Vascular Plants

Vascular-From the Latin vasculum meaning a vessel or duct

Vascular plants first appear in the fossil record during the mid-Silurian period, about 410 million years ago. Rhyniophyta is the earliest known division of these plants, represented by several genera.

Figure 1- Rhynia, the first known plant with a complete fossil.

Rhynia (Fig. 1) is the first known vascular plant for which a complete fossil has been found. These leafless and rootless plants consisted of stem tissue which branched along an axis and contained spore cases at the tips of the aerial branches.

Although true roots developed much later, a primitive "pipeline" system for support and fluid transport was an important development for these plants. The pipelinesystem, consisiting of ineterconnected strands of specialized cells, allowed transport of water, nutrients, and metabolites throughout the plant. This tissue was separated into two types, xylem and phloem. Xylem is specialized hollow cells which move water and dissolved minerals up from the roots to the rest of the plant. Xylem also provides support via the lignin which is incorporated into the cell walls, making the plant rigid. Phloem transports carbohydrates and other metabolites from the leaves to other parts of the plant.

Unlike most nonvascular plants, vascular plants have a dominant, conspicuous sporophytic generation which is nutritionally independent. This generation is the familiar organism we see when we look at trees or wildflowers. The gametophytic generation is small and incorporated into the sporophytic generation in angiosperms, although it may be independent in some vascular plants. Heterospory is unique to vascular plants; two different spores form egg-producing gametophytes and sperm-producing gametophytes. Vascular plants have also developed a waxy outer covering called the cuticle which protects from desiccation due to water loss. Specialized pores on the cuticle called stomata act as gateways for gas exchange.

Vascular plants are divided into 2 groups: