Vascular plants are the largest class of plants in the world and comprise nearly a quarter million species, with many as yet unclassified. By contrast, other classes make up less than 35 thousand species. From the giant sequoias and bristlecone pines to the tiny sweet alyssum flower and primitive ferns, vascular plants have evolved and spread throughout the world.
Vascular plants are characterized by their ability to move water and nutrients throughout the plant, unlike nonvascular plants, which have no mechanism or built-in structure for doing so. Water is moved upward from the roots to the stems, leaves and flowers through specialized cells that make up tubes called xylem.
Xylem are dead cells placed end to end to form tubes. Water moves by a process called transpiration, which operates on the physics of the surface tension of water.
Phloem moves sucrose manufactured in the leaves through the plant by another method. The phloem are also tube-like cells, but they are alive. Companion cells control the flow of sucrose into and out of the phloem tubes.
The first vascular plants were ferns evolved from mosses and liverworts about 400 million years ago. Within another 100 million years, the ferns had evolved into large trees up to 100 feet tall and 5 feet in diameter. From the fern plants evolved the seed-producing plants called gymnosperms approximately 350 million years ago. Gymnosperms make up our conifers, ginkgos and palm trees. It would be another 200 million years until the flowering plants--angiosperms--evolved into the numerous plant species found today.
Vascular plants reproduce by megaspores (female) and microspores (male) in sexual reproduction. The spores are joined to produce a gametophyte, a new organism. The gametophyte grows and produces an additional offspring that will be either a seed (in angiosperms and gymnosperms) or an adult plant (non-seed-producing plants only).
The vascular plants represent a leap in the evolutionary record of plants. Prior to vascular plants, only mosses, liverworts, hornworts and algae existed. These plants were unable to grow large since they had no way of moving food throughout their systems. Likewise, they were unable to support the complicated means of reproduction found in vascular plants.
Algae dominated the worlds oceans during the Precambrian period, more than 700 million years ago. They made the transition to land species during the Cambrian period and evolved into mosses, liverworts and hornworts over 300 million years. The first vascular plants--club mosses, horsetails and ferns--developed late in that period and eventually evolved into gymnosperms between 350 and 400 million years ago. The angiosperms were the last of the vascular plants to emerge approximately 140 to 150 million years ago.