How Did Land Plants Evolve?
A lot of attention is paid to the process by which fish-like animals first evolved primitive legs and adapted to life on land instead of in the water. However, we often forget that plants had already made this crucial transition much earlier in evolutionary history. Without land plants to provide food and habitat, it would have been impossible for the first land animals to come ashore.
It is believed that the earliest plants evolved from a photosynthetic protist that resembled green algae. Like algae, these early plants were highly dependent on water for structure, hydration, and even reproduction; their sperm cells needed water to swim through in order to reach egg cells. The thing that set these early plants apart from algae was their differentiated male and female reproductive parts.
Adapting to dry land presented a lot of challenges. The plants had to hold themselves upright without support from an aqueous environment, and they had to hold on to enough water to avoid drying out. Early land plants were small, low to the ground, and usually found in moist areas as they were still fairly reliant on water.
Eventually some plants evolved roots, which gave them better stability and allowed them to absorb water from the soil even when the air was dry. Vascular plants, (the most dominant form of plant life) evolved a vascular system, which you can imagine as a series of internal pipes that allows plants to move water and sugar between different tissues in their body.
This way water collected in the roots could be transported to the stem and leaves, while sugar from photosynthesis in the leaves could be shared with the roots that weren’t generating energy themselves.
Early vascular plants probably resembled something like modern ferns. Although they were better adapted to land, they were still dependent on water for reproduction. It wasn’t until more advanced plants evolved seeds and pollen that plants were able to reproduce independent of access to water.
Helga (photographer). (2014). Fern Green Plant [photography]. Retrieved from https://pixabay.com/photos/fern-green-plant-spring-340854/