1. Avoiding dehydration. Plants on land would have developed roots to acquire water from the soil. They also developed stomata on the underside of their leaves to reduce water loss. Adaptation to arid environments involved waxy coats and fleshy leaves for water storage.
2. Disease. Bacteria and viruses would have better opportunities for infection on land than in water.
3. Competition. Because they are immobile, land plants would be susceptible to being overgrown by other plants that were better adapted to the habitat.
4. Reproduction. New means of reproduction had to be developed. This would include development of cross-fertilisation to ensure a larger gene pool and therefore greater diversity. New means of dispersal eg. wind, insects.