestrial plants into the following basic divisions, in order of increasing development: Bryophyta, Pteridophyta (ferns and fern allies), gymnosperms (Cycadophyta, Ginkgophyta, Coniferophyta, and Gnetophyta), and angiosperms (Anthophyta, divided into dicots and monocots). In this tour, we will explore the evolution of the unique kingdom Plantae's terrestrial members. To make matters simpler, we will focus mainly on comparing the reproductive characteristics of each group. Other features also serve to illustrate the evolutionary relationships, but the reproductive differences are vivid and clear markers of the progression of terrestrial plant advancement. As you proceed, consider the increasing complexity of the plants as we progress from the most primitive Bryophytes to the most modern angiosperms. Most of all, enjoy yourself as you delve into the wonderful and fascinating world of plants.