Last week’s post about xylem explained how it carries water from the roots to the rest of a plant. But there’s another transportation problem that plants have to solve. As you know, plants make food (sugars) through photosynthesis. (See my previous posts on photosynthesis.) This food factory requires both chlorophyll and sunlight, and can only take place in the green parts of a plant. Usually this means the leaves, although cacti and other xeric species (such as this Palo Verde, above right) often have chlorophyll in their stems.
Through the chemical process of respiration, these sugars are used to fuel all other life processes—growth, reproduction, defense, etc.—just as they are in animals. We store extra sugars as glycogen and fat. Plants store their extra sugars in roots, bulbs, and tubers. Often the sugars are combined to become starches, as in potatoes. (“Does this dress make my tuber look fat?”) But how do they get there?
Just as our blood carries nutrients from our digestive tract (or storage areas, such as fat cells) to the rest of our body, so does sap carry sugars to different parts of a plant. In fact, sap is simply a mixture of water and sugar. And just as blood travels inside blood vessels, that sap travels inside tubes called phloem.
Phloem is usually paired with xylem in non-woody stems (below). In woody plants such as trees, they surround the xylem just under the bark (right). But while xylem only carries water in one direction, phloem can carry sap both to and from the leaves, stems, and roots. Remember these illustrations from last week?
Gardeners wanting to impress others with gigantic apples, or perhaps peaches, can take advantage of this arrangement of phloem in a tree branch. First, the grower removes all but one fruit from a large, leafy branch. Then, the base of that branch is carefully girded just deeply enough to sever the phloem but not the xylem. By blocking the way out, the gardener can ensure that all sugars produced by that branch’s leaves go only to feed that one remaining fruit. The result is an apple or peach many times its normal size. (Of course, this doesn’t help the branch at all, and it will eventually die.)
In early spring, deciduous plants grow new leaves by drawing on the sugars stored over the winter in the roots. Then, once these new leaves come “online” and start producing their own food, the flow goes the other way. It’s even possible to have one phloem tube transporting sugars in one direction while the tube right next door is flowing in the opposite direction.
One way we take advantage of this phloem flow is through the maple sugar industry. Maple trees must be tapped in early spring, siphoning off some of the sugar-rich sap that is rising from the trees’ roots. Then the sap’s water is boiled away, leaving either syrup or, if you keep boiling until all the water is gone, maple candy.
Since plants lack a heart (artichoke hearts don’t count), what keeps this flow moving? Cells with extra sugars (those in either root storage areas or, later in the season, sugar-producing leaves) actively pump sugar into the phloem tubes. Water flows in after the sugars through osmosis (so that the concentration of sugars on either side of the phloem membranes is equal). This creates a higher pressure inside the phloem, pushing the sap down the tube.
Similarly, in areas lacking sugar, the cells actively move sugar out of the phloem, creating a low pressure area that pulls in more sap. At least, this is how it all works in most plants. There are exceptions, mostly among more primitive plant species, which keep scientists guessing. No, we don’t have all the answers!
Just as blood carries more than sugar and water, so phloem carries more than sugar. Other chemicals hitch a ride on this mass transit system. For example, different parts of the plant communicate with one another by sending chemical signals through the sap, in the same way that hormones communicate via the circulatory system within our own bodies.
In addition to carrying sugars, both phloem and xylem contribute strength to plant stems while allowing them to stay flexible. Phloem and xylem cells contain what a nutritionist would call fiber. The cell wall is the part of a plant cell that adds strength to a stem, and these cells have a double cell wall, adding even more strength. Paper, linen, and cotton are made out of this fiber, and it’s an important part of our diet as well.
That’s enough for now. It’s time for that quiz I promised last week.
- Why is it a bad idea to use a string trimmer (aka weed wacker) to cut down weeds around the base of a young tree?
- Why are deciduous plants and perennials harmed more severely by a big hail storm that occurs in June compared to one that occurs in late September?
- Why do beets and carrots contain more sugars than spinach and broccoli?
I’ll provide the answers next week.
Maple sugaring photo by Dave Pape, Wikipedia