The closest living relatives of plants are green algae that grow in streams and ponds (Fig. 33.1). Some features found in plants are also present in the close algal relatives, including cellular structures such as plasmodesmata and the enzymes used to reduce CO₂ loss during photorespiration. But for the most part, plants are quite distinct from their aquatic progenitors. The reason is that, when plants moved onto land, they had to survive in a new medium, air. Algae can rely on water for hydration, support, and protection from ultraviolet radiation and temperature fluctuations, and they can use water currents to transport sperm and disperse offspring. Surviving on land required the evolution of new ways of growing and reproducing. Here, we highlight key adaptations that allowed plants to photosynthesize and reproduce on land, explored in Chapters 29–31, and discuss how plant diversity changed as new ways of growing and reproducing evolved.