Explain the Evolution of the Vascular Plant Body?

Vascular Plant anatomy reflects adaptation to life on land. In an aquatic environment, the photosynthetic surfaces are supported by the buoyancy of the water medium, and therefore do not require significant internal structural support. On the other hand, vascular plants have specialized photosythetic organs called leaves, which do require structural tissues to raise them off the ground and maximize their exposure to sunlight, as well as enable the exchange of gases with the atmosphere.

Another adaptation to life on land that plants have had to make is the prevention of desiccation, or drying out. Leaves have evolved a layer of waxy coating called the cuticle, which covers the epidermal cells to prevent excessive water loss. Since oxygen and carbon dioxide need to be exchanged with the atmosphere, special cells called guard cells form an opening (stomate) in the leaf and cuticle to control gas exchange.

Aquatic plants do not require root systems since necessary water and nutrients are absorbed directly from the surrounding water. In order to make their transition to life on land, plants evolved vascular tissues to transport the water, minerals and nutrients to the organs above the ground. The vascular tissue that carries water and minerals up from the roots to the stems, leaves and flowers is called xylem, and the tissue that carries photosynthetic products to the rest of the plant is called phloem.

Xylem tissue consists of cells whose cell walls are thickened with fibers of cellulose, a polysaccharide. These cellulose fibers are laid down much like layers of fiberglass, and become impregnated with lignin, making them rigid. Xylem tissue has a common name--wood. Wood is formed in stems and branches, and provides structural support for the upright portions of terrestrial plants.

There are two kinds of water conducting cells: tracheids and vessels. Tracheids form thick cells walls that have pores called pits through which water passes from tracheid to tracheid. The protoplasm of these cells dies in order for the water to travel through the cell. Most Gymnosperms usually have only tracheids, as opposed to Angiosperms, which tend to have both tracheids and vessels.

Vessel cells go through a development process whereby they eventually lose their end walls and their cytoplasm, and ultimately form long empty tubes of cells laid end to end. These vessels act like tiny capillary tubes that carry water and minerals upwards much like straws. Distinct bundles of xylem vessels form in some groups of vascular plants, and in other groups xylem forms the familiar rings, or growth rings in hard woods.

Along with xylem tissue, vascular plants also form cells whose specific function is to provide support for the above ground organs of the plant. Sclerenchyma cells also have thick secondary walls that are impregnated with the hardening substance called lignin. There are two types of sclerenchyma: fibers and sclereids. Fibers are long, thin pointed cells that form bundles for strengthening the stem. Rope and hemp are made from these types of tissues.

Sclereids vary in shape and are shorter cells which are found in the outside layers of seeds and in nutshells. Sclereids provide the hardness that characterize seeds and nuts.

Photosynthetic products from the leaves are transported to other plant organs for use and storage by vascular phloem tissues called sieve tube members. These cells differ from the water-transporting xylem cells in that they retain their end walls. However, the end walls, called sieve plates, do have pores through which food flows from sieve cell to sieve cell.

Another difference between water transporting xylem and food transporting phloem is that the protoplasm does not die and disappear, but instead remains within the cells, forming a continuous protoplasm. Interesting enough, organelles like the nuclei, ribosomes and vacuoles do disappear, leaving only the cytoplasm within the sieve tubes. Another difference is found in cells that accompany the sieve tube cells. These cells are known as companion cells, they do have functional nuclei, and are thought to somehow interact with their related phloem cells through cytoplasmic connections.