From the Casparian strip to the phloem, the vascular system is crucial to nutrient uptake and plant health. Grubbycup puts on his biology hat and explains the mechanisms of vascular plants.
Vascular plants include trees, plants with fruit, plants with flowers, dicots, monocots, land plants, and pretty much any plant that is isn’t a moss, liverwort, hornwart, or algae. They all have a vascular system (hence the name) including some form of xylem, and phloem. This system allows for the plant to absorb moisture and nutrients from the growing medium and move essential resources around the interior of the plant.
The Casparian Strip
Water and nutrients enter through the root hairs. Once inside the root hair, the liquid can move either by going along the spaces between the cells (apoplastic pathway), by entering a cell and moving from cell to cell through the adjoining cell walls (symplastic pathway), or a combination of the two (transcellular). Regardless of the type of pathway taken, they all lead to a layer of cells known as the Casparian strip.
The Casparian strip blocks the pathways between cells (which ends the apoplastic paths), and forces liquids to enter through a plasma membrane (in symplastic fashion) to cross the barrier. This plasma membrane can not only regulate the flow of liquid, but filters out a variety of contaminates and microbes. It selectively allows desired nutrients and water to pass through, while (mostly) preventing undesirable substances a pathway to the plant’s sapstream. The filtered nutrient solution passes through the Casparian strip to vascular bundles in the stele.
Trundle Over to the Vascular Bundles
Vascular bundles include both xylem and phloem, which are the main fluid pathways. In most dicots (plants with two seed leaves) there is a layer separating the two called the vascular cambium.
The xylem and phloem allow the plant to transport internal fluids. There are different arrangements for the xylem and phloem within the bundles, but in general the xylem tends to be toward the center of the stem while the phloem is closer to the bark. In leaves, the xylem side tends to be closer to the top or “face” of the leaf, while the phloem tends to be closer to the undersides of leaves.
Xylem from Start to Exit
The filtered water and nutrients pass through the Casparian strip to the xylem. The xylem is a series of long, connected dead cells (tracheids or vessel elements) that form “pipes” that carry water and nutrients from the roots up through the plant to the undersides of leaves where most of the water exits the plant through openings called stomata.
Each stoma opening has a pair of guard cells that can open and close the opening as needed. The opening and closing of the stomata resemble a pair of lips, which are closed when relaxed, and when swollen create pursed or “duck” lips to create an opening between them.
Read also: Take a Trip Along the Calcium Highway
The stomata not only allow water vapor and oxygen to escape, but they let in carbon dioxide. In times of high heat or in the absence of light (at night), they may close to prevent excess water loss. When the guard cells have closed, they cut off the supply of CO2, which retards plant growth.
Movement of water through the xylem is passive and does not require energy from the plant. The xylem tubes are small enough to take advantage of capillary action (with the help of some of water’s more unusual properties) to draw water up from the roots to replace what has been lost due to evaporation through the stomata.
This effectively creates “chains” of water that lead from the roots to the leaves. If an insufficient amount of water is available, at first these chains will shrink, causing the plant to droop. This can be seen in a slightly underwatered plant that starts to wilt. In the early stages, this can be corrected by watering the plant, which should respond fairly quickly (within an hour or so) by replacing the lost water and swelling back to normal appearance.
If the plant continues to dehydrate due to lack of water, the water chains will get thin enough to begin to break, causing air pockets to develop. Once enough of the chains have been broken, the plant will go into “terminal wilt,” will no longer respond to watering, and will die.
While it is usual to consider the xylem as carrying water, hormones, and nutrients, and the phloem as transporting water, hormones, and sugars, in some cases, such as with maple trees, sugars stored in the roots can use the xylem as a pathway to lift them up. Maple syrup is collected by piercing the xylem to collect this sugar sap, which is then boiled to condense and thicken.
Vascular land plants can grow taller than their non-vascular counterparts such as mosses and algae in part because the xylem gives strength and structure to the plant stems.
Can’t Cross the Cambium
The vascular cambium layer often forms as a cylinder along the stem (or trunk), with the xylem (wood) on one side and the phloem (bark) on the other. The vascular cambium has a concentration of meristem (building block) cells which are used to increase both the xylem to the inside and the phloem on the outside.
Read also: Seeds and Meristems
As an example, tree trunks add a yearly layer of wood (tree rings) formed from xylem, and the cambium grows a new layer of xylem to replace the old one. As the inner wood diameter increases, the cambium on the phloem side adds additional cells as needed.
Go with the Phloem
While the xylem carries water and nutrients in only one direction (from the roots to the leaves), the phloem carries sugars, hormones, amino acids, and relocating nutrients around the plant from stores (where the resources are in the plant) to sinks (where the resources are needed). For example, in the spring, sugars stored in the roots are moved to new growth sinks to plant growth. During photosynthesis the sugars made in the leaves act as stores that can be transported to sinks such as the roots (to prepare for the following spring) or to flowers and fruits.
When growing vascular plants, it is helpful to be at least acquainted with vascular systems. By understanding that guard cells close at night, it becomes apparent that supplemental carbon dioxide is better used during the day. By understanding that guard cells close in extreme heat, it is easier to comprehend why plant growth “stalls” during a heat wave. Each piece of the puzzle makes it easier to get the big picture.