From the Ground Up

by Derek Burch

Stems and Trunks

We have been looking at what soil conditions are needed for good root growth, leading to a look at how to water a plant in a pot. Now let's consider what happens to most of the water that the roots take in.

  


Plants have a very effective internal set up for moving things around. It is a liquid transport system that begins with the water that enters the root through thin-walled cells at the surface called root hairs. The water and dissolved substances pass from cell to cell into the center of the root where a complex of tubes begins that connects with every part of the plant.

The tubes are formed from dead, hollow cells stacked end to end. Not perfectly enough joined to be as continuous as a pipe in the plumbing of your house, but with very little in the end connections to interfere with easy flow. These cells make up a tissue called the xylem. In a plant that grows for more than a few weeks, new cells are continually added to the xylem on the outside of the core which already exists, and the new tissue increases the capacity of the system to carry liquids to meet the needs of the enlarging plant body that has to be served.

The oldest parts of the xylem are eventually taken out of service as liquid carriers of liquid, and may be plugged and used by the plant to store waste products that cannot otherwise be eliminated.

The transport system extends all the way into the leaves, branching into smaller and smaller divisions, so that no cell in the leaf is more than a few cell-widths away from a vein, as the divisions are now called. Picture a city expanding into its suburbs, with the main water supply being pumped through a network of pipes that change in size to match the consumption needs of progressively more distant areas. The xylem is the corresponding system in each plant.

The sewage disposal from our houses in the suburbs takes place in pipes that increase in size as more and more consumer's wastes flow into the system and back to the treatment plant. So it is with the collecting tissue in the veins, leading to the progressively larger transport system in the twigs and branches and tree trunks. This collecting tissue (called the phloem) carries not only waste from the cells of the leaf, but all the simple sugars that are formed by the leaves, using the energy of sunlight, which is their reason for being.
Wood
 In the trunk of the tree, the mature xylem forms the wood that we use as lumber. The cells that produce the new xylem are larger in the spring when growth is fast and smaller in the fall.The familiar "annual" rings formed in this way let us estimate the age of the tree, and are one of the elements that contribute to the characteristic grain pattern of certain tree species.Since many of the waste products stored in the disused xylem are toxic, the heartwood is often more resistant to fungus and insect attack.

The phloem connects the leaves with every other part of the plant, including the roots where some of the sugars cross back into the xylem to be carried up again to reach any cells not served by the downward flowing phloem. The sugars are broken down as an energy source for all the living processes in each cell of the plant, and are also used in conjunction with the minerals that come in with the water from the soil solution to make the myriad of compounds that make up the plant body and all the chemicals found in it.

The stems or trunks of the plant, then, are the connecting pieces between the roots and the leafy canopy. They are largely made up of cells that carries water and dissolved materials upwards, and another type of tissue that transports manufactured and waste products downwards.

They are also connecting or supporting pieces in the sense of holding the leaves up where they can intercept the energy of sunlight that the plant needs. Competition for light between crowded plants probably favoured the evolution of taller and taller species.

The internal makeup of the trunks is largely determined by the need to contain the conducting tissues, but the details of where exactly these lie depend on the type of plant. Forest trees are cylinders of woody tissue, while vines often have several separate strands running though them, or have a core of wood that is deeply ridged, which allows them to be more flexible. Palms never make wood. Their trunks have large numbers of separate strands of conducting tissue. The system works well for them, and is just the product of a different evolutionay pathway from the one that regular trees took.

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