The Vascular System

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Oh, my aching muscles...

By Marty Wingate

The inside of a plant is just as mysterious for most of us as our own insides: Complicated processes are carried out that somehow result in-for plants-roots, leaves, flowers, and fruit. How does it all work?

Fortunately for all of us, I'm no doctor, but I can explain to you how plants live and grow. And when we understand how plants do that, how they fare in our gardens makes much more sense.

Water and food travel between the roots and the leaves of a plant; water travels up from the roots in a system of tubes called xylem, while sugars made during photosynthesis travel down from the leaves through phloem.

In herbaceous plants, those that die back to the ground each winter, the vascular system above ground is replaced every year. In woody plants, new layers of xylem and phloem are added to old. This leads to heartwood in the center of a tree trunk-older layers of the vascular system-and a band of active xylem and phloem protected by a crusty bark.

Plants need oxygen-that's always a surprise to gardeners who begin to learn about the vascular system of plants and the uptake and output of various minerals, gases and water. The roots of a plant use oxygen, and so it needs to be available to them in the various sizes of pores in the soil structure. Oxygen is lost from the soil through compaction and when it is waterlogged. That is one of the reasons that the swamp maple, Acer rubrum, is such a popular choice as a street tree. It is native to wet soils and so has adapted to low levels of oxygen.

One of the functions of roots is to take up water from the soil and send it up to the leaves to be used during photosynthesis. Tiny root hairs absorb water, not the thick, woody anchor roots. Root hairs are worn out fast and are constantly being replaced.

Water is pulled up through a plant to the leaves through transpiration; it is used in photosynthesis and then released, along with oxygen, to the atmosphere through tiny pores called stomata that are on the undersides of leaves. Stomata create their own tiny moist atmosphere when open. As water is released through stomata, more is pulled up from the roots like liquid through a straw.

More water is lost from a plant during hot, dry or windy weather. Instead of water molecules hanging around each stomate opening, the moisture dries up or is whisked away. As water is lost, more water is pulled up from the roots.

Some plants have the ability to shut the stomata during such stressful times, and thereby reduce water loss. But they can't stay shut forever, or photosynthesis cannot take place. Of course, there are exceptions to this rule. In the desert, some plants have developed the ability to absorb light during the day, but carry out part of the photosynthetic process at night, when it's cooler and they won't lose as much water.

Gray-leaved plants have developed their own ability to reduce stress. Many, such as Artemisia and Senecio, are native to hot-summer areas. The silver-gray color of their leaves reflects much of the light and heat, while still absorbing enough light to carry out photosynthesis.

At the very least, we all remember the term photosynthesis from some science class somewhere in our past. And we probably also remember that photosynthesis keeps us all alive-no photosynthesis, no plants, no food, no us. Let's go a step deeperwithout having to rely on any memories of your organic chemistry classand see how photosynthesis works.

Chlorophyll is the green pigment we see in leaves. It is located in cells called chloroplasts. Here, photons of light are captured and, through a process involving water and carbon dioxide, sugars are made. The sugars are sent down to the roots of the plant through the phloem.

In a fast-motion sequence film, we could see how leaves orient themselves throughout the daylight hours in order to capture as many photons as possible. They look as if they are doing a leafy dance to the sun. But even in normal time we can spot the particular arrangements that help a plant's ability to photosynthesize.

Deciduous shade plants, for example, often have thin leaves that are spaced so that they overlap as little as possible; in this way they can capture more photons. Our native vine maple (Acer circinatum) growing in light shade is just such a plant. Evergreen plants, such as Leucothoe fontanesiana, also arrange their leaves in such a manner, but their leaves are thicker and have a waxy cuticle, which helps them last for more than one season.

Although photosynthesis is usually a leaf activity, some plants have adapted to harsh environments by shifting the light-gathering process elsewhere. Thin leaves wouldn't last long in the desert, and so cactuses have swollen stems called cladodes where photosynthesis takes place.

Photosynthesis is carried out and energy is made during the growing season, although these activities slow as the summer progresses. Because the low light of winter is not the best time for active above ground growth, most plants slow or shut down shoot and leaf growth then.

Deciduous plants shed their leaves, because it cost too much energy to keep them. The death of a leaf (senescence) occurs when the cells in vascular path break down. As chlorophyll dies off, other pigments can be seen better, such as the reds of anthocyanin and the oranges of carotene.

Autumn colors are a showy manifestation of what goes on inside a plant. But even those mostly unseen events have an effect on a plant, its health, and ultimately, our gardens.

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