When cells join together to make more complex organisms, they begin to develop specialized functions. They become dependent on one another to supply the energy and nutrients that they need to live. An immediate problem that they have to solve is how to share resources with one another. They need to develop vascular systems.      We know that in our own bodies, blood is circulated through veins and arteries. The blood cells bring oxygen and glucose to our cells and take waste products away. Plants also need to circulate fluids through their systems too. How do they do this?      When plants are single cells, they simply absorb water and nutrients through their cell walls. However, when cells are grouped together, the cells in the interior of the organism are cut off from these resources. The cells on the inside also have no way to dispose of the wastes that they once simply pushed out through the cell wall. The need to develop distribution systems had to be met before more complex organisms could develop.

      As we learned in The Evolution of Land Plants, the earliest land plants, such as the mosses, did not have vascular structures. Their cells absorbed moisture and minerals by letting liquids move slowly through their cell walls. This worked for them because mosses are very small plants. Mosses live in moist environments where water is freely available.

     The ferns developed vascular structures. They grew roots so that they could get moisture and minerals from the soil. These roots were hidden in darkness in the ground. They had no way of synthesizing the food that they needed for life and growth. Above the ground, the fronds of the ferns spread out to catch the light. However, without water, they could not live. Vascular structures allowed the different parts of the plant to contribute to the good of the whole.

These greenish cells with the arrows pointing down are called phloem (pronounced FLOW-em) The phloem is made up of cells which join to form a tube. They have open spaces in the cell walls where they join, and so are called sieve tubes.

Think phloem = food. Food from the leaves flows down the tubes of the phloem and nourishes the roots and other parts of the plant, such as the growing areas underneath the bark.

The cells that make up the phloem are modified to serve as conduits. Their cell walls become thickened, and they are surrounded by helper cells which support them. These cells are alive, but they have lost most of their original contents so that the nourishing liquid can flow through them.

These plant tubes are very thin, being made from single cells. Large plants have many of them. They transport water that the leaves release into the air as a byproduct of photosynthesis. On hot days leaves loose a lot of moisture, and must be supplied with enough water to keep them from wilting. Water has to get up to the top of even the tallest tree. The height to which water can be raised on your planet will determine the maximum height of your trees.

 The Evolution of Land Plants