Today’s (and next week’s) botany lessons are brought to you by “xylem” and “phloem.” You may remember these terms from a biology class, but I bet you haven’t used them lately. It’s time you did. You’ll be a better gardener as a result. Besides, I used to teach biology, and once a teacher, always a teacher. There will even be a quiz at the end.
Like all forms of life, plants have to solve a very important problem: how do you move nutrients and water from one part of your body to another? Animals (at least the more complicated ones) have a circulatory system, but what do plants do?
They too have circulatory systems. Their systems are comprised of two types of vessels:
- Phloem carries food (carbohydrates created by photosynthesis) from the leaves and green stems where it’s made, to the non-photosynthetic parts of the plant (such as non-green stems) where it’s used to fuel life, and to the roots, where it’s stored for later use. You can think of phloem as “feeding tubes” although their structure is of course more complicated.
- Xylem carries water and soluble minerals from the roots, where they are absorbed from the soil, to the leaves and green stems, where they’re used in photosynthesis. I think of xylem as straws, but their structure is more complicated too. You can look it up if you’re curious.
It’s easy to remember which is which—phloem carries “phood” while water is carried by xylem (wxy…z). Got it?
You can easily see some xylem by putting the stem and leafy top of a celery stalk into water with lots of red food coloring in it. As the water evaporates from the leaves, more water is drawn into the xylem. After a while, the leaves will turn red! (A more dramatic demonstration uses napa cabbage leaves and colored water, but we were all out of napa cabbage.)
Now cut the celery stalk to see a cross section. The red dots are cross-sections of xylem full of red water. And you know those “strings” that always get caught in your teeth when you eat celery? Yup, that’s the xylem. The next time you eat celery, you can impress everyone by mentioning that you have to go pick the xylem out of your teeth!
This is also one way florists turn flowers all sorts of unnatural colors, such as chrysanthemum corsages in your school colors for homecoming, or those green daisies you buy for St. Patrick’s Day.
In herbaceous plants (those that aren’t woody), the phloem and xylem may be distributed throughout the stem, as in monocots, or arranged in a ring of bundles (like one of those underground utility cables), as in dicots. Celery strings are actually xylem bundles.
In a woody shrub or tree, the phloem is forms the living, inmost layer of bark, while the xylem is inside that. As the tree grows, new xylem cells are formed outward from the old ones. After they’ve grown, they die. These dead xylem tubes pack the interior of the trunk. As they age, they’re filled with resin and become wood.
How does the xylem move water against gravity from underground to end up high in the leafy canopy of a tall tree? Most botanists believe the primary force is “cohesion-tension”—a fancy way of saying “surface tension.” Water molecules tend to stick to one another (it has to do with the hydrogen bonds in each water molecule), so when one molecule evaporates or is used up at the top of the tree, the rest are pulled upward. The water molecules stick to the cells of the xylem, which helps them climb. And, as water pressure builds up in the roots (after a rain, perhaps), that pressure also propels water upward into the rest of the plant. You can see that a combination of factors all work together to get that water into the leaves.
There are numerous pests and diseases that interfere with a plant’s xylem. “Damping off,” the foul killer of defenseless seedlings, is actually a fungus that grows inside the xylem, clogging it. That’s why your little plants wilt and die, and why it’s important to use a sterile potting mix when starting seeds indoors.
Cytospora is another genus of fungi that kill by blocking the xylem. These fungi live in many trees popular in the Colorado landscape. You might recognize a Cytospora infection as bright orange seepage on the trunk (as with this aspen, right). It spells doom by clogging both the xylem and phloem, effectively girding the trunks from the inside.
Insects also damage a plant’s xylem. For example, Squash Vine Borers are the larvae of small moths that tunnel into the stems of squash plants. They effectively block the flow of water through the xylem, and the leaves wilt, then collapse and die.
Even the gardener can injure or kill a plant simply by crushing fragile stems. Always hold small transplants by a leaf, rather than squeezing the stem, or simply cup your hand and gently dump the seedling into your palm.
Next week’s topic is “Phloem is for phood.” See you then.