Mosses were among the first plants to invade the land more than 400 million years ago, about when amphibians also ventured out of the water. Like amphibians, mosses need water for their sperm to swim to their eggs. Ancient, wetlands were the proving ground for plants to evolve the many adaptations required for survival on land. Mosses continue to rely on wet habitats today, and are most abundant in bogs or rainforests. Mosses lack the complex internal transport systems (vascular tissue) of most plants, and instead directly absorb water and nutrients. Because of this lack of vascular tissue mosses are not large in size – most are less than 20 cm high– but they have diversified into more than 9,500 species. Some mosses harmlessly live on other plants (epiphytes) such as colonizing the branches of living trees. They absorb water and nutrients from the atmosphere. During drought, a moss may shrivel up, but then rehydrate and recover under more favorable conditions when water becomes available. A few mosses have escaped their dependence on moisture, evolving to live in dry environments.