Most plant owners get caught up on one thing- watering their beloved plants. Sometimes they're busy, sometimes its laziness; but do "plant moms" know what happens when they get to the point of no return? Typically, a thirsty succulent will spring right back after a little drink; but what does it mean when a plant can't recover, even after taking a long bath? This process is known as water transportation, something that every gardener, farmer, or house plant enthusiast should be aware of. The knowledge of the inner workings of water transportation can be life or death for plants at home.

Xylem and Water Transport

Water is stored and transported in hollow tubes throughout the plant body called xylem. Primary xylem is found in all vascular plants as a conduit of water and minerals, constructed of dead, hollow cells that allow water to move freely throughout the cylindrical tube. The cell walls are thickened with lignin, providing the structure necessary for water flow and prevents water from escaping.

When a plant is wilting, there is not enough water to fill the xylem and hold it upright, even with support from lignified cell walls. Plants extract water from the soil, pulling it upwards through the plant and ultimately out of the leaves, which is referred to as transpiration.

This entire process exhibits high amounts of negative pressure as water is pulled through the plant and out of small openings in the leaves called stomata. All known vascular plants possess root hairs, which are fine, unicellular growths on the epidermal layer of the root. These root hairs soak up water and nutrients and pull them into the root, where they find the xylem and begin the journey upwards.

To picture the structure of xylem, imagine a long, narrow straw that might be placed in a soft drink. A large bundle of these straws makes up the xylem we typically see in a vascular plant. Figure 1 depicts a microscopic view of a cross section from a carrot (Daucus carota subsp. sativus) root. The large, red circles represent the xylem. The xylem is hollow to allow for the transportation of water from the root to the rest of the plant.

Xylem structure can look different depending on plant type and placement in the plant. Vascular bundles (which include xylem, phloem, and vascular cambium) appear differently in roots versus stems, as well as monocots versus dicots. Figure 2 highlights the differences in structure between various plant types.

Water is able to defy gravity and move upwards throughout the plant by principles of cohesion and adhesion. Cohesion is the ability of water molecules to stick to each other, adhesion is the ability of water molecules to stick to other things (xylem walls). Both of these properties are possible with hydrogen bonds. Water molecules are able to stick to each other and cell walls to pull themselves towards the leaves.

Much like humans, water molecules don't like to be crowded and will find a place with low osmotic pressure. Most molecules prefer to move from a high to low concentration gradient, if many water molecules are being absorbed at the root, they move farther away from each other. As you move up the plant, water potential decreases, which drives molecules in that direction. Measured in megapascals (MPa), the atmosphere has a water potential 500x lower than the roots of a plant, explaining the need for water to move upwards and out of the plant.

Embolism

As water moves towards a lower water potential, substantial negative pressure is placed internally. If there is limited access to water, this could result in embolism within a plant. Embolism occurs when, instead of pulling water through the xylem, air gets trapped in the vessel, clogging up the system. An air bubble will prevent any water from flowing through and shuts down the xylem permanently. Additionally, embolism can spread throughout the plant as air bubbles rise and make the gap larger.

What can be done to prevent a blockage? The good news is that plants have developed strategies to avoid embolistic fates and their owners have systems to ensure safety. Angiosperms have developed margo torus features that diverge water around an air bubble as pictured in Figure 4. This can provide a safety net but should not be taken advantage of as it is not a foolproof plan. Plant owners should still take careful consideration into watering schedules and plant needs. Bottom watering can be especially helpful for house plants to simulate drawing in water from the ground at a necessary pace.

Knowledge of how water moves, where it is moving, and when it is not moving is great for any plant owner to have. Extensive backgrounds in biology are not necessary for every phytophile, but can increase the longevity and happiness of precious house plants!

Written By Riley Abernathy, Purdue University Bachelor of Science in Sustainable Food and Farming Systems Student

If you are interested in winning a $5000 scholarship towards higher education in an agriculture related field, check out our scholarship page for more information. Our scholarship program started back in 2016 in honor of our founder Demetrios Agathangelides. Demetrios immigrated to the United States from Greece and attended Utah State University, graduating with a degree in Plant Science. With a love of seeds and an appreciation for education he continued to teach in seminars and local talk shows. Today we honor him by awarding a scholarship to a deserving student in the agricultural sciences.

References:

Abernathy, R. (2025). Carrot Cross Section.

Claßen-Bockhoff, R., Franke, D., & Krähmer, H. (2020). Early ontogeny defines the diversification of primary vascular bundle systems in angiosperms. Botanical Journal of the Linnean Society 195(3), 281–307. https://doi.org/10.1093/botlinnean/boaa084

Delzon, S., Douthe, C., Sala, A., & Cochard, H. (2010). Different hypotheses of air-seeding through a conifer bordered pit membrane. In Plant, Cell, & Environment. https://www.researchgate.net/publication/45269011_Mechanism_of_waterstress_induced_ cavitation_in_conifers_bordered_pit_structure_and_function_support_the_hypothesis_of_seal_capillary-seeding_Cavitation_in_conifers

Isasa, E., Link, R. M., Jansen, S., Tezeh, F. R., Kaack, L., Sarmento Cabral, J., & Schuldt, B. (2023). Addressing controversies in the xylem embolism resistance–vessel diameter relationship. New Phytologist. https://doi.org/10.1111/nph.18731

Lens, F., Gleason, S. M., Bortolami, G., Brodersen, C., Delzon, S., & Jansen, S. (2022). Functional xylem characteristics associated with drought‐induced embolism in angiosperms. New Phytologist 236(6), 2019–2036. https://doi.org/10.1111/nph.18447

Ma, Y.; Liu, H.; Yu, Y.; Guo,L.; Zhao, W.; Yetemen, O. RevisitingSoil Water Potential: Towards a BetterUnderstanding of Soil and PlantInteractions. Water 2022,14, 3721.https://doi.org/10.3390/w14223721