Vascular plants contain tissues that transport water and nutrients to all parts of the plant. All higher plants have a vascular system and they make up the majority of plants today.
The migration to land by plants probably occurred 450 million years ago. The first plants were non-vascular green algae, and lacked true leaves, stems and roots. Without a vascular system to move nutrients and water, these primitive plants gathered their needed moisture and nutrients by direct contact with the soil. From these simple plants, vascular plants emerged. The fossil remains of the first vascular plants date to 410 million years ago. They had leaves, stems and roots, and they quickly became the dominant form of plant life.
The vascular structure of plants encompasses roots, stems and leaves. Roots allow plants to draw moisture and nutrients from the soil. The development of roots allowed plants to spread out over larger geographical areas and exist in drier locations. The vascular system also provides a support structure, allowing plants to grow taller and for more surface area for photosynthesis.
How the System Functions
Vascular plants contain conducting tissues called xylem and phloem, as well as roots and a shoot system made up of leaves and stems. Water and nutrients enter the plant through the roots and are carried by the xylem to all parts of the plant. The shoot system, made up of leaves and stems, specialize in photosynthesis, producing sugars that are carried to all parts of the plant by the phloem. Primary plant growth occurs close to the tip of the roots and stems.
Xylem and Phloem
Xylem tissue has rigid, persistent walls that contain cellulose. When the cells die, the structure remains and continues to move water and nutrients upward. Their persistence allows them to be seen in fossil records. A good example of xylem is wood. As the plant grows, more cells form around the outside, strengthening the plant structure and thickening the stem and roots. Phloem is made up of living cells only, and sits on the outside of the xylem. The phloem moves the sugars created by photosynthesis around the plant as needed. In trees, the phloem exists just under the bark. If the phloem dies or is severely damaged, the whole plant will die. That is why the removal of bark around a tree will kill it.
Not all vascular plants produce seeds. Seedless vascular plants also have a vascular structure and roots, stems, and leaves, but they reproduce in a very different way, using spores. Millions of years ago, they were the primary form of plant life, but today are only represented by a small group of plants that include club mosses, scouring rushes, horsetails and ferns.
Vascular plants that produce seeds are divided into two groups, gymnosperms and angiosperms. Angiosperms include all the flowering plants and make up the largest plant group today. Gymnosperms produce seeds that are not enclosed in an ovule. Plants in this division include conifers, cycads, ginkgo trees and Gnetales.