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Life on Earth originated in the oceans. Plants, like animals, evolved through a number of stages which enabled them to live and reproduce successfully on land.
Algae
Around 420 million years ago, some marine algae developed the ability to live outside the water by growing waxy cuticles that kept them from drying out. However, the algae that had these cuticles still needed to enter the ocean in order to reproduce sexually. Algae reproduce both asexually, by division, as well as asexually. Sexual reproduction - and the genetic variation that it brings - is necessary for evolution to occur.
Mosses and Liverworts
Mosses and liverworts were the first plants to use the wind to assist in reproduction. These plants alternate between sexual reproduction and asexual reproduction with each generation. During sexual reproduction, the egg is attached to a stem. Sperm cells are released in the water and swim to the egg in order to fertilize it. The egg that is fertilized remains attached to the parent plant.
The next generation reproduces asexually. The plants of this generation consist of a thin stem with a hollow capsule in which many small spores are produced. When the atmosphere dries up, the wall of the capsule breaks and the spores are distributed by the wind. Those that land on a suitable site grow to become new plants. These plants cannot grow very high. They have soft cells that aren't strong enough to allow their stems to stand up straight
Plants with Upright Stems
Over 400 million years ago, the first plants that could stand up appeared on Earth. These plants, which could grow to heights up to a few centimeters, had long cells with thick walls that allowed water to travel up a stem. They had a horizontal stem that lay along or just below the ground. The upright stem was an extension of the horizontal stem. This meant that these plants could only survive in places where there was a constant supply of aboveground water.
The First Trees - With Roots and Leaves
Three groups of plants - ferns, clubmosses and horsetails - went on to develop roots. This allowed them to absorb water from underground sources. These plants had strong, woody vessels in their stems. This allowed the water that the roots absorbed to be transported up the stems. The evolution of rigid stems allowed plants to grow very tall. Because green plants depend on sunlight for photosynthesis, there was intense competition to grow taller. If a plant isn't as tall as its neighbors, it will be blocked from receiving sunlight and may die. As a result of this competition for height, these plants grew very tall and became the first trees.
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