Vascular plants comprise most of the plants people see everyday. From the tallest trees to the tiniest flowers, they are complex structures with roots for gathering water and nutrients and leaves for gathering energy from the sun. For much of Earth's history, plants were single-celled organisms, like moss, with no roots or leaves and absorbed nutrients directly through their cells. The evolution of a vascular system allows for the incredible diversity seen in plants today.
Evolution of the Vascular System
Up until 425 million years ago, plants were simple structures like moss and algae. Because these plants absorbed water through their cells, the environments they could colonize were limited to those with water readily available. As these plants moved out of the water and onto land, they had to adapt to environments where water wasn't consistently available. The development of roots allowed these early plants to find water underground and thrive in relatively dry environments.
The first vascular plants didn't have leaves like plants today, they had what resembled leafless stalks. As more and more plants evolved, competition for sunlight brought about the formation of leaves.
The Vascular System
Plants absorb nutrients in two ways -- through the roots along with water, and through the leaves as sunlight, which is then photosynthesized into nutrients. The two main structures in a plant's vascular system are the xylem and phloem. The phloem carries nutrients from the leaves down to the rest of the plant. Xylem carries water and nutrients up from the roots to the rest of the plant.
Much like the blood vessels in our bodies, the xylem vessels are hollow tubes perfect for allowing the flow of nutrients. The xylem vessels in all plants are strengthened by lignin, the substance that also makes up tree bark. At maturity, all xylem is dead. In plants the xylem, even though it is not alive and growing, still functions. In a tree, when the xylem vessels die they cease to serve as conduits for nutrients and simply add strength in the form of wood. You can see old xylem vessels in the rings of a tree.
Photosynthesis is the process by which plants convert sunlight to sugars. Leaves collect sunlight and convert it to sugar, which then needs to be delivered to the rest of the plant. Unlike the xylem, the phloem is made up of living cells. These cells are stacked end to end and are called sieve-tubes. The sieve-tubes are held together by sieve-plates and together with companion cells they guide sugars to the rest of the plant.