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
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
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.
we see diagrams of the vascular structures found in modern trees.
These greenish cells with the arrows pointing down are called
(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
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 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.
blue-filled cells are known as the xylem (pronounced z-eye-lem).
They are dead cells, and they form a strong, hollow pipe for
bringing water and minerals up from the ground. They, too, have
thickened cell walls, which are strengthened by incorporating
lignin, the fiber that gives strength to wood.
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
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