The term vascular means relating to vessels that carry fluids. In humans, the vascular system consists of the heart and the blood vessels. Veins and arteries carry blood to all the cells and the movement of blood is powered by the muscular heart. the blood carries oxygen, water and nutrients to the cells and removes carbon dioxid and other waste products. In higher plants, the vascular tissues are called xylem and phloem. Xylem moves nutrients and phloem moves water from the roots to the leaves and back, ensuring that all the plant cells get what they need to survive and grow.
There is no heart of course. Movement occurs because of diffusion as fluids and nutrients flow from areas of high concentration to low. There is also a pumping action that occurs as leaves lose water through transpiration and this pulls water up from the roots. The most primitive plants are the single celled algae and the seaweeds. These plants are termed nonvascular because they have no specialised tissues for moving fluids and foods through the body of the plants.
Single celled plants don’t need a vascular system because water, nutrients and waste products can move in and out across cell membranes. The holdfasts of seaweeds are not true roots because all they do is anchor the plant to the sea floor. There is no movement of water or nutrients from the holdfast to the rest of the plant so the stalk that lifts the seaweed fronds to the surface and the light is not a true stem either.
Similarly, mosses and ferns have no vascular tissues. These are the highest forms of the nonvascular plant world. Seaweeds, because they are found in water, can continue to deliver water and nutrients directly across cell membranes to the cells. Mosses and ferns are limited in how big they can get and the environments in which they can live because they lack vascular tissues. Most are small or restricted to wet habitats. For plants to conquer new habitats in the distant past, they had to evolve some form of system to deliver food and nutrients to the cells. This led to the development of xylem and phloem and with these tissues, came true roots, stems and leaves. This development allowed plants to cover the earth in trees, shrubs, bushes, herbs and grasses.
Trees, in particular, are a good example of why vascular tissues are a necessity in higher plants. The roots can access minerals and water but the whole plant needs them. In a tall tree like a redwood, the leaves are up to hundreds of meters above the ground. Without the connection of the trunk with the xylem and phloem to move the minerals and water up, the leaves would die and correspondingly the roots would not be able to access the sugars produced by photosynthesis in the leaves. The vascular tissues are what ties the whole tree together and allows it to function. Unlike in ferns, whose stems are solely for lifting the leaves up above ground level, the trunks of trees and the stems of all higher plants use their vascular tissues to move water and nutrients around which allows them to become much larger than was possible before this major evolutionary advance. for more information:
http://www.ehow.com/facts_5657026_difference-between-vascular-nonvascular-plants.html
