Walls Prevent Collapse Under Tension

close up photograph of budding yellow flower

Biological Strategy

Plants

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Xylem vessels and tracheids of vascular plants prevent their own collapse while under tension via helical thickening of their walls.

“In young plants, often in addition to the epidermis, the cells specialized for conducting water from root to leaves and shoots, have a mechanical function. The xylem vessels and tracheids are elongated cells (in the case of vessels, the vessel itself is composed of a series of shorter ‘vessel elements’ forming an axially elongated structure). These cells have thickened walls which help prevent their collapse when water in them is under tension through the pull of the transpiration stream (Fig. 3). The drying effect at the leaf surface promotes water movement from the roots through the plant body. The first formed conducting cells of the xylem consist of rather thin-walled, elongated cells that have to extend with the growth in the length of the stem. Their collapse during the time they are needed to function is prevented by specialised thickening in their walls. This takes on the form of a series of annuli, or of a spiral (helical) winding…The tracheids and vessels formed after extension growth is complete tend to have thick, rigid walls with either thin areas (pits), as in both tracheids and vessel elements, or clear openings between cells in line, as in vessel elements alone. These facilitate water movement from cell to cell. Even here, some of these cells in a range of species have an additional thickening on the inner side of their walls.” (Cutler 2005:99-100)

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