Although it is not certain when plants first arose, it appears that they did so during a time when the Earth's climate was changing. Likely, those areas where plants evolved was subject to periods of flooding and periods of drying, and characteristics that enabled some species to better survive during the dry periods evolved slowly. Adaptation to the drier conditions eventually enabled early plants to colonize the land. To fully appreciate the huge advantages that terrestrial migration had for plant development, it is necessary to understand the differences between aquatic and terrestrial environments with respect to requirements for plant growth.
In order for plants to photosynthesize and produce the proteins, lipids, and carbohydrates necessary for growth, they require light energy. Light energy available to organisms living beneath the water's surface is greatly reduced. The blue and especially red wavelengths of light that are absorbed by photosynthetic pigments do not penetrate deep beyond the surface of the water; therefore, photosynthetic organisms living in an aquatic environment do not receive the full amount of light energy radiated from the sun. Photosynthetic organisms growing on land do not face this problem. Photons emitted from the sun can directly strike light-absorbing surfaces and the full range of useful wavelengths are available for photosynthesis. In the aquatic environment, many large algae compete for sunlight (similar to the competition for sunlight in modern forests). For early plants first moving into terrestrial environments, there was no competition for access to light.
It is important to remember the fundamental role that plants play within an ecosystem. Plants and other autotrophs are the basis for supporting heterotrophic life. Prior to colonization of the land by plants, there was little basis for support of animal or fungal life. As mentioned above, the vast majority of life existed in the ocean, including herbivores that depended on algae for food. Another great advantage to the terrestrial migration of early plants was the lack of herbivores on land. Compared to life in the ocean, the terrestrial environment provided free access to sunlight and freedom from damage by larger organisms that could crush or eat the developing plants. However, the conditions on land were not completely hospitable for early plants.
The major challenge for early plants first migrating onto land was the lack of water. In an aquatic environment, desiccation is generally not a problem and there is no need for any protective covering to prevent water loss. Lacking any protection from the dry terrestrial environment, early plants probably dried out very quickly and would have been limited to very moist environments.
The ancestors of early plants were dependent on water, not only to maintain their moisture content but also for structural support. The buoyancy of water supports upright growth of giant marine seaweeds (e.g., kelp) Consider the seaweeds that are often found washed up on the beach. Although these algae are no longer alive, when held beneath the water their upright form is restored. In a terrestrial environment, the surrounding media is air rather than water. Air does not provide any support for upright growth. The transition to land required changes in structural features, and, as will be discussed later in this tutorial, adaptations for structural support are key features used in plant classification.