The transport system in plants
Only a Hobbit would be able to demonstrate that a plant’s transport system is a tree’s ability to walk to the Tower. In the real world, a plant’s transport system moves water and nutrients throughout the plant.
The main source of water and mineral salts is the soil in which the plant is rooted. Like a ring of bendable straws, the plant transports ions and water from its root hairs to the tips of branches and the ends of leaves. In ‘reverse’, the plant brings photosynthesis products from the leaves to the rest of the plant.
Unlike an animal, these are not real arteries, but rather masses of cells called xylem and phloem cells. And unlike the pumping heart of an animal, the liquid is moved by osmosis, the difference in concentration which water tries to equalize. They are found in roots, stems and leaves. Both xylem and phloem cells grow from cambium cells which are present in the stem.
Xylem cells transport upward from the roots only. This is demonstrated by unique colored carnations, which are soaked in colored ink. The cells line up like tubes which are wide and have thick walls. They bring ions and water to the tips of the plant, so the plant may grow branches and create buds, flowers, cones and fruit. These cells live about one year then die and need to be replaced.
Phloem cells create “sieve tubes” which can transport in both directions – water from the ground, and sugars (mostly sucrose) and nutrients such as amino acids which are created by photosynthesis. Their end walls have small holes with strands of cytoplasm running vertically through them. Companion cells alongside the phloem cells control the movement and direction. When the yearly ring of xylem cells dies in a tree, the phloem continues as sap. Since phloem transports in both directions, it has a lot of sugars from some trees such as the maple tree, which can be tapped to make maple syrup.
The root hair cells of the root absorb water and mineral salts from the surrounding soil, which are transported throughout the plant to the root cap cells, which protect the growing tip of a stem or branch. The osmotic pressure between the xylem cells gives the plant some structure, so that the stems of flowers don’t wilt as long as there is water being passed along. In trees, these cells ‘line up’ as a ring; as the tree grows outward, the old xylem cells die, and new ones are created along the bark. This is why when people cut a ring around a tree below its bark, the tree will die – there is no completion of the transport system; nutrients cannot reach the roots or branches.
When the water and mineral salts reach the leaves, the water evaporates off the leaves, causing a constant pull upwards. This is called transpiration. The leaf can control the rate of evaporation by opening and closing its stomata (pores). If there is no water, the leaf shrivels and dies. Light stimulates the opening of the stomata. Other environmental factors also affect the rate of transpiration, such as temperature, humidity and even wind.
Deciduous plants are those which seem to hibernate in the winter. As the weather gets colder, transportation of water and nutrients slows down. Winds cause fast transpiration and sunlight is in low supply. Sugar gets stored; depending on the sugars being created by a tree, this causes the colorful fall foliage. When the leaves can no longer photosynthesize, they fall off, and the place where the leaf was is sealed off, as are the tips of the branches and stems. This keeps the water pressure steady but unmoving. As the spring sun warms the plant, these caps are dropped and growth resumes. Leaves grow and darken as they photosynthesize, flowers pop up and form nuts or fruit, and the cycle resumes.
By understanding how and why a plant circulates water, mineral salts, sugars and amino acids, people can understand how to care for the trees and plants around them, and spot the signs that there is a problem with the transport system.