Vascular plants have a mechanism for moving water and food throughout the plant. Specialized cells arranged in bundles transfer water up from the roots to the stems and on to the leaves. Other specialized cells within the bundles move simple sugars manufactured in the leaves to places of active growth, reproduction or storage. The evolution of vascular plants allowed for species larger than a few inches to develop into many of the plants we know today.
The specialized cells that transfer water up from the roots are arranged in tube-like structures that start in the rootlets, travel through the stems and into the leaves. This tissue is called xylem and is the primary means of moving water up from the roots. Water molecules form weak bonds with each other in long strings. When a water molecule evaporates from a leaf pore (called stomata), it pulls molecules below it up, and with it, all the water molecules below it as well. Xylem is arranged in bundles and is comprised of non-living cells arranged end to end. The movement of water through the xylem is called transpiration.
Two cell types create phloem and unlike xylem, phloem is comprised of living cells. Sieve cells are arranged in long tubes. Each cell has an end plate with pores that allow food in the form of sucrose to pass through it. Sieve cells have no nucleus and are controlled by companion cells which do have a nucleus. Movement of food through the phloem is by osmotic pressure. Sucrose dissolved in water (sap) fills a sieve cell, increasing pressure. The liquid moves to a place of lower pressure, either roots or regions of active growth. In the roots, sucrose is usually converted to starch.
Xylem and phloem form bundles of tubes throughout the plant in the roots, stems and leaves. A number of xylem tubes and phloem tubes are in each bundle. The vascular tissue (xylem and phloem) is bounded by sheath cells which form the exterior of the bundles. These bundles appear as the veins you see in leaves and are present not only in the leaves, but throughout the plant. Although the parts appear very different, the vascular tissues in the roots, stems and leaves are all the same.
Larger and Older
The evolution of vascular plants and their ability to move water and nutrients from one part of the plant to another allowed plants to grow much higher and live much longer than earlier plants. It took less than 100 million years (a relatively short period) for simple ferns to develop into giant forest trees 5 feet in diameter and 100 feet tall. By contrast, simple moss can never grow more than a few inches tall; it lacks the ability to move food and water throughout itself because it has no vascular system. Today there are trees reaching up to 400 feet high and others have been alive more than 4,000 years.
All plants share the same ancestry--green algae. Some 700 million years ago, algae made the move from ocean to land. From algae evolved the simple mosses and liverworts. The club mosses, which are not mosses at all, evolved from the liverworts. The club mosses were likely the very first vascular plants and first appeared approximately 400 million years ago. From the club mosses evolved the horsetails and ferns and from those came the seed-producing plants which include everything from tiny flowers to the giant redwoods.